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SPECIAL TOPICS |
* American Institute for Goat Research, Langston University, Langston, OK; and
College of Veterinary Medicine, Oklahoma State University, Stillwater, OK
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Abstract |
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 Top Abstract INTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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Key Words: goat • research
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INTRODUCTION |
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TopAbstractINTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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). By 1997 that number had decreased by over 50% to 829,000 (USDA, 2004), and in 2008 there are only an estimated 210,000 Angora goats in the United States (NASS, 2008). From 1987 to 2008, the number of meat goats increased from 415,000 to 2.5 million and the number of dairy goats rose from 129,000 to over 305,000 (USDA, 1994; NASS, 2008). The introduction of Boer goats and elimination of the mohair subsidy contributed to this change in production, although other factors such as the increasingly diverse ethnicity of the American public are also important. The changing landscape of goat production during the past 20 yr has also affected the focus of much of the goat research undertaken in the United States. Fiber research has lessened, whereas studies concerning milk and meat production have increased. In 2008, goats are raised in large numbers in many states. Thus, the need for locality-specific studies on topics such as feedstuffs and management systems has expanded the breadth of research. Considerable advancements in goat research have been made in understanding nutrient requirements, the growing problem of internal parasitism in light of increasing anthelmintic resistance, and the natures of goat meat and milk. Future research areas must be carefully considered and focused on most important limitations to profitable and efficient production of goats and use of their products. |
NUTRITION/FEEDING MANAGEMENT |
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TopAbstractINTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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Limitations
Recently the National Research Council (NRC, 2007) summarized research concerning energy and protein requirements of goats. Most attention was given to a series of articles from Langston University published in a special issue of the journal Small Ruminant Research (2004; volume 53, issue 3). The recommendations from this work and those of NRC (2007) were derived by regressions with databases of treatment mean observations derived from literature. The observations in most cases were categorized into biotypes according to selection for different productive purposes [i.e., dairy, Angora, meat (50% or more Boer blood), local or indigenous]. This was done under the assumption that selection might have affected nutrient requirements. In some cases biotype did influence requirement estimates. However, there are several limitations of this approach. For example, the number of observations for meat goats was less than for indigenous/local goats, for which intensive selection by humans has not occurred. Furthermore, the number of intact males was greater for the meat goat classification than for indigenous/local goats. It is possible that such disparities influenced the detection of significant differences in some nutrient requirements, such as a greater MP need by meat vs. local/indigenous goats for growth.
Among other shortcomings of this work is that potential effects of sex on nutrient requirements could not be addressed because sex was not reported in some articles and often values presented were averages for 2 or more sexes. Hence, a 15% greater ME requirement for maintenance (MEm) of intact males vs. females and male castrates and no difference in MEm between females and male castrates were assumed. Future research should address existence and quantification of differences among goat sexes in energy and protein requirements.
Metabolizable energy and MP requirements for ADG, milk production, and mohair fiber growth were determined by regressing estimates of intake against levels of production. An attribute of this approach is relatively large numbers of observations available for use, but there are disadvantages as well. For example, numerous assumptions were necessary to derive ME and MP intakes, and ADG was on a full or unshrunk BW basis. Because of the wide array of dietary conditions encompassed by the database, often with differences in dietary concentrations and levels of intake of ME and MP, when ME intake was regressed against level of production to assess energy requirements, energy may not have been the primary determinant of, or limitation to, production level. The same concerns are valid for MP. Therefore, casual relationships determined were assumed to be directly related to requirements.
Nutrient Requirement Knowledge Gaps
Both NRC (2007) and Sahlu et al. (2004) indicated areas in which additional research is needed to more accurately describe energy and protein requirements of goats for improved feeding management practices. One pertaining to protein is efficiency of MP use for different physiological functions. For instance, there is currently a large range in efficiencies of MP use for pregnancy assumed by different committees and for various ruminant livestock species. Examples are 0.85 for the Agricultural and Food Research Council (AFRC, 1993, 1998), 0.65 for NRC (2000), 0.7 for Standing Committee for Agriculture (SCA, 1990), and 0.33 for NRC (2001) and Sahlu et al. (2004).
Energy requirements of NRC (2007) are for ME. To eventually advance to a NE system, research is needed to address body composition and efficiency of ME utilization for different physiological functions. For example, in a Web-based nutrient requirement calculation system for goats for applying NRC (2007) recommendations (available at http://www2.luresext.edu), assumptions based on relatively small amounts of literature for body composition (AFRC, 1993, 1998) are employed to address tissue mobilization for maintenance with low ME intake, mobilization of tissue for support of lactation and mohair fiber growth, and use of dietary energy for BW gain by lactating goats. It is assumed that the energy concentration in tissue with accretion and mobilization is constant regardless of level of condition or fatness. However, recent research suggests that the composition of tissue being accreted may differ from that being mobilized within specific ranges of BCS and also that tissue composition early in realimentation or restricted feeding periods may differ from that later (Ngwa et al., 2007).
The MEm requirement of goats, as of other ruminant livestock species, is affected by nutritional plane. The NRC (2007) method of adjusting MEm for nutritional plane is similar to the NRC (2000) adjustment for beef cattle with levels of energy intake less than MEm. This method should be tested with various goat biotypes/ genotypes, and if found inaccurate, more appropriate means should be developed. In this regard, at least for some genotypes like the black Bedouin (Brosh et al., 1986; Silanikove, 1986, 1987; Choshniak et al., 1995) and Boer goats (Asmare et al., 2006), the degree to which MEm can decline with restricted energy intake appears greater than the maximum level of 20% assumed by NRC (2007).
Another area in which research is needed is the characterization of factors influencing, and establishing simple means for predicting, the grazing activity energy cost (GAEC). Estimates of the GAEC have been quite variable, including values as high as 100% of the MEm requirement of animals in confinement (Lachica and Aguilera, 2003). Current descriptions of GAEC in nutrient requirement prediction systems of committees such as the NRC (United States), AFRC (United Kingdom), and SCA (Australia) are not based on direct measurements while grazing and have not been validated. The lack of studies addressing GAEC is in part because of the difficulty of study under free-moving field conditions. However, recently techniques and methods have been developed that may allow such research. Examples are heart rate to estimate heat production, GPS collars and GIS software programs for horizontal and vertical distances traveled, and monitors to characterize grazing activities (Animut et al., 2005; Berhan et al., 2005; Brosh et al., 2006; Brosh, 2007; Patra et al., 2008a,b).
Potential effects of acclimatization on energy requirements of ruminant livestock species have been acknowledged in most sets of nutrient requirement recommendations. However, the most common method of adjustment of NRC (1981), also presented for goats by NRC (2007), was derived with cattle and cold temperatures. Hence, research on potential effects of acclimatization on energy requirements of goats should be conducted. There should be consideration of humidity in addition to ambient temperature, an assumed linear effect of temperature on MEm, use of a mean thermoneutral zone temperature, and a mean temperature in the preceding 1-mo period, etc. However, besides the difficulty of research in this area regarding necessary facilities, goat breeds vary considerably in relevant characteristics such as insulating fiber, thus necessitating study with different goat genotypes.
The NRC (2007) discussed general factors influencing ad libitum intake by goats and other small ruminant species. However, equations for use in practical production or experimental settings were not presented. Luo et al. (2004) developed equations for predicting intake by growing, mature, lactating, and Angora goats. Levels of productivity such as BW change, milk energy yield, and mohair fiber growth were among the independent variables. Hence, such equations cannot be used to predict feed intake for subsequent projection of level of production. Furthermore, treatment mean observations used by Luo et al. (2004) were with goats under confinement conditions. Future research to develop feed intake prediction equations with goats should consider typical conditions with free movement.
Goat mineral and vitamin nutrition has been recently reviewed by NRC (2007). Sufficient information was available to estimate requirements of most macrominerals by the factorial method, but this was not true for many microminerals. In some cases requirements were derived based on data for other ruminant species. Mineral requirements may be the next research frontier in goat nutrient requirements.
Future Research for Emerging/Increasing Production Practices
Goats are typically produced with most nutrients derived relatively inexpensively from grasses, forbs, and browse plant species. Luginbuhl (2007) recently discussed common pasture systems used in meat goat production. However, with the ever increasing demand for goat meat in the United States, interest in rapid-growth meat goat production systems (i.e., feedlots) has risen. Research of most appropriate management practices for meat goat feedlots is needed, such as economical dietary feedstuffs, minimum fiber requirements, health management practices, product quality relative to consumer preferences, etc. Scientists trained in nutrition and feeding practices with other ruminant livestock species, such as beef cattle, surely will participate in these investigations. But, for most efficient research programs it is recommended that there be collaboration with scientists having previous specific experience with goats.
Goats are commonly used for vegetation management, as reviewed by Hart (2001). Similarly, co-grazing of goats and other ruminant species has been practiced throughout history. Co-grazing of goats and sheep was recently reviewed by Animut and Goetsch (2008). Benefits of co-grazing may not be fully appreciated, and means to maximize them have not been extensively studied. Furthermore, as anthelmintic resistance of internal parasites increases, co-grazing of goats with cattle as an internal parasite management strategy may increase in prevalence. Apart from benefits in internal parasite management realized by co-grazing of cattle and goats, there are many other favorable attributes. Grazing by ruminants itself decreases fire hazards, although grazing by goats is particularly advantageous for minimizing fire danger and vegetation management because of relatively high consumption of browse and woody plant species. Advantages of co- vs. mono-species grazing are derived primarily from differences in preferences for particular plant species and parts, abilities or willingness to consume forages that are not highly preferred and would have greater adverse effects on the other species, and physical capabilities to gain access to specific types of vegetation. Hence, the degree to which total stocking rate or carrying capacity is greater for co- vs. mono-species grazing increases with increasing vegetation diversity and, concomitantly, decreasing dietary overlap. Reasons why co-grazing is not practiced in some areas include a simple lack of knowledge or appreciation of the attributes, greater management skills and knowledge necessary for 2 or 3 species vs. 1, possible decreases in production efficiencies such as in purchase of smaller lots of supplies, and additional production inputs for raising of small ruminants such as increased fencing requirements and protection from predation. Of these, in the United States fencing and predation are of particular importance and areas where research is required.
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HEALTH |
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TopAbstractINTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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Successful goat production depends on maintaining animal health. Benefits from proper management practices such as in nutrition and breeding are only realized with healthy animals. Although several diseases can affect the health and productivity of goats, some of special concern for goats in the United States include caseous lymphadenitis, caprine arthritis encephalitis, Johne’s, enterotoxemia, and scrapie.
Caseous Lymphadenitis
Caseous lymphadenitis is a chronic, contagious bacterial disease affecting sheep, goats, and horses around the world. The causative agent is the bacteria Corynebacterium pseudotuberculosis (Smith and Sherman, 1994; Williams, 2001; Anderson et al., 2002). The disease is manifested mainly in 2 forms, external involving superficial lymph nodes and internal involving visceral lymph nodes and organs (Smith and Sherman, 1994; Williams, 2001; Anderson et al., 2002). Efficacy of vaccines depends on the protective effects of antibodies against exotoxins. Currently available vaccines mainly activate humoral responses by limiting spread of infection without elimination (Brogden et al., 1996; Williams, 2001). Research to produce DNA and whole cell vaccines have shown some promise (Chaplin et al., 1999; Williams, 2001).
Caprine Arthritis Encephalitis
Caprine arthritis encephalitis (CAE) is a chronic degenerative disease seen in adult goats, manifested as polyarthritis, interstitial mastitis, or pneumonia. Occasionally, CAE may be characterized by acute leukoencephalomyelitis in 2- to 6-mo-old kids (Perk, 1995; Stehman, 1996; Reilly et al., 2002; de la Concha-Bermejillo, 2003). The disease is caused by nononcogenic retroviruses, which are closely related to the maedi-visna virus affecting sheep. Goats are natural hosts for the CAE virus, but recent work has shown that CAE and maedi-visna viruses are interspersed in sheep and goats, supporting the theory of potential cross species transmission (Perk, 1995; de la Concha-Bermejillo, 2003).
Johne’s Disease
Johne’s disease, or paratuberculosis, in goats is caused by Mycobacterium avium ssp. paratuberculosis. Although many features of infection caused by M. paratuberculosis are similar across ruminant species, the clinical presentation and interpretation of diagnostic tests differ between cattle and other ruminants (Stehman, 1996). Research in Norway has indicated that a vaccine for Johne’s disease in goats eliciting local defense in the intestine where the bacterium invades may be possible in the future (Olsen, 2007).
Recently this bacterium has received increasing attention because of evidence suggesting a relationship with Crohn’s disease in humans (Uzoigwe et al., 2007). Crohn’s disease is a gastrointestinal disease with histopathological findings similar to Johne’s disease in cattle. A high percentage of people with Crohn’s disease are infected with M. avium ssp. paratuberculosis, although it is not known if the association is causal or coincidental.
Scrapie
Scrapie is a fatal degenerative disease affecting the central nervous system of sheep and goats. Most scientists believe that the causative agent of scrapie is an abnormal form of a naturally occurring cellular prion protein known as PrP scrapie. It is thought that this abnormally conformed prion protein, PrP scrapie, serves as a template to influence a geometrical conformation change in the normal PrP cellular protein produced by the exposed animal. After a period of months and more often years, it causes nervous system dysfunction and, eventually, death. The PrP protein may be found in the nervous system, spleen, lymph nodes, placenta, intestines, blood, pancreas, ovaries, and liver of infected sheep (USDA, 2008).
The gene that encodes the normal prion protein has polymorphism at codons 136, 154, and 171, which influences the ability of the prion cellular protein structure to be geometrically altered by the PrP scrapie template when the animal is exposed. At this time, no such polymorphisms have been identified in goats; therefore, all goats are currently assumed susceptible to scrapie (USDA, 2008). Current testing procedures include histopathology and immunohistochemistry, genetic testing for susceptibility, and live animal tests like third eyelid biopsy, rectal tissue biopsy, and capillary electrophoresis using monoclonal antibodies (Shulaw, 2008).
Enterotoxemia
Enterotoxemia is another important disease of goats caused by gram positive bacteria called Clostridium perfringens type C or D (Rings, 2004). The disease is caused by an abrupt diet change favoring proliferation by these bacteria producing toxins that degrade the integrity of the intestines (Fernández-Miyakawa and Uzal, 2005). Recent research indicated that intestinal content, preferably ileal, should be used in diagnosing suspected cases of enterotoxemia because no toxin was found in abdominal fluid or urine (Layanaa et al., 2006). Type C bacteria affect very young kids and older adult animals, and type D bacteria usually affect young rapidly growing kids. Vaccination of goats does not allow the same level of protection that occurs in sheep (Schoenian, 2008).
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INTERNAL PARASITE CONTROL |
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TopAbstractINTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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Internal parasitism poses a major problem in the small ruminant industry. The USDA National Animal Health Monitoring System reported that 74% of sheep in the United States encountered gastrointestinal nematode parasitism (USDA-APHIS-VS, 2003). The level in goats may be more severe because goats are more susceptible to nematode parasites than sheep (Lloyd, 1987; Lightbody et al., 2001). In clinical cases during 1993 to 2000 at the Auburn University Veterinary Medical Teaching Hospital, 91% of the goats were examined and treated for reasons related to internal parasite diseases (Pugh and Navarre, 2001). Parasitism significantly reduces performance of animals, including decreased BW, fertility (older age at first kidding and longer intervals of kidding cycles), birth weights and growth rates, and markedly increased mortality of kids (Gatongi et al., 1997).
The current management of internal parasitism mainly relies on uses of anthelmintics. This practice is challenged by widespread anthelmintic resistance in the United States and other parts of the world (Zajac and Gipson, 2000; Terrill et al., 2001; Mortenson et al., 2003; Kaplan, 2004; McKellar and Jackson, 2004). Anthelmintic resistance in goats seems to be more prevalent than in other species of animals. Alternative approaches to internal parasite control include enhancing resistance by nutritional manipulation, genetic selection, and vaccination; feeding or browsing forages with helmintic-suppressing properties; and avoiding contaminated pastures by grazing management. Other strategies include adding nematode-trapping fungi or copper oxide wire particles (COWP) to the diet to kill adult worms in the abomasums or parasitic larvae on pasture (nematode-trapping fungi). Also, the FAMACHA eye color chart system is a simple and rapid means of assessing anemia caused by Haemonchus contortus, which allows for selective treatment of individual animals.
COWP
The effects of minerals on immunity to internal parasitism have been reviewed recently in detail (McClure, 2008). Here we briefly review the application of COWP in treatment of parasitism. The COWP, a supplementary form for copper deficiency in small ruminants, has been found to be effective in treatment of internal parasites in sheep under a wide array of experimental conditions (Bang et al., 1990; Knox, 2002; Burke et al., 2004, 2007a; Burke and Miller, 2006). The research in goats has produced mixed results. Glennon et al. (2004) observed a 32 to 41% reduction in fecal egg counts (FEC) in yearling goats rotationally grazing bermudagrass (Cynodon dactylon) pasture receiving 5 g of COWP and a 57% reduction with a 10-g dose. In a subsequent study with strip-grazing of bermudagrass pasture, Luginbuhl et al. (2006) found that COWP at 2.5 g did not have any effect on FEC in weanling goats. However, with 2 studies in cooler months in Georgia and 4 trials in warmer spring and summer months in Arkansas, Burke et al. (2007b) observed that COWP capsules were effective as an anthelmintic for up to 21 d after dosing and doses as small as 0.5 and 5 g were effective for weanling and mature goats, respectively. Furthermore, COWP are more effective against H. contortus (Burke et al., 2007b) and less effective against mixed natural infections with Teladorsagia, Trichostrongylus, or Trichuris (Pomroy and Adlington, 2006). Nevertheless, more studies are needed in the area to use COWP capsules as an alternative anthelmintic.
Selection of Resistant Animals
Resistance to internal parasites in goats has been less thoroughly studied than in sheep. Some breeds of goats are more resistant to internal parasites than others. For example, the Small East African goat genotype had less FEC and greater packed cell volume as compared with Galla goats throughout the year, and the difference was even greater during lactation (Baker et al., 1998, 2001). Similarly, in one recent report Kiko and Spanish breeds appeared more resistant than Boer goats (Browning et al., 2007). Baker and Gray (2004) discussed evidence for greater relative resistance of indigenous goats in Thailand and the Philippines, compared with Nubian derived breeds and crosses. Pralomkarn et al. (1997) also observed that Thai native goats had much less FEC than crosses of 50 and 75% Anglo Nubian. Only 8% of a trickle infection became established in the Thai native goats as compared with 17% for Anglo Nubian crosses. Even in the same breed, FEC of individual animals are not normally distributed, but are rather overdispersed with a positive skewness (Hoste et al., 2001). A few individual animals of the population have most of the worms (Costa et al., 2000). Typically 20 to 30% of goats in a herd have 70 to 80% of the worms, and FEC are relatively repeatable (Hoste et al., 2001). Vlassoff et al. (1999) observed a considerable individual variability in FEC of Angora goats after natural or experimental challenge infection. Repeatabilities of FEC in goats are 0.22 to 0.41, similar to sheep, and FEC heritability in goats is moderately low (average of 0.29 and range from 0.19 to 0.37; Rohrer et al., 1991; Mandonnet et al., 2001).
The mechanism for heritability is not well established. Genes for the major histocompatability complex of the immune system in sheep have been identified as candidates (Benavides et al., 2002). The genetic trait measured (FEC) may be affected directly by genetic effects on the immune system (Fakae et al., 1999), indirectly through diet selection for improved nutrition or grazing habits that reduce exposure to infective helminth larvae (Hutchings et al., 2007).
Vaccination
Early research showed that sheep could be vaccinated with X-irradiated H. contortus larvae to develop immunity (Jarrett et al., 1959). In subsequent research immunoprotection against H. contortus was conferred after immunization with a cysteine protease-enriched protein fraction (Ruiz et al., 2004). A degree of immunity in goats in some instances can be developed by natural parasite infection (Pomroy and Charleston, 1989a,b). There has been a tremendous effort in Australia and New Zealand to develop a vaccine for control of helminths. Although most vaccines would be specific for only one species of helminth, this would still be effective because usually only one species predominates in a given season. In a review of the prospects of a vaccine for sheep parasites, Pomroy (2000) stated that many promising results had been realized, but no successful vaccines were available yet.
Forages with Anthelmintic Properties
Effects of condensed tannins on gastrointestinal nematodes have been reviewed in detail by Hoste et al. (2006). Condensed tannins extracted from various forage species have reduced FEC, hatch rate of eggs, development to L3 larvae, and motility and migration of L3 larvae (Molan et al., 1999, 2000; Min et al., 2005; Iqbal et al., 2007). Furthermore, tannin-containing plants may inhibit the development of larvae attached on leaves (Niezen et al., 1998). Lambs with heavy parasite burdens gained more BW and had less FEC and worm burdens after grazing a tannin-containing forage (sulla, Hedysarum coronarium) as compared with lambs grazing alfalfa (Medicago sativa; Niezen et al., 1995), although Pomroy and Adlington (2006) did not observe any effect on worm numbers in the gastrointestinal tract of animals fed sulla (H. coronarium) for 10 d. Paolini et al. (2003) confirmed a reduction in parasite fecundity when Quebracho tannins were administered to goats. Min et al. (2004) showed that consumption of fresh (grazed) sericia lespedeza [Lespedeza cuneata (Dum-Cours) G. Don] (4.6% tannin) for 10 d reduced FEC by 72% and fecal egg hatch by 40%. In addition, FEC rebounded to prefeeding levels when sericea lespedeza feeding ceased, indicating that short-term exposure to sericea lespedeza did not kill the worms. Shaik et al. (2004) observed similar results in goats fed sericea lespedeza hay.
When goats consume sericea lespedeza on a long-term basis, FEC was decreased by 80% (Min et al., 2005) and worm burden was lessened by 75% (Shaik et al., 2006; Terrill et al., 2007). Goats have been grazed throughout the summer on sericea lespedeza without requiring deworming. Pasture contamination was reduced dramatically as indicated by tracer animals that had 88% fewer H. contortus and 100% fewer Teladorsagia circumcincta in the abomasum (Min et al., 2005). Shaik et al. (2006) also observed a reduction not only in FEC (80%) but also in adult worms in the abomasum and small intestine. Although the reduction in FEC could have been caused by a reduction in fecundity or a direct effect of condensed tannins on adult nematodes, reduced numbers of adult worms suggest a more direct effect. There are some differences in the effect of tannins on different species of gastrointestinal nematodes (Paolini et al., 2004), which is not surprising given that tannins are a class of compounds that differ in molecular weight, structure, and active sites.
Grazing Management
One factor with sheep and goats grazing together is that goats have been shown to ingest twice as many trichostrongylid larvae as do sheep (Jallow et al., 1994). This was attributed to goats not avoiding their fecal piles as sheep do and greater susceptibility of goats to internal parasites as compared with sheep. Grazing management can be an important management factor in the level of infectivity. It is possible to circumvent reinfection if animals are removed from a paddock after 3.5 to 6 d of grazing, depending on the time required for infective larvae to appear, as determined by environmental factors; in hot ambient temperatures, this may be as short as around 20 d, but more typically during summertime in the United States it will be about 40 to 50 d (Barger et al., 1994; Pomroy et al., 2002). Alternatively, pastures can be decontaminated by grazing with another species of animal (e.g., sheep grazed with cattle; Rocha et al., 2008) harvest of hay, or cultivation if sown annual forages are being used. But, there are also other reports such as that by Bairden et al. (1995) where grazing alternative species was beneficial in the short term on reducing pasture contamination; however, after 2 yr it appeared that strongyloid species adapted to the alternative host, mitigating the usefulness of this strategy.
Pasture Contamination Control
Certain chemicals, such as nitrogenous fertilizers, have been found to be toxic to L3 H. contortus larvae in laboratory settings, but subsequent field studies have failed to show similar effectiveness (Mankau and Mankau, 1975; Howell et al., 1999). Meanwhile, biological approaches have shown promise. Nematophagus fungi are able to parasitize nematodes. Some of those that can withstand the harsh gut conditions in animals are considered to be potential agents to control nematode parasites in ruminants. Larsen et al. (1994) screened fecal samples from grazing livestock for the presence of nematophagus fungi and identified Duddingtonia flagrans as a common isolate. This fungus has been shown to be effective against H. contortus, Trichostrongylus colubriformis, T. circumcincta, and Muellerius capillaris larvae in feces from sheep and goats (Waghorn et al., 2003; Paraud et al., 2005). Pena et al. (2002) showed that feeding 105 spores of D. flagrans per kg of BW to sheep for 7 d reduced infective larvae in the fecal culture by 95%. Wright et al. (2003) observed a 90% reduction in pasture contamination of H. contortus by feeding 5 x 107 spores of D. flagrans per animal and a 55% reduction in pasture contamination by T. circumcincta. This resulted in a 44% reduction in FEC. A study by Terrill et al. (2004) showed that daily feeding of 2.5 x 105 spores/kg of BW to sheep reduced L3 larval development by 80%. In a second experiment, Terrill et al. (2004) showed that it was necessary to feed the spores every day to obtain satisfactory control. Because this technique appears so promising, there is an urgent need for research to develop a delivery system for spores so they do not have to be fed every day.
Future Research
Our epidemiology knowledge needs to be synthesized in a simple producer-oriented model into which weather, animal, and pasture conditions could be considered to predict level of pasture infection and infection level in animals. This would identify areas in which deficiencies of knowledge exist and where research should be focused. It would also be a great tool to assist in making pasture management decisions.
In the face of anthelmintic resistance, alternative approaches of control have been explored extensively. Nutritional manipulation is feasible and promising as reviewed by Hoste et al. (2005, 2008). Because of the importance of some dietary biologically active components, such as AA and fatty acids, in regulation of immunity, this area warrants further research. More studies on use of COWP to treat internal parasites in goats are needed to substantiate the effectiveness and define dosage. Genetic selection for resistant populations is a promising approach, and there is need to establish breeding programs utilizing major goat breeds available in the United States. More research on tannin-containing plants and nematophagus fungi are required to fine tune dosages and develop strategies of use in practical goat production settings. Mechanisms of immune responses to internal parasites are not fully understood, and work is needed to demonstrate how the host reacts to parasite infection and whether parasites manipulate immune outcome of the host.
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GOAT MEAT |
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TopAbstractINTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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Of great importance to the future of the US goat industry is the perception of consumers, particularly ones not familiar with goat meat, regarding its acceptability and their willingness to purchase goat meat or its products. Rhee et al. (2000a) found that two-thirds of surveyed Texas A&M University faculty and graduate students had not seen goat meat offered in a supermarket or restaurant, and over one-half of respondents had never consumed goat meat. More than 60% of respondents reported a changed perception of goat meat after learning of the nutritional quality of goat meat as compared with traditional meats. No sensory differences were found between cabrito smoked sausage (Cosenza et al., 2003a) and cabrito snack sticks (Cosenza et al., 2003b) made with 0 or 3.5% soy protein added. Rhee et al. (1999) found no differences in blends of goat, lamb, mutton, spent hen, and beef with corn starch (approximately 82 to 85% corn starch and 12 to 15% meat) cooked/puffed via an extruder to test whether meats with undesirable flavors or toughness could yield expanded extrudates with similar qualities as beef.
Palatability and Tenderness
Palatability and acceptance of a goat meat product depends on many factors including background of sensory panelists and presentation (Griffin et al., 1992; Rhee et al., 2003), percentage goat meat vs. beef in a product (James and Berry, 1997), and age of the animal (Smith et al., 1978; Kannan et al., 2003), among other factors.
Aging goat carcasses from 7 to 14 d resulted in increased meat tenderness (Kannan et al., 2003; King et al., 2004; Kouakou et al., 2005), although Kannan et al. (2002a) suggested that aging chevon cuts for more than 4 d had no additional effect on tenderness. High voltage electrical stimulation of cabrito carcasses resulted in a more rapid pH decline than low voltage stimulation or traditional aging and resulted in improved tenderness at 1 and 3 d postmortem with no further change after 14 d (McKeith et al., 1979).
Breed and Sex Differences
Angora goats have recorded less BW and carcass weight and smaller LM than Spanish or Boer x Spanish goats (Riley et al., 1989; Oman et al., 2000). Goats with Boer genetics have shown a tendency for heavier weight of primal cuts than Spanish goats (Cameron et al., 2001a). Johnson et al. (1995b) reported decreased percentages of feet, pelt, liver, and heart for female Nubian x Florida Native, Florida Native, and Spanish x Florida Native goats than for intact males or castrates. Cameron et al. (2001b) found no sex differences in dressing percentage in Boer x Spanish goats. Whereas sex has been reported to affect marbling and fatty acid ratios in goat meat, sex did not affect cholesterol, vitamin, or mineral contents of meat (Johnson et al., 1995a). The latter findings agree with those of Park (1988, 1990) and Park and Attaie (1988) who found few differences in macro- or micro-mineral concentrations, nonheme, heme, and total Fe or Fe/Zn ratio between sexes or breeds (Alpine and Nubian).
Diet
Concentrate feeding of goats has generally resulted in greater BW and carcass weight compared with goats raised on pasture (Oman et al., 1999; Hopkins-Schoemaker, 2006; Ryan et al., 2007), although Johnson and McGowan (1998) reported few differences in carcass traits of Florida Native 8-mo-old kids raised either intensively or semi-intensively when expressed relative to HCW. Diets based largely on wheat middlings, soybean hulls, and corn gluten meal did not yield different carcass grades, but carcass weights were greater for the soyhull and wheat middlings diets than for the hay/ soybean meal control treatment, with the corn gluten meal diet being intermediate (Moore et al., 2002). Ryan et al. (2007) found no differences in carcass weight or subprimal or LM sensory traits in goats fed 50, 70, or 90% concentrate diets.
There have been very few comparisons of effects of different forage types on carcass characteristics of goats. Recently, Lema et al. (2007) noted that Maton rye (Secale cereale) and TRICAL-336 triticale (Triticum secale) for wintering goats in Tennessee resulted in greater ADG and chevon production than Kentucky-31 tall fescue (Festuca arundinacea), and chilled carcass weight and dressing percentage were greater for rye than for fescue.
The majority of research on goat meat production has been with traditional meat breeds and studies using dairy kids or breeds are rare (Potchoiba et al., 1990; Kannan et al., 2006). Lupton et al. (2008) found that carcass weight tended to be greater for Angora goats raised in a traditional feedlot or a raised, slatted floor facility than those raised on pasture.
Tissue Lipid Concentrations
In a review, Banskalieva et al. (2000) noted that diet can affect the type of lipids found in goat tissues. Lee et al. (2008a, b), feeding a 50% alfalfa meal, 35% corn grain supplement to forage-fed goats, reported that supplemented goats had greater concentrations of oleic acid in muscle lipids. Greater concentrations of linoleic acid in subcutaneous fat were also found in supplemented goats, in agreement with findings of Hopkins-Schoemaker (2006). Hay-fed goats had less total lipids in LM than supplemented goats and had greater linolenic acid concentrations in muscle lipids and subcutaneous fat (Lee at al., 2008b).
Rhee et al. (2000b), studying the effects of dietary fatty acid on animal tissue fatty acid content, reported that total unsaturated dietary fatty acids were greater in a grain-based diet than in forage plants, with the major unsaturated fatty acids being linoleic and linolenic in range plants and oleic and linoleic in the grain diet. However, oleic acid constituted over two-thirds unsaturated fatty acids in intramuscular fat of goats regardless of diet. The percentage of stearic acid was greater in range plants than the grain-based diet but was not different in intramuscular fat of goats consuming either diet.
Scant scientific literature exists on the CLA content of goat meat and attempts to influence CLA concentrations. Lee et al. (2006) raised Boer goats and cross-bred (Dorset x Suffolk) lambs on pasture with a corn-based grain supplement and sampled LM of carcasses of both species. Goat meat had a greater cis-9, trans-11 CLA content than lamb (0.17 vs. 0.12%) but a similar amount of trans-10, cis-12 CLA (0.11 vs. 0.10%).
Preslaughter Stress
Transportation, isolation, a strange environment, and animal density increase preslaughter stress in goats as indicated by increased plasma cortisol concentration (Kannan et al., 2000, 2002b, 2003, 2007b). Creatine kinase activity, plasma concentrations of glucose and urea nitrogen, and meat pH were also reported to be affected. Environment during preslaughter holding and isolation were postulated as being more potent stressors for goats than feed deprivation (Kannan et al., 2002b). Stress was not increased in animals subjected to a 1-min spray washing before slaughter to test the efficacy of spray washing in reducing microbial counts on skin and carcass (Kannan et al., 2007a). Seaweed supplementation of goats for 3 wk before simulated preslaughter stress decreased lipid peroxidation and increased glutathione peroxidase activity, suggesting that the seaweed extract increased anti-oxidant status (Kannan et al., 2007b).
Shelf Life
Meat color is an important factor in consumer acceptability, as is ultimate taste. Chevon cuts packaged on Styrofoam trays covered with polyvinylchloride film showed maximum discoloration within 4 to 8 d (Oman et al., 1999; Kannan et al., 2001). Oman et al. (1999) did not detect breed difference in meat cut storage traits among Boer x Spanish, Boer x Angora, Spanish, and Angora goats. Kannan et al. (2002a) noted similar shear values of cooked meat samples between Styrofoam and vacuum packaging.
Future Research Needs
There is a need to continue integrating a meat quality component into breeding and genetic research as well as nutrition and management studies. Discerning effects of breed, sex, age, time of slaughter, diet, or a combination of these on palatability, flavor, and tenderness attributes will lead to a better understanding of the type of meat goat needed to appeal to a wider portion of the meat-consuming public. Developing reliable, easy-to-use methods to select live animals for increased muscle mass is necessary to form selection indices for faster genetic progress on muscling attributes. Additional research is also warranted on how fatty acids in goat meat, particularly unsaturated fatty acids such as CLA, can be further influenced and their relation to human nutrition. Convenience and choice are important in American diets, and research to formulate value-added products such as ready-to-cook goat meat and a broader array of goat meat snack items could expand the current goat meat-consuming public to include more traditional consumers and increase the value of goat meat. Research on the issues of preslaughter stress and carcass storage and packaging and effects as they relate to color, flavor, tenderness, and other meat quality attributes will help achieve a consistent quality for retail goat meat products.
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GOAT MILK AND PRODUCTS |
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TopAbstractINTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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The somatic cell count (SCC) in goat milk, commonly regarded as an indicator of udder health and milk quality, has been a major issue over the last 2 decades. Besides udder health, research has identified many other factors affecting SCC in goat milk, such as breed, parity, stage of lactation, milking procedure, morning vs. evening milking, sample shipping, testing methods (automated machine counting vs. pyronin-Y methyl green direct microscopic counting), testing laboratory, and milking methods (hand vs. machine milking; Zeng and Escobar, 1995, 1996; Zeng et al., 1996, 1997, 1998a, 1999). In addition, Zeng (1996) stated that it is imperative to use goat milk SCC standards for monthly calibration for more accurate assessment of SCC in goat milk if an automated counter is used. Paape et al. (2007) monitored the SCC distribution of goats enrolled in the Dairy Herd Improvement program over a 5-yr span (2000 to 2004) and reported that non-infectious factors such as parity and stage of lactation had major impact on SCC. Because of the now extensive scientific evidence, the National Conference on Interstate Milk Shipment (NCIMS) in the United States has established a separate SCC limit of 1,000,000/mL for grade A goat milk, whereas the limit for grade A cow milk is presently 750,000/mL. The SCC limit for goat milk will continue to be unchanged, whereas that for cow milk may be further decreased to 400,000/mL.
Antibiotic Residue Testing
Antibiotic residues in goat milk are of great concern to dairy goat farmers, processors, regulators, and consumers. Their presence in goat milk above tolerance (safe) concentrations interferes with product processing such as of cheese and yogurt and more importantly presents potential health risks to the consumer. In the mid-1990s, a series of studies (Zeng et al., 1996, 1998b) was carried out in the American Institute for Goat Research to validate antibiotic residue test kits for dairy goats following the FDA-approved protocols. Among the test kits examined, SNAP (IDEXX Laboratories Inc., Westbrook, ME), Penzyme (Cultor Food Science Inc., Milwaukee, WI), Charm BsDA, and Charm II Tablet Sequential tests (Charm Sciences Inc., Malden, MA) were reported to meet the FDA sensitivity and specificity requirements at tolerance and detection concentrations. Therefore, dairy goat producers in the United States now have "goatside" test kits (SNAP and Penzyme) for quick screening of antibiotic residues in goat milk within 10 and 20 min, respectively, and for better control of health risks in goat milk products. In addition, the withholding times of goat milk not to be used for human consumption have been established for specific drugs commonly used in treating mastitic goats (Zeng et al., 1996, 1998b), although few drugs have been specifically approved for use in dairy goats without veterinarian permission (Contreras et al., 2007).
Incentive Payment Programs
Premiums have long been given to goat milk with high fat and protein concentrations, and low SCC and bacterial counts in many European countries where the dairy goat industry is more developed and the testing for these quality parameters is more accessible and consistent (Pirisi et al., 2007). In the United States, price premiums are implemented primarily by individual processing companies on regional bases (Haenlein, 2001). Guo et al. (2004) used composition variables of goat milk and cheese from a commercial cheese plant to derive cheese yield predictive formulae, whereas Zeng et al. (2007) examined cheese yield potential of goat milk varying in composition in a designed experiment. Because total solids, fat, and protein concentrations in goat milk were strong predictors of cheese yields, such yield predictive formulae can be used as tools to help establish price incentive systems to further the development of the US dairy goat industry.
Future Research Focal Points
There will continue to be a strong demand for traditional and authentic goat cheeses. To support legal protection and promote enforcement of high quality goat cheese standards, uniform national standards and identities of goat cheeses must be established. Research is needed to explore the application of genomics and proteomics in understanding the dynamic biochemical and microbiological reactions in goat milk cheese. The molecular interactions that determine cheese texture and functionality require study to improve consistency of goat cheeses.
Because of the increased production and consumption of goat milk cheeses in recent years, the recovery, utilization, and disposal of goat cheese whey can no longer be ignored. Major advances in economically processing cow milk and whey into various components have been in the use of membrane processing, particularly reverse osmosis, ultrafiltration, microfiltration, and ion-exchange technology (IDF, 2004). Membrane processing systems allow the separation of milk and whey components into various fractions (Johnson and Lucey, 2006). Such technologies must be explored in treating goat cheese whey and producing whey cheese, whey protein concentrates and isolates, and other functional ingredients. The feasibility of membrane filtration for separation of bacteria, particularly spore formers, from goat milk to increase the shelf life of pasteurized milk should also be studied.
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MOHAIR AND CASHMERE FIBER |
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TopAbstractINTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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Angora goats have great potential for mohair fiber growth and preferentially partition nutrients for that purpose. Thus, nutrition is very important in fiber production as well as for other important physiological functions. Because of the high protein content of mohair fiber, research conducted has focused on amino acid supplies as well as regulation of fiber growth. Several studies have shown the positive effects of dietary protein supply on rate of fiber growth and fiber diameter (Hart et al., 1993; Sahlu et al., 1993; Davis et al., 1999). Sulfur-containing amino acids are of particular importance. Grégoire et al. (1996) did not observe differences in mohair fiber growth among diets that included soybean meal, soybean meal plus ruminally protected methionine (e.g., Mepron, Degussa, Hanau, Germany), fish meal, or corn gluten meal. Conversely, Litherland et al. (2000) observed greater mohair fiber growth with a diet containing corn gluten meal and greater ADG for one with fish meal.
Angora goats are able to continue producing mohair fiber during periods of feed shortage or nutrient restriction, although the nature of partitioning regarding other competing physiological processes and the severity of nutrient restriction are not well understood. Puchala et al. (2007) recently conducted a study with growing Angora wethers. In the first 12-wk phase ME intake was provided through alfalfa pellets to support values of tissue and mohair fiber growth of 0 and 0, 15 and 1.5, 30 and 3.0, 45 and 4.5, 60 and 6.0, and 75 and 7.5 g/d, respectively. In the subsequent 12 wk alfalfa pellets were consumed ad libitum. Findings indicated that growing Angora goats partition nutrients to maintain mohair fiber growth with limited ME intake and decrease energy expenditure to lessen the ME requirement for maintenance, resulting in compensatory tissue growth upon realimentation. Similarly, Sahlu et al. (1999) found that mohair fiber growth by lactating Angora goats was not affected by increasing protein concentration, whereas milk production increased. Findings of Puchala et al. (2003) suggest that this was due to an elevated concentration of prolactin during lactation and that in late gestation and early lactation prolactin decreases AA use by active follicles to partition nutrient availability to other tissues (i.e., fetus or milk production).
Puchala et al. (2001) studied effect of bovine ST and thyroid hormone status on mohair fiber growth by growing Angora goats and concluded that exogenous ST administration does not appear to influence mohair fiber growth, regardless of thyroid hormone status. Similarly, bovine ST did not change mohair production regardless of age and physiological stage of Angora goats (Davis et al., 1999). Conversely, thyroxine administration had substantial effect on mohair fiber growth despite decreased feed intake and ADG, implying a major role in regulation of mohair fiber growth. Pierzynowski et al. (1997) and Puchala et al. (1998) demonstrated that thyroid hormones increased uptake of AA by skin of Angora goats, whereas insulin had a limited effect. Small peptides do not appear involved in regulation of mohair fiber growth, as methionine-leucine and lysine-leucine function through supply of free AA used for mohair fiber protein synthesis.
Cashmere
In contrast to mohair, cashmere fiber must be separated from guard hair and is produced by many breeds of goats, with the Spanish genotype the most important in the United States. Cashmere must be less than 18.5 µm in diameter and at least 3.175 cm in length to be considered a luxury fiber.
Nutrition has limited effects on cashmere fiber production. Ivey et al. (2000) studied effects and interactions of ad libitum consumption of diets differing in concentrations of CP (10 and 15%) and ME (2.00, 2.35, and 2.70 Mcal/kg; DM basis) on growth and cashmere production in an 84-d, fall-season experiment. The increased concentration of CP increased fiber diameter; however, there were no clear effects of protein or energy on cashmere production or the ratio of guard hair to cashmere fiber. Galbraith (2000) studied effects of supplementation with urea or fish meal to provide CP above the maintenance requirement on cashmere growth in Siberian and Australian cashmere goats. Total fleece growth and the ratio of guard hair to cashmere fiber were not influenced by supplementation. Similar findings were observed by Restall et al. (1994) and Russel (1995).
Wuliji et al. (2003) suggested that melatonin administration for spring breeding may be an effective means of extending the cashmere growth phase in Spanish goats. Melatonin, given orally or in a slow release implant, increased fiber growth rate (27%), fiber elongation, and cashmere yield in the spring months, although fiber diameter was also increased (7.5%). These changes were accompanied by a delay in the initiation of fall growth, but this did not affect annual fleece weight.
Because of limited effects of nutrition on cashmere production and low availability and high cost of melatonin treatments, genetic selection has been used to improve cashmere production. Diaz et al. (1999) studied the problem of restricted selection of cashmere goats in a closed nucleus scheme with a herd of 200 breeding females and concluded that the resulting cumulative level of inbreeding varied from 0.045 to 0.101, and there was no clear pattern of the relationship between level of inbreeding and variance of response. Zhou et al. (2003) studied effects of nongenetic factors on production traits of Inner Mongolia cashmere goats in China and concluded that the estimation of breeding value for cashmere production would be more precise following adjustment for age structure, age of dam, sex, type of birth, herd, and their interactions.
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BREEDING, GENETICS, AND REPRODUCTION |
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TopAbstractINTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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Before 30 yr ago, scant breeding and genetics research was conducted on goats, especially in the United States. Shelton (1978) stated in a major review article on the subject that "The goat contributes most in tropical regions (within 30° of the equator)." Except for research on dairy goats in Europe, primarily in France, this was an accurate statement. Some of the summary statements of Shelton (1978), shown below as subheadings, will be used as a springboard to examine advances and research findings of impact on breeding and genetics of goats since that time.
"The most important product from the goat is milk with meat a close second. Other products are minor."
This is no longer true in the United States as was noted in the Introduction. Meat has clearly passed milk, which is now a distant second. In 1993, the year that the Boer goat was introduced into the United States, USDA only tracked the Texas goat inventory, which had almost all of the goats in the United States. In 1993, there were 2.0 million goats in Texas with 1.8 million being Angora goats (NASS, 2007). In 2007, there were 3.6 million goats in the United States with 260,000 being Angoras and 3.0 million classified as meat goats, primarily Boer goats. In 2007, there were 6.4 million goats in South Africa (FAO, 2007) with 2.6 million being Boer goats and 3.8 million being Angora goats. There are now more Boer goats in the United States than there are in South Africa.
"Reproductive rate is a problem only with the Angora goat, but increased reproduction with any type of goat would contribute to improved efficiency."
Reproductive rate is still not a problem and goats continue to be one of the most fertile domestic species. Therefore, reproduction research has tended to concentrate on assisted reproductive technologies (Amoah and Gelaye, 1997; Whitley and Jackson, 2004; Holtz, 2005; Sohnrey and Holtz, 2005). In the 1970s and 1980s, research focused on hormone delivery via sponges for estrous synchronization and AI. Recent progress has been made in collection and handling of gametes, in vitro fertilization, estrus control, embryo transfer and conservation, manipulation of gametes and embryos, transgenesis, and pregnancy detection (Holtz, 2005). The advent of the Boer goat has increased the use of multiple ovulation and embryo transfer in goats, which has led to further research on this subject. The demand for the Boer goat has also propelled in vitro fertilization techniques for goats. Kids have been produced, as a result of refined techniques of in vitro maturation and fertilization of recovered oocytes, resulting in successful culture and transfer of embryos. Advances also have been made in the old stalwarts of estrous synchronization and AI, especially in meat goats for the former (Whitley and Jackson, 2004; Sohnrey and Holtz, 2005). In meat goat management systems, methods of estrous synchronization that have been the subject of recent research studies have included techniques as simple as altering the photoperiod or the manipulation of the buck effect and as complex as varying timed hormonal treatments combined with timed AI (Whitley and Jackson, 2004).
"Genetic studies of goats are limited, but this should not limit improvement programs."
Genetic studies are still limited in meat goats; however, this has changed considerably for dairy goats and will be discussed further under the next statement. For meat-producing animals, performance testing has been a central component for genetic improvement to enhance output and profitability at all levels of production, from small-holder to large livestock enterprises (Holst, 1999). In the United States, the performance testing of meat goats has followed the same paradigm as outlined in South Africa (Olivier et al., 2005) and incorporating the beneficial aspects of bull, boar, and ram performance testing in the United States. The first performance test for meat goats was established at San Angelo, Texas, in 1995 and was followed by others in Oklahoma, Georgia, Kentucky, New York, Pennsylvania, and California. Outside of the end-of-test reports, few results on performance testing have been reported (Gipson et al., 2005, 2007).
As the meat goat industry develops, performance testing and genetic evaluation will be necessary for the industry to make progress. There may be a shift from feeding of pelleted concentrate-forage diets to grazing in performance tests. Expected progeny differences for growth traits and even for resistance to internal parasites of meat goats should be developed.
"Even within temperate regions, there is little evidence of progress in breeding for milk production."
Genetic progress for milk production has been slow; however, that lack of progress is not the fault of tools available but in the lack of interest of dairy goat producers in using new and refined genetic evaluation information on animals. The research on enhancing genetic evaluations in dairy goats has paralleled that of dairy cattle, and these enhancements have refined dairy goat genetic evaluations. First, projection and adjustment factors specifically for dairy goats were developed. Wiggans et al. (1979) developed projection factors based on production on the last day of sampling for dairy goat milk and fat records. These projection factors enabled dairy goat producers to fairly compare records of goats at all stages of lactation. These projected records also allowed more accurate culling and breeding decisions and were a first step toward genetic evaluations of goats, which was to come later. Multiplicative age-season adjustment factors for milk and fat yields of 5 breeds of dairy goats were estimated and published (Iloeje et al., 1980). These adjustment factors reflected the changes in milk and fat yields associated with age and season of freshening. Before the development of these adjustment factors, goat records were adjusted using age factors for dairy cows (Iloeje et al., 1980). In the early 1980s, genetic evaluations of production traits soon followed, and in 1989, genetic evaluation of dairy goats was extended to include protein yield and Oberhasli and experimental breeds (Wiggans, 1989). Due to a smaller sample size, the animal model system implemented for dairy goats differed from the one for dairy cattle in that all breeds were processed simultaneously. In 1997, type traits were included in the genetic evaluation (Luo et al., 1997). Later refinements to the animal model were made, and genetic progress has continued to chart production and type trait progress (Wiggans and Hubbard, 2001). The most recent advancement in the genetic evaluation of production traits in dairy goats has been the incorporation of the test-day model (Andonov et al., 2007).
Primarily in dairy goats, research emphasis upon major genes as well as quantitative genetics has evolved rapidly (Barillet, 2007). Marker- or gene-assisted selection has been applied to the 
s1-casein (s1-cas) gene in goats. The knowledge of major genes affecting dairy traits in goats has been well established for protein, and more precisely casein content, which is the main component of milk protein. So far, 7 variants corresponding to 14 alleles have been identified in many European breeds (Grosclaude et al., 1987; Grosclaude and Martin, 1997) and classified according to their synthesis rate of s1-cas as strong (A, B, C), intermediate (E), weak (F, G), and null (O) when s1-cas is absent. The quantitative effects of goat s1-cas variants on dairy traits were studied and have indicated that the difference for protein content between the extreme genotypes AA (favorable) and FF was 4.5 g/kg. Barbieri et al. (1995) compared estimates of genetic parameters for Alpine goats using an animal model including or excluding the s1-cas fixed effect. The heritability of protein content changed from 0.66 to 0.34 when the s1-cas effect was accounted for, showing that variance at this major gene represents about 50% of total genetic variance of protein content in this goat population.
"Little has been done on the development of the goat as a meat animal."
Present day reality stands in stark contrast to this statement. With the advent of the Boer goat into the United States, the research arena has exploded with information about the Boer goat. The first major research was a simulation study because real production information quantifying performance of Boer goats in the United States was lacking (Blackburn, 1995). A simulation study was performed to determine how Boer goats would compare with Spanish goats in 2 locations with varying high, medium, and low forage conditions. This simulation study was the first to show American meat goat producers that the effective use of the Boer goat will depend on the forage resource base and producers’ ability to provide inputs, such as supplementation, to the goat production system (Blackburn, 1995).
Following the simulation study noted above were studies estimating genetic parameters, which could be used in improvement programs and genetic evaluations (Schoeman et al., 1997). Although the 2 data sets used in this South African study were small, they provided important information on variance components in the Boer goat. The results indicate that genetic improvement could be obtained by selecting on both direct and maternal breeding values (Schoeman et al., 1997).
"Also, research on crossbreeding for milk or meat production is limited."
Crossbreeding for milk or meat production has still not been well-studied. Shortly after the arrival of the Boer goat in the United States, a study evaluated the growth rate, feed consumption, and carcass traits Spanish and Boer x Spanish cross kids (Waldron et al., 1995). Boer x Spanish cross kids were heavier than Spanish kids by 0.26 kg at birth and 0.67 kg at 90 d of age. The Boer x Spanish kids weighed more than Spanish kids at 8 mo of age by 2.1 kg on pasture and by 4.1 kg in the feedlot. Boer x Spanish kids exceeded purebred Spanish kids in postweaning growth rate in the feedlot (168 vs. 132 g/d). Crossbreeding studies have been summarized in a recent review article (Shrestha and Fahmy, 2007).
Other Issues
Three decades ago, transgenic animals and cloning were not on the goat research horizon. The introduction of specific genes into the genome of farm animals and its stable incorporation into the germ line has been a major technological advance in agriculture (Wheeler, 2003). Animal pharming, the process of using transgenic animals to produce human drugs, is becoming more common. Global demand continues to grow for human proteins and vaccines, which serve numerous therapeutic purposes such as treatments for cystic fibrosis, hemophilia, osteoporosis, arthritis, malaria, and human immunodeficiency virus. Transgenic animals can also produce monoclonal antibodies (antibodies specifically targeted toward disease proteins) used in vaccine development. Currently, transgenic goats are producing in their milk human protein C and antithrombin 3 for the treatment of thrombosis, glutamic acid decarboxylase for the treatment of type 1 diabetes, and Pro542 for the possible treatment of human immunodeficiency virus. Recently, scientists spliced spider genes into the cells of lactating goats to manufacture silk in milk.
Even with these advances in transgenics, there is an urgent need to conserve genes in situ so that they will be available for traditional introgression or transgenics. Therefore, studies in genetic diversity have become increasingly important. During the past decade, a large number of genetic diversity studies in domestic livestock based on microsatellite loci have been conducted (Baumung et al., 2004). Livestock breeds are usually studied because of a long history of isolation, unique phenotypic qualities, or evolution within a unique environment. However, a lack of detailed recommendations for goat genetic distance studies dictates an urgent need to develop genetic marker lists for that species (Baumung et al., 2004). Preservation of genetic diversity determines not only breed survival, but also adaptation to changing environments. A genetic diversity study concluded that "Breed conservation decisions cannot be limited to genetic diversity alone but should also consider phenotypic performances and non-profit values for the society in their cultural aspects" (Glowatzki-Mullis et al., 2008).
Ancillary to genetic diversity is the potential for selection for QTL. Advances in molecular genetic technology have made available very dense marker maps based on SNP on other livestock species and these will be available for goats in the future (Van der Werf et al., 2007). Currently, the goat map is sparser than the sheep map and contains about one-half the number of markers known in sheep: 731 loci with 271 genes and 423 microsatellites. The last published linkage map for goats contains 622 mapped markers, with the coverage of the whole goat genome far from complete. Although the sparsity of the goat map makes it difficult to develop a good homology between the maps, about two-thirds of the mapped goat markers can also be linked to the sheep map (Maddox, 2008).
In addition to selection upon production traits or even type traits, there is increasing interest in selecting for genetically healthier animals (Bishop and Morris, 2007). There is well-documented evidence for between-animal genetic variation in resistance to disease and, in the case of some infectious diseases, resistance to infection. These genetic differences between animals will lead to opportunities to breed animals for enhanced resistance to disease and internal parasites. This latter aspect is becoming increasingly important because anthelmintic resistance of internal parasites is widespread throughout the world and is probably underestimated (Carta and Scala, 2004).
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SUMMARY |
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TopAbstractINTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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1 Corresponding author: goetsch{at}luresext.edu
Received for publication July 14, 2008. Accepted for publication September 3, 2008.
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TopAbstractINTRODUCTIONNUTRITION/FEEDING MANAGEMENTHEALTHINTERNAL PARASITE CONTROLGOAT MEATGOAT MILK AND PRODUCTSMOHAIR AND CASHMERE FIBERBREEDING, GENETICS, AND...SUMMARYLITERATURE CITED |
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toxin in the intestine of sheep. Am. J. Vet. Res. 66:251–255.[CrossRef][Medline]This article has been cited by other articles:
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  A. T. Ngwa, L. J. Dawson, R. Puchala, G. D. Detweiler, R. C. Merkel, Z. Wang, K. Tesfai, T. Sahlu, C. L. Ferrell, and A. L. Goetsch Effects of breed and diet on growth and body composition of crossbred Boer and Spanish wether goats J Anim Sci, September 1, 2009; 87(9): 2913 - 2923. [Abstract] [Full Text] [PDF]
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