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A decade of developments in the area of fat supplementation research with beef cattle and sheep¹

Department of Animal Science, University of Wyoming, Laramie, 82071-3684

	Abstract

Top Abstract INTRODUCTION SUPPLEMENTATION CONSIDERATIONS RUMINAL VFA, DIET DIGESTIBILITY,... SUPPLEMENTING FAT TO PERIPUBERAL... SUPPLEMENTING FAT TO LACTATING... SUMMARY AND CONCLUSIONS LITERATURE CITED

 
Supplementing ruminant animal diets with fat has been investigated as a means to influence a variety of physiological processes or to alter fatty acid composition of food products derived from ruminant animals. Several digestion experiments have been conducted with beef cattle and sheep to elucidate the effects of supplemental fat on utilization of other dietary components. Negative associative effects are not likely to be observed in ruminants consuming forage-based diets with supplemental fat at 2% of DMI. Inclusion of supplemental fat at 3% of DM is recommended to obtain the most benefit from the energy contained within the fat and other dietary components in high-forage diets. For ruminants fed high-concentrate diets, supplementing fat at 6% of diet DM is expected to have minimal impacts on utilization of other dietary components. Although there is greater potential to supply the ruminant animal with unsaturated fatty acids from dietary origin if fat is added to high-concentrate diets, incomplete ruminal biohydrogenation of C18 unsaturated fatty acids results in an increase in duodenal flow of 18:1 trans fatty acids regardless of basal diet consumed by the animal. The biohydrogenation intermediate 18:1 trans-11 (trans-vaccenic acid) is the likely precursor to cis-9, trans-11 CLA because the magnitude of increase in CLA content in tissues or milk of ruminants fed fat is much greater than the increase in CLA presented to the small intestine of ruminants fed fat supplements. Duodenal flow of trans-vaccenic acid is also substantially greater than CLA. Increasing unsaturated fatty acids status of ruminants imparts physiological responses that are separate than the energy value of supplemental fat. Manipulating maternal diet to improve unsaturated fatty acid status of the neonate has practical benefits for animals experiencing stress due to exposure to cold environments or conditions which mount an immune response. Supplementing fat to provide an additional 16 to 18 g/d of 18:2n-6 to the small intestine of beef cows for the first 60 to 90 d of lactation will have negative impacts on reproduction and may impair immune function of the suckling calf. Consequences of the suckling animal increasing its intake of unsaturated fatty acids because of manipulation of maternal diet warrants further investigation.

Key Words: beef cattle • digestion • fat supplementation • metabolism • reproduction • sheep

	INTRODUCTION

Top Abstract INTRODUCTION SUPPLEMENTATION CONSIDERATIONS RUMINAL VFA, DIET DIGESTIBILITY,... SUPPLEMENTING FAT TO PERIPUBERAL... SUPPLEMENTING FAT TO LACTATING... SUMMARY AND CONCLUSIONS LITERATURE CITED

 
Ruminant animals fed conventional diets typically consume limited amounts of fat. Nevertheless, induction of essential fatty acid deficiencies in ruminants has been difficult to demonstrate (Palmquist et al., 1977). The ruminant’s ability to make very efficient use of essential fatty acids (Noble, 1984) has likely contributed to relatively modest interest in lipid nutrition of ruminants. Because of its caloric density, the primary function of fat in diets consumed by ruminants is to provide energy (NRC, 2007).

Interest in the area of supplementing ruminant animal diets with fat has increased over the past decade. To illustrate this point, we listed the manuscripts published in the Journal of Animal Science in which investigators reported outcomes of fat supplementation for ruminants since Dr. Hess began his career at the University of Wyoming (Table 1). The average number of papers published annually from July 1996, through December 1999, nearly doubled from January 2000, through July 2007. The lowest number of manuscripts published on the topic since the turn of the decade was 9, which was comparable to the greatest number of papers published in a single year among the previous 3.5 yr. The increase in interest noted above stemmed from the concept that manipulation of ruminant animal diets via supplementation with lipid is a means to influence a variety of physiological processes or alter fatty acid composition of food products derived from ruminant animals. The primary objective of this review is to summarize research conducted by the authors on the inclusion of fat in diets fed to various classes of ruminant livestock.

	SUPPLEMENTATION CONSIDERATIONS

Top Abstract INTRODUCTION SUPPLEMENTATION CONSIDERATIONS RUMINAL VFA, DIET DIGESTIBILITY,... SUPPLEMENTING FAT TO PERIPUBERAL... SUPPLEMENTING FAT TO LACTATING... SUMMARY AND CONCLUSIONS LITERATURE CITED

Our overarching goal has been to develop fat supplementation programs that will lead to improvements in the sustainability of beef and sheep production systems. The first step in this process was to conduct multiple digestion trials to determine the level of dietary fat that does not have negative effects on diet digestion. Therefore, initial experiments conducted at the University of Wyoming focused on identifying the level of supplemental fat that had minimal impact on utilization of other macronutrients under a variety of dietary situations. Another main objective of simultaneous interest was to investigate how metabolism of dietary fatty acids by ruminal microflora affected supply of fatty acids available for metabolism by the ruminant animal. The premise for the second objective was that dietary fatty acids had been associated with improvements in reproduction or could be incorporated into various tissues, thereby altering the composition of food products produced by the animal. Whenever possible, we attempted to balance experimental diets to be isocaloric and isonitrogenous to avoid confounding experimental outcomes associated with changing the animal’s energy and protein status. Thus, responses by animals maintained under normal production practices could be directly attributable to the dietary fat.

	RUMINAL VFA, DIET DIGESTIBILITY, AND FATTY ACID SUPPLY

Top Abstract INTRODUCTION SUPPLEMENTATION CONSIDERATIONS RUMINAL VFA, DIET DIGESTIBILITY,... SUPPLEMENTING FAT TO PERIPUBERAL... SUPPLEMENTING FAT TO LACTATING... SUMMARY AND CONCLUSIONS LITERATURE CITED

 
A brief overview on the fate of supplemental fat in the ruminant’s digestive tract is necessary because dietary fat contributes both directly and indirectly to production of ruminal VFA, diet digestibility, and supply of fatty acids to the animal. Ester linkages of dietary fatty acyl glycerol undergo rapid and extensive hydrolysis by microbial lipolytic enzymes in the rumen to form glycerol and free fatty acids (Jenkins, 1993). Glycerol may then be metabolized by the ruminal microorganisms to produce VFA (Nagaraja et al., 1997). Fatty acids liberated upon lipolysis may exert antimicrobial effects in the rumen (Palmquist and Jenkins, 1980), which results in a shift in molar proportions of VFA (Doreau and Chilliard, 1997). Unsaturated fatty acids have more potent antimicrobial effects and promote greater inhibition of ruminal fermentation than do saturated fatty acids (Jenkins, 1993). Unsaturated fatty acids with 18 carbons are transformed to a variety of trienoic, dienoic, and monoenoic isomers as they are biohydrogenated (Bauman et al., 2000), with 18:0 being formed as a result of complete biohydrogenation (Jenkins, 1993). Accumulation of 18:2n-6 in the rumen inhibits complete biohydrogenation (Jenkins and Adams, 2002). There is negligible disappearance of long-chain fatty acids from the rumen (Jenkins, 1993) and the small intestine is the primary site of fatty acid absorption. Quantifying fatty acids actually reaching the duodenum for absorption is critical for determining potential transfer into animal tissues or milk (Merchen et al., 1997) because dietary lipids transformed in the rumen (60%) and lipids synthesized by ruminal microorganisms (35%) constitute 95% of total lipids reaching the duodenum (Jenkins, 1994).

High-Forage Diets
Supplementation strategies should be developed to be compatible with livestock production systems in the western United States (DelCurto et al., 2000). Forages are the primary source of nutrients for most livestock operations in Wyoming. Although the use of supplemental fat for dairy cows (Palmquist and Jenkins, 1980; Jenkins, 1993, 1994; Palmquist, 1994) or for ruminants consuming relatively low-roughage diets (Doreau and Chilliard, 1997; Merchen et al., 1997) were well documented, considerably less attention had been directed toward ruminants consuming forage-based diets. Therefore, one of the early foci was to determine the level of supplemental fat that could be used in forage-based diets without adversely affecting ruminal fermentation and digestibility of other dietary components. Soybean oil was initially used as the fat source because beef cows fed supplemental soybean oil had greater metabolic and reproductive responses than beef cows fed tallow or fish oil (Thomas et al., 1997).

Ruminal VFA.
Studies summarized in Table 2 indicate that results of our in vitro experiments (Whitney et al., 2000; Brokaw et al., 2002) were in partial accord with results obtained in vivo (Whitney et al., 1999; Gould et al., 2000; Brokaw et al., 2001). Soybean oil added to corn-based supplements in bromegrass hay-based diets generally decreased the acetate:propionate ratio. At first glance, our observations were consistent with reports in which an increase of unsaturated fatty acids within the rumen decreased the acetate:propionate ratio (Jenkins, 1997; Jenkins et al., 2000; Jenkins and Adams, 2002). Doreau and Chilliard (1997) noted that a decrease in acetate:propionate ratio in the rumen of animals fed supplemental fat was accompanied by reduced digestion of OM, primarily the fibrous fraction. This was consistent with our observations when dietary soybean oil was added to forage-based diets at 6% of DM. An alternative explanation is necessary for diets containing supplemental soybean oil 3% of diet DM. In our experiment, cracked corn was used as the carrier for supplemental soybean oil. Supplemental cracked corn has decreased acetate:propionate ratio and increased molar proportions of butyrate (Hess et al., 1996). An increase in molar proportions of both propionate and butyrate occurred during ruminal fermentation of glycerol (Rémond et al., 1993). Thus, we proposed that alterations in ruminal proportions of VFA are largely related to fermentation of the cracked corn used as the carrier of the supplemental fat. Molar proportions of butyrate increased when cracked corn was used as the carrier for supplemental soybean oil, and linear decreases in molar proportions of butyrate were observed as the level of dietary cracked corn decreased. Although the contribution of butyrate arising from fermentation of glycerol cannot be discounted, reduced molar proportions of butyrate accounted for an increase in molar proportions of propionate when either cracked high-linoleate or high-oleate safflower seeds replaced supplemental cracked corn (Scholljegerdes et al., 2004b). An increase in ruminal propionate also resulted from fermentation of glycerol because an increase in molar proportion of propionate could not be explained by reduced ruminal fiber digestion in heifers receiving intraruminal infusion of 300 mL of soybean oil/d while consuming grass hay (Krysl et al., 1991).

Forage OM intake was not affected by feeding supplemental fat at 1.5 to 1.74% of diet DM (Brokaw et al., 2000, 2001). Forage intake and diet digestibility also were not affected in steers offered switchgrass hay and canola seeds to provide approximately 4% of diet DM as crude fat (Leupp et al., 2006). However, supplementing corn oil to steers grazing fescue pasture at 2 to 3 times the dietary levels used in our grazing experiments decreased forage DMI and total DE intake, and reduced total DMI reflected forage substitution rates >1 (Pavan et al., 2007). A forage substitution rate of 0.5 for heifers grazing bromegrass pastures was reported by MacDonald et al. (2007) when dried distillers grains was supplemented to provide approximately twice the dietary level of fat used in our grazing studies. Those authors, however, noted that supplementing corn oil that provided a comparable level of fat as dried distillers grains did not affect forage intake. In our experiments, DE for diets with supplemental fat would be comparable or greater than the corn-based supplements if one accounts for greater energy value of fatty acids disappearing from the small intestine (Scholljegerdes et al., 2004b). Moore et al. (1986) demonstrated that dietary DE increased when fat replaced wheat straw at 4% of the diet, but DE did not differ between the 2 and 4% fat diets. Comparable effects on digestibility of major dietary nutrients were observed in meat goats fed orchardgrass hay-based diets (Luginbuhl et al., 2000). We interpret collective results summarized thus far to indicate that an optimal inclusion rate for supplemental fat is less than 3% of DM if the goal is to maximize use of forage-based diets. Supplemental fat should be limited to 2% of DMI or less if the goal is to prevent substitution of forage consumption with intake of supplemental fat. Supplemental fat should not exceed 4% of DMI if the goal is to increase dietary DE with the provision of fat.

Fatty Acid Supply.
As mentioned previously, dietary lipids are modified extensively as they undergo biohydrogenation in the rumen. In all experiments in which we supplemented vegetable oil and determined flow of fatty acids to the duodenum in animals fed high-forage diets, complete biohydrogenation of dietary fat resulted in a substantial increase in duodenal flow of 18:0 (Table 2). Due to incomplete biohydrogenation, however, we also observed an increase in unsaturated fatty acids flowing to the duodenum in animals receiving supplemental fat. Of particular interest, duodenal flow of trans-vaccenic acid (TVA) increased regardless of the type of supplemental fat added to forage-based diets. Less complete ruminal biohydrogenation of C18 unsaturated fatty acids occurred in cattle fed a fishmeal-based supplement resulting in increased duodenal flow of TVA, but not CLA, in cattle fed a diet in which fatty acids from the supplement were included at 1.7% of OM (Hess et al., 2007). In a review of the literature, Khanal and Olson (2004) concluded that the greatest concentration of CLA in milk fat was achieved when fish oil was included at 2% of the diet DM. Because intestinal supply of 18:1 trans-11 far exceeds that of 18:2 cis-9, trans-11 CLA, it seems reasonable to suggest that 18:1 trans-11 serves as a precursor for conversion to 18:2 cis-9, trans-11 CLA via ⁹-desaturase in mammary tissue (Bauman et al., 2003). Duodenal flow of 20:5n-3 was approximately 11% of 20:5n-3 intake in cattle fed the greatest quantity of supplement, and 22:6n-3 was not detected in duodenal digesta in our study. This latter response may explain why Burns et al. (2003) did not detect an increase in endometrial content of 22:6n-3 in beef cows fed fishmeal. In contrast, Mandell et al. (1997) noted at least a 4-fold increase in 22:6n-3 of LM from steers fed fishmeal. Perhaps the increase in 22:6n-3 observed by Mandell et al. (1997) resulted from desaturation of 22:5n-3. Hess et al. (2007) reported a trend for increased duodenal flow of 22:5n-3 in cattle fed the fishmeal-based supplement. Our collective findings are particularly intriguing considering the heightened interest in modifying composition of food products derived from ruminant animals fed high-roughage diets (French et al., 2000; Ponnampalam et al., 2001, 2002; Rule et al., 2002; Daniel et al., 2004; Noci et al., 2005; Pavan and Duckett, 2007) and the potential to augment reproductive efficiency (Williams and Stanko, 2000; Funston, 2004) by manipulating fatty acids available to the animal.

Changing Forage:Concentrate
Associative effects between single sources of forage and concentrate have been well documented (Kromann, 1973; Woody et al., 1983; Hart, 1987; Ørskov and Ryle, 1990). The level of forage in the diet is also expected to affect flow of unsaturated fatty acids to the small intestine of ruminants (Jenkins, 1994; Loor et al., 2003). Data are lacking on interactions among dietary components in ruminants fed supplemental fat within a broad spectrum of forage to concentrate ratios. Thus, we evaluated effects of dietary forage:concentrate on fatty acid flow to the duodenum (Kucuk et al., 2001), site and extent of nutrient digestion (Kucuk et al., 2003), and fatty acid composition of mixed ruminal bacteria (Kucuk et al., 2007). Diets varying from 18.4 to 72.9% forage were supplemented with soybean oil to maintain 6.4% of diet DM as crude fat. Ruminal VFA patterns changed (as expected) as dietary forage:concentrate changed. Likewise, ruminal biohydrogenation of C18 unsaturated fatty acids increased as dietary forage level increased. Despite more extensive biohydrogenation of 18:3n-3, duodenal flow of this fatty acid increased as forage intake increased (Kucuk et al., 2001). This observation supports the findings of French et al. (2000), who reported that 18:3n-3 in intramuscular lipid increased as forage in the diet increased from 10 to 100%.

An increase in total fatty acid concentration of mixed ruminal microbes with increasing levels of dietary forage was due, in large part, to nearly a 3-fold increase in 18:0 (Kucuk et al., 2007). This contributed to an increase in 18:0 of microbial origin flowing to the duodenum (Kucuk et al., 2001). Greater than predicted total-tract OM digestibility for 59.4 and 72.9% forage diets (Kucuk et al., 2003) could not be attributed to postruminal ruminal disappearance of fatty acids, which decreased as forage level in the diet increased (Kucuk et al., 2001). The change in magnitude of postruminal disappearance of total fatty acids mirrored the decline in postruminal disappearance of SFA, although postruminal disappearance of unsaturated fatty acids also decreased as forage in the diet increased. Zinn et al. (2000) concluded that decreasing ruminal biohydrogenation will increase postruminal digestibility of fat. Thus, there is greater potential to supply the ruminant animal with unsaturated fatty acids from dietary origin if fat is supplemented to animals fed high-concentrate diets. Furthermore, energy available from digested fatty acids would be expected to increase as forage:concentrate changes from 72.9:27.1 to 18.4:81.6 because changes in intestinal fatty acid digestion account for most of the variation in NE values for fat (Plascencia et al., 2003). Our results are in contrast to those reported by Zinn and Plascencia (1996) who suggested that energy value of yellow grease was greater for high-forage vs. low-forage diets. In addition to using different species and feeding levels, forages used by Zinn and Plascencia (1996) were provided at 30 and 10% of the diet DM and fat was added at 6% of diet DM. Level of feed intake may be important in responses to supplemental fat (Elliott et al., 1997; Sackmann et al., 2003). Clearly, additional research is warranted on associative effects with high-fat diets ranging in forage to concentrate ratio.

High-Concentrate Diets
Brandt and Anderson (1990) concluded that interactions of fat with other dietary components in high-grain finishing diets required further investigation. Several studies on the subject were published in the Journal of Animal Science from 1996 through 2006 (Zinn and Shen, 1996; Zinn and Plascencia, 1996; Richards et al., 1998; Plascencia et al., 1999; Ramirez and Zinn, 2000; Zinn et al., 2000; Andrae et al., 2000; Kucuk et al., 2001, 2004; Nelson et al., 2001, 2004; Plascencia et al., 2003; Sackmann et al., 2003; Hristov et al., 2004, 2005; Jordan et al., 2006a; Atkinson et al., 2006). Results from our laboratory (Kucuk et al., 2004; Atkinson et al., 2006) indicate that supplementing fat up to 9.4% of DM does not affect digestibility of other dietary components. Less extensive ruminal biohydrogenation of C18 unsaturated fatty acids resulted in greater flows of these fatty acids to the duodenum as dietary soybean oil increased (Kucuk et al., 2004). Increased duodenal flow of 18:2n-6 that occurred from increased dietary oil could be accounted for by increased flow of esterified 18:2n-6 (Atkinson et al., 2006). The presence of biohydrogenation intermediates in the esterified fraction of duodenal digesta suggested that reesterification by the ruminal microbiota contributed to duodenal flow of esterified fatty acids. An increase in duodenal flow of TVA from microbial esterified lipid (Atkinson et al., 2006) as dietary fat increased was consistent with an increase in TVA concentration of mixed ruminal microbes (Kucuk et al., 2007) as dietary fat increased. Consistent with the observation of Elliott et al. (1999), Atkinson et al. (2006) noted that the extent of ruminal lypolysis decreased as the level of dietary fat increased. Contrary to the results of Elliott et al. (1999), small intestinal lipolysis did not impede digestion of esterified fatty acids because Atkinson et al. (2006) observed a linear increase in fatty acids disappearing from the small intestine as the level of dietary fat increased. Results of our site and extent of digestion studies with sheep fed high-concentrate diets support the notion that feeding greater levels of unsaturated fatty acids will increase unsaturated fatty acids available for incorporation into ruminant-derived food products.

Feed Processing.
Grains included in finishing diets are often processed to increase availability of nutrients, particularly starch. Disappearance of starch after a 12-h ruminal incubation period was 3.3 times greater for extruded grain than for dry-rolled grain (Gaebe et al., 1998). Feeding an extruded grain-based concentrate fortified with canola oil increased 18:1n-9 in lamb carcasses (Awadelseid, 1994). However, the presence of lasalocid may have contributed to reduced biohydrogenation of unsaturated fatty acids (Van Nevel and Demeyer, 1995). Using sheep, we evaluated the influence of lasalocid and soybean oil before or after extruding the concentrate portion of the diet on ruminal VFA and digestion (Burgwald-Balstad et al., 1998; Hess et al., 1999). Ruminal disappearance of OM, N, and starch were not affected by dietary addition of fat or lasalocid. An increase in acetate:propionate in sheep fed diets with lasalocid and oil added before extrusion suggested that extrusion rendered the ionophore less effective. However, Zinn (1988) reported an appreciable negative effect of monesin on ruminal acetate:propionate in steers fed diets with 4% yellow grease. Our observations were consistent with Zinn (1988), who noted no significant interactions between supplemental fat and monensin on metabolism of fat in the rumen or on composition of fatty acids entering the small intestine. Duodenal flow of total fatty acids was less in sheep fed diets in which fat was added before extrusion than with sheep fed the diet in which fat was added after extrusion. This response was attributed to loss of fat due to processing because flow of total fatty acids from microbial origin was not affected by timing of oil addition. Duodenal flow of unsaturated fatty acids was not affected by timing of oil addition or supplemental lasalocid. Thus, supplementing vegetable fat likely alters fatty acid composition of the ruminant animal’s tissue regardless of when it is added during feed processing. Addition of lasalocid also did not affect the composition of fatty acids reaching the small intestine of ruminants fed high-concentrate diets.

Moisture content may alter the need for, or benefit from, grain processing (Owens et al., 1997). Ensiled grains generally produce carcass measurements intermediate to those of cattle fed dry-rolled and steam-flaked grains, and adding 2% supplemental fat to the diet seemed to maximize LM area (Owens and Gardner, 2000). High-oil corn has been advocated as a dietary replacement for supplemental fat from oilseeds (Dado, 1999). Andrae et al. (2000) suggested that dry-rolled high-oil corn may have enhanced energy value because of its lipid content and a total tract starch digestibility that was greater than conventional corn. Unlike like conventional corn, energy values of high-oil corn are not expected to improve with ensiling because neither ruminal nor total-tract starch disappearance increased by ensiling high-oil corn (Scholljegerdes et al., 2001). Feeding high-oil corn increased duodenal flow of 18:2n-6 (Duckett et al., 2002), a response not influenced by ensiling (Bolte et al., 2001). Thus, incorporating high-oil corn in finishing beef cattle diets increased 18:2n-6 in the LM (Andrae et al., 2001).

Predicting changes in fatty acid composition of food products derived from ruminants based on profile of supplementary fatty acids is complicated by the form in which the supplement is fed (NRC, 2007). Feeding unprocessed whole oilseeds may reduce effects of supplemental fat on digestibility of other major dietary components because the seed coat is presumed to protect the oil from ruminal metabolism (Baldwin and Allison, 1983). Ruminal disappearance of safflower seed DM increased from 12.0 to 56.5% after 48 h of incubation in situ (Lammoglia et al., 1999a), but crushing (Hussein et al., 1995), extruding (Ferlay et al., 1992; Albro et al., 1993), and grinding (Leupp et al., 2006) oilseeds had minimal impact on digestibility of high-forage diets. Although unsaturated fatty acids within whole seeds are shielded from ruminal biohydrogenation, the seed may remain intact during passage through the small intestine resulting in reduced digestibility of fatty acids (Aldrich et al., 1997b). Gibb et al. (2004) attributed greater deposition of unsaturated fatty acids in target tissues of feedlot steers fed whole vs. rolled sunflower seeds to reduced biohydrogenation in the rumen; however, those authors also reported that rolling sunflower seeds did not enhance digestibility of the oil. Feeding feedlot heifers 8% flax increased 18:3n-3, and rolling or grinding increased intramuscular 18:3n-3 to a greater extent than with whole seeds (Maddock et al., 2006).

Because of limited, and often conflicting, information on altering the fatty acid content of ruminant-derived food products by feeding oilseeds processed to different degrees, a study was conducted to evaluate fatty acid digestion in lambs fed supplemental safflower fat in the form of whole seeds, cracked seeds, and oil extracted from seeds (Price et al., 2007). Ruminal biohydrogenation of high-linoleate safflower fatty acids was extensive regardless of the physical form fed. However, because ruminal biohydrogenation was not complete, feeding safflower fatty acids increased duodenal flow and intestinal disappearance of 18:2n-6 and biohydrogenation intermediates. Greater intestinal digestibility of 18:2n-6 fed in the form of oil (Price et al., 2007) may explain why milk produced by goats had greater percentage of 18:2n-6 when supplemented with sunflower oil compared with whole sunflower seeds (Chilliard and Ferlay, 2004). Similarly, a 10-fold increase in duodenal flow of 18:1 trans-10 in lambs fed oil supports an apparent shift in the biohydrogenation pathway suggested by Hristov et al. (2005) who fed feedlot steers high-linole-ate safflower oil at 5% of dietary DM. Moreover, results of Price et al. (2007) are consistent with studies in which we (Bolte et al., 2002) and others (Boles et al., 2005) demonstrated that 18:2n-6 and biohydrogenation intermediates increased in muscle tissue of feedlot lambs fed either cracked safflower seeds or safflower seed oil.

	SUPPLEMENTING FAT TO PERIPUBERAL HEIFERS

Top Abstract INTRODUCTION SUPPLEMENTATION CONSIDERATIONS RUMINAL VFA, DIET DIGESTIBILITY,... SUPPLEMENTING FAT TO PERIPUBERAL... SUPPLEMENTING FAT TO LACTATING... SUMMARY AND CONCLUSIONS LITERATURE CITED

 
Fat supplementation has exerted beneficial effects on reproduction that were attributed to dietary energy (DelCurto et al., 2000). Thomas et al. (1997) reported that ovarian follicular growth and development were enhanced in beef cows supplemented with supplemental soybean oil, which was independent of dietary energy. Hence, responses to supplemental soybean oil were evaluated in peripuberal heifers (Whitney et al., 2000; Brokaw et al., 2001). In an experiment in which heifers were fed individually, Whitney et al. (2000) attributed increased ADG and G:F for heifers fed soybean oil at 3% of DM to an increase in serum glucose arising from greater ruminal production of propionate. In a concurrent experiment using heifers fed in pens, animals fed the 3% soybean oil diet did not experience greater ADG but conceived approximately 11 d sooner than heifers fed diets with 0 or 6% soybean oil. The combination of confined spaces and interruptions in normal behavior associated with feeding heifers individually may have contributed to the lack of difference in days to conception among dietary treatments in the first experiment of Whitney et al. (2000). In 2 subsequent experiments, Brokaw et al. (2001) compared hand-fed supplements used in the 0 and 3% soybean oil diets of Whitney et al. (2000) to a self-fed supplement formulated to be equivalent to the hand-fed soybean oil supplement. Decreased ADG for heifers offered the selffed supplement occurred because daily supplement consumption was less than half of the target amount. As a result, pregnancy rate decreased from an average of 93.2% for heifers hand fed supplement to 72.4% for heifers offered the self-fed supplement in the first experiment. Despite reduced ADG for heifers self fed supplement in the second experiment, pregnancy rate was not affected by diet. Heifers offered the self-fed supplement were only 57.8% of mature BW at the onset of breeding in the first experiment but achieved 62.0% of mature BW by the beginning of the breeding season in the second experiment.

In a summary of 9 experiments with a total of 16 dietary supplements and 21 possible dietary fat comparisons, Hess et al. (2002) noted that fat supplementation increased overall pregnancy rates from 63.8% for the 373 non–fat-supplemented heifers to 73.6% for the 363 heifers fed supplemental fat (only one study used fat supplementation to increase dietary energy). Brokaw et al. (2002) reported that daily cost was $0.05 per animal more (4.67% increase) for heifers hand fed fat vs. heifers hand fed a control supplement. The additional cost of feeding fat to peripuberal heifers would be recuperated easily if supplemental fat improved pregnancy rate by 15.4% (Hess et al., 2002). A return on investment would be realized if heifers conceived earlier in the breeding season. Based on 0.97 kg/d ADG for calves suckling primiparous heifers in the University of Wyoming herd (Alderton et al., 2000; Bottger et al., 2002), heifers conceiving 11 d sooner (Whitney et al., 2000) would be expected to wean nearly 10.7 kg more calf. The additional calf weight would be worth $25.95 at $110/cwt. Replacement heifers would need to be fed for 519 d in this scenario before fat supplementation would not be economically feasible. Nonsignificant, numerical trends for improved first-service conception rates (Howlett et al., 2003), and pregnancy rates to AI (Funston, 2004) for heifers fed whole soybeans were consistent with observations summarized by Hess et al. (2002).

The feasibility of feeding fat to replacement heifers was recently questioned by Funston (2004). Based on a summary of 6 reports, Funston (2004) suggested the limited benefits of fat supplementation in well-developed replacement heifers indicates that supplemental fat is probably only warranted when fat supplements are priced comparably to other energy sources. We agree with this contention. Although a greater percentage of heifers fed fat tended to reach puberty before the breeding season (Lammoglia et al., 2000), puberty tended to be achieved at a later age if heifers were heavy and fed supplemental fat (Garcia et al., 2003). Rhodes et al. (1978) and Lammoglia et al. (2000) reported that ultrasonographic measurements of backfat thickness increased with supplemental fat by d 92 and 112, respectively. The number of heifers reaching puberty on test was reduced in the study of Rhodes et al. (1978). Therefore, caution should be exercised to not overcondition heifers by feeding supplemental fat.

It is also possible that reproductive response to supplemental fat is affected by the length of time heifers are fed fat. Lammoglia et al. (2000) hypothesized that a feeding period of 60 d before the beginning of the breeding season may have been more suitable than the prolonged feeding period used in their experiment (162 d). This hypothesis was based partially on the observation that backfat thickness was similar among treatments after 56 d of feeding and the leaner heifers (Piedmontese) responded more favorably to supplemental fat. Hess et al. (2002) recommended feeding supplemental fat to beef heifers for 60 to 90 d before the onset of the breeding season because dietary fat-induced increases in serum cholesterol concentrations of beef heifers appeared to plateau between d 56 and 88 after feeding (Lammoglia et al., 2000; Whitney et al., 2000; Lloyd et al., 2002).

	SUPPLEMENTING FAT TO LACTATING BEEF COWS

Top Abstract INTRODUCTION SUPPLEMENTATION CONSIDERATIONS RUMINAL VFA, DIET DIGESTIBILITY,... SUPPLEMENTING FAT TO PERIPUBERAL... SUPPLEMENTING FAT TO LACTATING... SUMMARY AND CONCLUSIONS LITERATURE CITED

 
Reproduction
Since the report by Williams (1989), evidence has accumulated on the use of dietary fat as a nutraceutical to positively influence reproductive events in beef cows (Williams and Stanko, 2000). Because 18:2n-6 was touted as the causative agent for many of the beneficial responses, and we had noted increased 18:2n-6 in plasma of reproducing beef cattle fed fat (Whitney et al., 2000; Alexander et al., 2002), a series of studies was conducted to investigate the influence of supplementing fat high in linoleic acid on reproductive responses in beef cows. Extensive details of our experimental results were described in a previous review (Hess et al., 2005). It was concluded that supplementing cracked high-linoleate safflower seeds decreased first-service conception rates (Hess, 2003) because fewer cows had functional corpus luteum (Grant et al., 2003), which may be related to an increase in PGF₂ (Grant et al., 2005) or perturbations in the IGF-I system (Scholljegerdes et al., 2004a). Subsequent ² analysis of data from experiments in which we fed cracked high-linoleate safflower seeds to young beef cows during early lactation revealed that pregnancy rate was reduced (P = 0.06) from 93.6% for cows (n = 47) fed control diets to 80.4% for cows (n = 45) fed high-linoleate safflower seeds. Using ruminal biohydrogenation values reported by Scholljegerdes et al. (2004b), we suggest that increasing intestinal supply of 18:2n-6 by 16 to 18 g/d during the first 60 to 90 d postpartum exerts deleterious effects on reproduction. Scholljegerdes et al. (2007) reported that primiparous cows fed high-linoleate safflower seeds for the first 33 d postpartum displayed increased concentrations of 18:2n-6 in the oviduct but not in other uterine tissues. A trend for greater concentrations of 18:3n-3 in plasma was consistent with greater concentrations of 18:3n-3 in endometrial tissues of cows fed high-linoleate safflower seeds. Despite an increase in plasma concentration of 20:5n-3 for cows fed high-linoleate safflower seeds, concentration of 20:5n-3 in uterine tissues was not affected by diet. Intercaruncular concentration of 20:5n-3 was negatively correlated with serum concentrations of PGF₂ metabolite on d 33 postpartum. We suspect that desaturation and elongation of 18:3n-3 to 20:5n-3 in uterine tissues was greater in cows fed control, resulting in reduced PGF₂ synthesis in bovine endometrium (Burns et al., 2003).

Provision of supplemental fat to increase intestinal supply of 18:2n-6 by 16 to 18 g/d notwithstanding, ² analysis of 324 individual observations from literature reports using 170 beef cows fed fat revealed an equivocal effect on pregnancy rates (Hess et al., 2005). This result was reaffirmed by Martin et al. (2005) who reported that pregnancy rate was not affected by feeding whole corn germ to cows for approximately 45 d postpartum. Therefore, fat supplementation during the postpartum period should not be recommended as a nutritional strategy to improve pregnancy rates of beef cows. Fat supplementation should only be utilized if cost-effective fat sources are available (Funston, 2004).

Cow Metabolism
Bottger et al. (2002) reported that feeding high-linoleate safflower seeds permitted cows to maintain greater BCS, whereas feeding high-oleate safflower seeds increased milk fat synthesis. The apparent ability for cows to prioritize the use of energy for different metabolic processes depending on fatty acids composition of the fat source prompted Lake et al. (2005) to evaluate the interaction between BCS at parturition and postpartum supplementation of cracked high-oleic or high-linoleic acid safflower seeds on cow and calf production. Contrary to our hypothesis, lipid supplementation did not affect cow or calf performance (Lake et al., 2005), had minimal impacts on cow metabolic hormones or metabolites (Bottger et al., 2002; Lake et al., 2006b), and did not influence activity of lipogenic enzymes in s.c. adipose tissue collected from the cows (Lake et al., 2006d).

Plasma (Lake et al., 2007; Scholljegerdes et al., 2007) and milk fatty acid composition (Murrieta et al., 2006; Lake et al., 2007) reflected alterations in postpartum diet that were generally anticipated based on flow of fatty acids to the duodenum of cattle fed similar diets (Scholljegerdes et al., 2004b). Scholljegerdes et al. (2007) suggested that an apparent lack of concurrence between duodenal supply of TVA and concentrations of TVA in plasma with a concomitant increase in plasma concentrations of 18:2 cis-9, trans-11 was due to activity of stearoyl-CoA desaturase in the intestinal mucosa (Archibeque et al., 2005) or tissues, such as the mammary gland, that rapidly removed TVA from circulation (Bauman et al., 2001). Lake et al. (2007) indicated that the decrease in plasma TVA observed in cows fed high-linoleate safflower seeds as lactation progressed did not result from an increase secretion of TVA into milk. Because the metabolic demands of lactation seemed to prevent the deposition of exogenously-derived fatty acids in adipose tissue through d 90 of lactation, Lake et al. (2007) surmised that TVA was selectively incorporated into tissues other than milk and s.c. adipose tissue. Nevertheless, feeding beef cows diets high in 18:2n-6 during early lactation will increase milk output of both TVA and CLA (Murrieta et al., 2006; Lake et al., 2007). Less medium-chain fatty acids and more 18-carbon fatty acids in milk were indicative of reduced de novo fatty acid synthesis in the mammary gland of beef cows fed supplements containing safflower lipid (Murrieta et al., 2006; Lake et al., 2007). An increase in 18-carbon fatty acid in milk of cows fed safflower fat could also be explained by greater uptake of dietary fatty acids, which was supported by a trend toward greater lipoprotein lipase mRNA in mammary cells collected from cows fed high-linoleate safflower seeds (Murrieta et al., 2006). A lack of dietary fat-induced abundance of other lipogenic enzyme mRNA transcripts collected from mammary somatic cells (Murrieta et al., 2006; Murrieta, 2007) indicates that exogenously derived fatty acids diluted fatty acids synthesized de novo. This apparent dissociation between milk fatty acid composition and mRNA abundance for lipogenic enzymes may also be related to less substrate for milk fat synthesis in cows fed safflower seeds, which is supported by less digestion of NDF by cattle consuming cracked safflower seeds (Scholljegerdes et al., 2004b). Overall, feeding fat to lactating beef cows resulted in substantial changes in blood and milk fatty acids but the metabolic demands associated with lactation seem to override the potential to partition nutrients toward body energy reserves during early lactation.

Calf Metabolism
An improvement in the essential fatty acid status of the neonatal ruminant could provide practical benefits to the animal (Noble et al., 1978). Lammoglia et al. (1999a, b) demonstrated that calves born to cows fed high-linoleate safflower seeds during gestation responded to cold stress by increasing rectal temperature, which was maintained for a longer period than calves born to cows not fed supplemental fat. Feeding high-linoleate safflower seeds to late-gestational ewes also improved neonatal lamb survivability (Encinias et al., 2004). Serum IgG concentrations were greater in calves born to fat-supplemented cows (Small et al., 2004). Dietz et al. (2003) reported that serum IgG concentrations tended to be increased when calves born to fat-supplement cows exposed to a cold environment; however, this result was not observed in calves born during milder conditions. Similarly, prepartum fat supplementation did not affect apparent cold tolerance if calves were exposed to milder conditions around the time of calving (Lammoglia et al., 1999b; Dietz et al., 2003). This lack of response by the neonate exposed to less harsh environments is consistent with the lack of prepartum dietary fat response on calf vigor scores (Alexander et al., 2002; Small et al., 2004).

Lammoglia et al. (1999a, b) attributed greater cold tolerance of calves from lipid-supplemented dams to increased availability of glucose for metabolism and heat production. Because milk is the calf’s sole source of nutrients during the early suckling phase, changes in milk fatty acid output also should be considered when beef cows are fed lipid supplements. Total milk fat output did not differ due to lipid supplementation (Lake et al., 2005); therefore, calf plasma and adipose tissue fatty acids (Lake et al., 2006c) were reflective of alterations in long-chain fatty acids of milk from beef cows fed safflower seeds (Lake et al., 2007). Dietary fatty acids may influence lipogenic activity such that fatty acid synthesis is downregulated (Azain, 2004). Maternal dietary treatment did not affect abundance of mRNA transcripts for lipogenic enzymes in calf s.c. adipose tissue (Murrieta, 2007). Greater serum concentrations of glucose, coupled with similar insulin concentrations in calves suckling lipid-supplemented dams, supports the concept that an apparent glucose-sparing effect was related to decreased insulin-stimulated uptake of glucose compared with control calves (Lake et al., 2006b).

Fatty acids play important roles in metabolic regulatory functions. Calves used in the experiment of Small et al. (2004) had greater plasma concentrations of CLA within 24 h after birth. In a review, McGuire and McGuire (2000) noted that CLA is a modulator of the immune system. Calves from cows fed fat for 61 d prepartum had enhanced responses to ovalbumin antigen challenge at approximately 90 d of age (Small et al., 2004). In contrast, calves suckling cows fed a high-linoleate supplement had a decrease in total antibody production in response to ovalbumin and appeared to have a delayed response to antigen challenge (Lake et al., 2006a). Lake et al. (2006c) reported that greater cis-9, trans-11 CLA in adipose tissue from calves suckling cows fed high-linoleate safflower seeds was due to an increase in cis-9, trans-11 CLA of milk from cows fed the high-linoleate (Lake et al., 2007). A positive relationship (r² = 0.82) between milk TVA and cis-9, trans-11 CLA in s.c. adipose tissue of calves indicated that endogenous synthesis of cis-9, trans-11 CLA also occurred in the adipose tissue from increased milk supply of TVA. Nonetheless, maternal dietary treatment did not affect calf plasma concentrations of TVA and CLA (Lake et al., 2006c). Lake et al. (2006c) suggested that changes in membrane fluidity and signal transduction associated with increased 18:2n-6 of calves suckling cows fed a high-linoleate supplement may have altered immune function. Supplementing fish oil to grazing cattle may boost the proliferative response of lymphocytes (Wistuba et al., 2005). Wistuba et al. (2006) reported a decrease in plasma 18:2n-6 and a concomitant increase in plasma 20:5n-3 in grazing cattle fed fish oil. Perhaps the unexpected decrease in antibody production in response to ovalbumin observed by Lake et al. (2006a) was also associated with reduced 20:5n-3. Lake et al. (2006c) reported that plasma concentrations of 20:5n-3 were less in calves suckling cows fed high-linoleate safflower seeds than in calves suckling cows fed the control diet. Response to stimulation with concanavalin A by lymphocytes from calves used in the study of Wistuba et al. (2005) depended on the carrier of the supplemental fat. We conclude that more research is needed to elucidate the effects of supplemental fat on immune function in ruminants.

	SUMMARY AND CONCLUSIONS

Top Abstract INTRODUCTION SUPPLEMENTATION CONSIDERATIONS RUMINAL VFA, DIET DIGESTIBILITY,... SUPPLEMENTING FAT TO PERIPUBERAL... SUPPLEMENTING FAT TO LACTATING... SUMMARY AND CONCLUSIONS LITERATURE CITED

 
Supplementing fat to beef cattle and sheep can be an effective strategy to increase energy density of the animal’s diet. Optimal levels of fat in the diet depend on goals set for the production unit. Limiting supplemental fat to 2% of dietary DM will help prevent negative associative effects for ruminants fed high-forage diets. The energy density of high-forage diets will not be increased if supplemental fat exceeds 4% of DM. However, ruminants fed high-concentrate diets may receive up to 6% supplemental fat in the diet without ill effects on utilization of other dietary components. Extensive ruminal biohydrogenation of dietary C18 unsaturated fatty acids did not preclude the ability to alter unsaturated fatty acid status of ruminants fed fat supplements. Feeding vegetable and fish-based fat supplements will increase biohydrogenation intermediates available for metabolism by a variety of ruminant animal tissues. Positive effects on reproductive processes in beef cattle fed fat have been attributed to changes in unsaturated fatty acid status rather than changes in energy per se. Developing replacement heifers should be fed supplemental fat for 60 to 90 d before the breeding season. Feeding fat to overconditioned heifers seems to delay attainment of puberty. Cow pregnancy rate will not be improved with provision of supplemental fat during the early postpartum period, and increasing intestinal supply of 18:2n-6 by 16 to 18 g/d has a negative influence on beef cow reproduction. Manipulating maternal diet to improve unsaturated fatty acid status of the neonate has practical benefits, especially for neonates exposed to harsh environmental conditions or foreign antigens. More research, however, is required to understand consequences of altering unsaturated fatty acid status of calves suckling cows fed fat supplements during lactation.

	Footnotes

 
¹ Presented by B. W. Hess in partial fulfillment of the ASAS Early Career Achievement Award received at the annual meeting of the American Society of Animal Science, San Antonio, Texas, July 8 to 12, 2007. The authors express sincere gratitude to the many students, technicians, and colleagues who assisted with various studies described in this paper.

² Corresponding author: brethess{at}uwyo.edu

Received for publication August 30, 2007. Accepted for publication December 9, 2007.

	LITERATURE CITED

Top Abstract INTRODUCTION SUPPLEMENTATION CONSIDERATIONS RUMINAL VFA, DIET DIGESTIBILITY,... SUPPLEMENTING FAT TO PERIPUBERAL... SUPPLEMENTING FAT TO LACTATING... SUMMARY AND CONCLUSIONS LITERATURE CITED

 


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