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Division of Agriculture, West Texas A&M University, Canyon 79016 and Texas Agricultural Experiment Station, Amarillo 79106
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Key Words: concentrate feeding • diet adaptation • starch intake
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INTRODUCTION |
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TopAbstractINTRODUCTIONIMPLICATIONSLITERATURE CITED |
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; Owens et al., 1998) that may impose long-term consequences or may be lethal (Krehbiel et al., 1995; Nagaraja and Chengappa, 1998).
Counette and Prins (1981) proposed a fitting definition of when a ruminant may be considered adapted to consuming concentrates as the point when an animal can be fed a given diet, without adverse effects, at a level of feed intake that would elicit acidosis in the nonadapted animal. In the feedlot, nutritionists must consider a number of variables to develop management practices to optimize the cost of weight gain both during diet adaptation and the overall feeding period. Owens and Goetsch (1986) have described the relationships between diet forage:concentrate and ruminal kinetics, whereas Kaufmann et al. (1980) provided a brief overview of ruminal events during diet adaptation. Although our knowledge of the etiology of acidosis is quite extensive (Owens et al., 1998), less attention has been directed to defining the magnitude and frequency of increases in cereal grain consumption during adaptation that can be expected to result in optimum cattle performance. The objective of this paper is to review the influence of grain adaptation procedures on performance by feedlot cattle and on ruminal fermentation and microbial changes.
Effect of Diet Composition and Management on Performance
The inherent stressors of fasting and dehydration during the processes of procuring and transporting feeder cattle from the point of origin to the feedlot can markedly affect feed intake shortly after arrival. Data summarized by Loerch and Fluharty (1999) indicate that the capacity for ruminal fermentation is restored within a few days of arrival. After cattle arrival, some process would be employed to transition cattle ultimately to a diet containing approximately 5 to 15% of DM as a forage source during the finishing period.
Bartle and Preston (1992) adapted steers to 90% concentrate diets based on steam-flaked sorghum by feeding diets containing approximately 65, 75, and 85% concentrate for 7 d each. The transition between diets was made less dramatic by offering feed on the first day of each new diet to provide the same amount (Mcal) of NEm as they were offered on the last day of the previous diet. Steers were either allowed ad libitum access to the diet; DMI was limited to 2.1, 2.3, 2.5, and 2.7 x NEm on wk 1 through 4 (based on initial BW), or DMI was limited to 2.3, 2.5, 2.7, and 2.9 x NEm (based on initial BW) on wk 1 through 4. During the first 28 d, cattle that were initially limited to a maximum of 2.1 x NEm consumed 6% less DM and tended to gain weight more efficiently (8.5%) than cattle on the remaining treatments. The authors noted that cattle fed using the 2.1 x NEm starting point were frequently not fed to appetite during the first 28 d. Gain efficiency over the entire finishing period tended to be improved (4%) for cattle initially limited to 2.1 x NEm, although the greater frequency of clumped ruminal papillae at slaughter by these cattle compared with the other treatments argues against attributing improved performance to less metabolic disturbance during adaptation.
Using individually fed cattle previously fed a corn silage diet, Burrin et al. (1988) subsequently fed a 75% concentrate diet for 6 d followed by a 95% concentrate diet based on high-moisture corn to cattle receiving graded concentrations of monensin. Steer DMI was reduced approximately 60% across treatments on the fourth day of feeding the 95% concentrate diet compared with DMI on the first day of feeding this diet. In a second study, pens of cattle previously fed corn silage were allowed ad libitum access to the diet during adaptation. The DMI over the 21-d adaptation period was similar between cattle adapted from 55 to 95% concentrate over either 14 or 7 d. However, ADG and gain efficiency during the 21-d adaptation were reduced 10 and 9%, respectively, for cattle introduced to the finishing diet more rapidly.
Bierman and Pritchard (1996) adapted cattle to a 92% concentrate diet by either allowing ad libitum access to 45, 65, 75, and 82% concentrate diets over a period of 11 d or by restricting intake of the final diet initially (to 1.74% of initial BW) followed by gradual increases until ad libitum intake was achieved. Steer ADG did not differ among treatments during the first 29 d, but cattle fed restricted quantities of the final diet initially consumed 20% less DM and were 19% more efficient. It is likely that reduced NEm as well as other factors linked to feed intake restriction were components of the response (Sainz, 1995), but it is not clear to what extent reduced metabolic challenge contributed to the performance improvement. Across the entire 121-d feeding period, steer ADG was similar but cattle fed the final diet initially were 11% more efficient.
Choat et al. (2002) adapted yearling steers by feeding 70, 75, 80, and 85% concentrate diets based on steam-flaked corn for 5 d each followed by a 90% concentrate diet until slaughter. Steers receiving diets with increasingly less forage were initially offered the first diet at 1.5% of BW and feed offered was increased by 0.45 kg/ d when no feed remained the following day. Remaining cattle were initially offered 1.25% of BW of the 90% concentrate diet and feed offered was increased by 0.23 kg/d when no feed remained the following day. Cattle fed the finishing diet from d 1 consumed less DM (22%) and gained less during the first 28 d; however, overall ADG and gain efficiency did not differ among treatments. In a second study, calves were offered 65, 75, and 85% concentrate diets for 7 d each, followed by a 92.5% concentrate diet until slaughter or offered a restricted quantity of the final diet on d 1. For calves fed the final diet initially, procedures were similar to those in the first experiment. However, calves fed the series of diets were initially offered 2% of BW and feed offered was increased as in the first experiment. Calf ADG during both the first 28 d and the entire feeding period was reduced 8% by restricting intake of the final diet during adaptation compared with changing diet composition. However, DMI by calves fed the final diet initially did not approach that of ad libitum cattle until approximately 40 d on feed; this occurred near d 28 in the first study. Variation in DMI across days for a pen was lower through at least d 15 in both studies for cattle fed the final diet initially. Yearling steers were adapted to 95% concentrate diets by Weichenthal et al. (1999) by either allowing ad libitum access to 65, 75, 82, and 90% concentrate diets for approximately 6 d each or by offering 1.77% of BW of the 95% concentrate diet on d 1 and increasing feed offered by 0.23 to 0.45 kg/d over 24 d. Overall DMI was reduced 6% and gain efficiency was increased by 8% by restricting intake of the final diet initially.
Bunk management is inherently involved in the execution of adaptation procedures in the feedlot. Although some debate exists on the performance implications of variable feed intake patterns during the feeding period, observations from accumulated data on pens of cattle (Peters, 1995; Preston, 1995) indicate that allowing cattle ad libitum access to feed between approximately 5 and 14 d on feed (when appetite increases rapidly) during adaptation is commonly followed by a substantial reduction in subsequent feed intake. Tremere et al. (1968) determined the length of time between the beginning of grain feeding and the occurrence of reduced grain intake by individuals offered ad libitum access to hay while introducing grain at various rates (an additional 5 to 10 g/kg of BW0.75 daily) in a series of studies. Increasing the quantity of additional grain consumed daily shortened the duration until reduced feed intake from 18 to 10 d. Increasing feeding frequency from once to twice daily extended the time until reduced grain intake and increased the quantity of grain that was consumed before the reduction occurred. Reduced feed intake occurred at 70 to 75% concentrate across studies by Tremere et al. (1968).
The established relationship between NEm intake and diet NEm concentration (NRC, 1996) would predict a reduction in DMI of less than 10% for a 300-kg year-ling fed a diet containing 2.2 Mcal of NEm/kg (e.g., 90% concentrate) compared with a diet containing 1.8 Mcal of NEm/kg (e.g., 50% concentrate). It is likely that pen-level reductions beyond those explained by diet NE are related to ruminal conditions of multiple individuals in the pen in light of data indicating that reduced feed intake from excess grain consumption by unadapted cattle was highly related (r = 0.83) to average daily ruminal pH the preceding day (Brown et al., 2000). Data presented by Pritchard and Bruns (2003) suggest that employing management practices to reduce or avoid dramatic reductions in feed intake by pens of cattle during the feeding period can improve cattle performance relative to ad libitum access to the diet.
Few data from performance studies have been available to assist nutritionists in developing and refining adaptation procedures in the feedlot. In the experiments reviewed, cattle generally began grain adaptation with diets containing 55 to 70% concentrate or at a greater initial starch intake by restricted feeding of the final diet. Feedlot performance was generally reduced when ad libitum access to adaptation diets was allowed and the length of time until the final diet containing 92 to 95% concentrate was fed was less than approximately 14 d. However, the number of cattle involved in these studies does not allow insight into the frequencies of metabolic disturbances (e.g., bloat, sudden death) that are a tangible reality in production. The process of offering a limited quantity of the finishing diet initially and gradually increasing feed intake shows promise for improving production efficiency, but strategies will be needed in commercial feedlots to attenuate cattle interest in feeding early in the feeding period for routine use.
Individual Carbohydrate Tolerance
The quantity of feed consumed by a pen of cattle is the net result of a feed consumed by individual occupants. A growing body of evidence suggests considerable diversity in the ability of animals to cope with ingested grain. Of 3 steers dosed with 70 g of finely ground grain/kg of BW (Dougherty et al., 1975), 1 steer was euthanized, 1 survived acute acidosis, and ruminal pH of the third steer was not observed below 5.5 with the sampling schedule used, but the steer did display severe diarrhea.
Brown et al. (2000) fasted steers previously fed either forage or a 50% concentrate diet. Steers previously fed forage were either challenged with 3.5% of BW as steam-flaked corn or offered the 50% concentrate diet, whereas steers previously fed the 50% concentrate diet (for 28 d) were either challenged with 1.5% of BW as rolled corn:rolled wheat or offered ad libitum access to the 50% concentrate diet. Steers were then classified as experiencing either acute or subacute acidosis, or not affected if they displayed an average daily ruminal pH <5.0, 5.1 to 5.6, or <5.6, respectively, on at least 1 d. On average, subsequent feed intake was greatest by steers that were refed the 50% concentrate even though total ruminal lactate peaked at 5 to 15 mM. Feed intake by steers previously fed forage before offering the 50% concentrate diet increased linearly during the study, whereas intake of the 50% concentrate diet by steers challenged with steam-flaked corn was dramatically reduced on d 3 and gradually recovered by d 10. Three of the 5 steers challenged with steam-flaked corn exhibited classical clinical features of acute acidosis (average ruminal pH <5.0, ruminal lactate <90 mM) accompanied by nearly complete avoidance of feeding the following day. However, 1 steer challenged with steam-flaked corn was remarkably classified as not affected; total ruminal lactate peaked at 14 mM and DMI the following day was numerically greater than the mean of steers classified as subacute (5.1 vs. 4.5 kg/d). Of the 5 steers fed forage followed by ad libitum access to the 50% concentrate diet, 2 were classified as experiencing subacute acidosis. This finding has relevance for production because this treatment was selected to reflect a likely scenario for feedlot cattle arriving in commercial facilities (fed forage, fasted, and refed) and some metabolic disturbance would be expected by offering ad libitum access to a similar diet after arrival.
More recent data using continuous pH measurement have provided additional insight into responses by individuals. Bevans et al. (2005) transitioned heifers from a 40% concentrate diet to a 90% concentrate diet by feeding 5 diets with increasing grain over 15 d or by feeding 1 diet containing 65% concentrate for 3 d only. Mean ruminal pH did not differ during the first day that the 65% concentrate diet was fed; however, the variability in time that ruminal pH was <5.6 on that day was greater for heifers switched directly to this diet. The separation between treatments for time below pH 5.6 became more amplified on d 2 and 3 after introduction of the 65% concentrate diet. Similarly, the variation in time below pH 5.2 was greater on d 1, 3, and 4 of feeding the 90% concentrate diet for heifers receiving only the 65% concentrate diet previously. Treatment did not influence mean DMI or variation in DMI, but mean feed intake across all heifers was 8 and 17% lower on the second day of feeding the 65 and 90% concentrate diets, respectively. The similar variation in DMI across treatments was reflective of the range in feed intake on the second day of feeding the 90% concentrate diet (0.6 to 11.2 kg/d for heifers allowed 3 d of adaptation, 5.3 to 12.7 kg/d for heifers allowed 15 d of adaptation). Few metabolite concentrations differed, but these data should be interpreted cautiously due to widely ranging DMI.
The graphic representation of DMI and ruminal pH of heifers classified as either coping well or poorly with grain introduction (Bevans et al., 2005) suggests that cattle that can effectively regulate intake during adaptation gradually consume more DM during the progression of diets. These data also highlight an insidious, repeating cycle of overconsumption followed by a pronounced reduction in ruminal pH by cattle that cope poorly with grain introduction. The unique biological features that differentiate cattle into these 2 categories are not presently clear, but differences in either microbial populations or metabolic capacity (i.e., growth rate and VFA disposal) seem possible and other potential variables have been proposed (Schwartzkopf-Genswein et al., 2003).
Microbial Populations and Metabolites
Describing the general patterns of microbial shifts is of interest to gain insight into the relationship between underlying biology and performance observations discussed previously. Although ionophores were fed in performance studies discussed previously, data to be discussed in the section were frequently obtained from animals that did not receive an ionophore. The continuing development of molecular-based techniques to identify ruminal bacteria suggests that the vast majority have not been cultured to date, but data are accumulating rapidly. However, only a small number of studies are available employing the use of molecular techniques to examine microbial shifts during the grain adaptation process. Because the intent here is to focus on the implications of procedures during grain adaptation, relevant studies using culture-based identification will also be discussed.
Tajima et al. (2000, 2001) studied 13 culturable species using molecular techniques from cattle switched from a diet containing 21% grain to a diet containing 82% grain. Clone sequences from cellulolytic bacteria were dramatically reduced 3 d after the diet change, and generally decreased further by 28 d after the change. Clone libraries containing sequences with greatest identity to Prevotella species and Streptococcus bovis were transiently increased on d 3 but approached initial values on d 28, whereas sequences identified as Selenomonas ruminantium peaked on d 3 and were double the number found before the diet change on d 28. Anaerovibrio lipolytica representation remained relatively unchanged. Forage-fed cattle were fed 45 and 60% barley diets for 2 d each, followed by a 75% barley diet in a study by Klieve et al. (2003). Megasphaera spp. were not detectable using real-time PCR standardized with pure culture representatives of the targeted DNA when hay was fed; these bacteria first emerged (104 cells/mL) after 2 d of feeding the 75% concentrate diet or after 5 d of grain feeding (butyrate concentration also increased at this time), and reached the equivalent of 108 cells/mL after approximately 12 d of grain feeding. Numbers of S. bovis were relatively stable (107 to 108 cells/mL) throughout the study, whereas B. fibrosolvens declined steadily and approached the limit of detection after 10 d of grain feeding. Despite the challenge, average ruminal pH remained above 6.0 and the authors indicated that only 1 animal experienced pH below 5.6 (Klieve et al., 2003). Ruminal pH from this animal declined to 4.8 on the second day of consuming the 45% barley diet and feed was refused for 2 d. The population of S. bovis increased 100-fold within the first 24 h, B. fibrosolvens declined more rapidly, and M. elsdenii generally increased more rapidly than in unaffected animals. It is not clear from these data whether the aberration resulted primarily from a microbial predisposition or if feeding behavior or some aspect of animal physiology played a more influential role.
Brossard et al. (2004) fed lambs alfalfa hay or increased the percentage of ground wheat in the diet from 0 to 60% in 6 d. The concentration of total 16S rRNA and the percentage of rRNA from S. bovis were unchanged by the transition, whereas 16S rRNA from M. elsdenii was not detectable (less than approximately 106 cells/mL) in either lambs fed hay or those fed the wheat-based diet. Total protozoal numbers were also unchanged by feeding the wheat-based diet. However, S. ruminantium 16S rRNA (% of total rRNA) increased 7-fold after 1 wk from the beginning of wheat introduction compared with hay-fed lambs. The molar percentage of acetate was reduced, the percentage of butyrate was increased, and the percentage of propionate was not different between lambs fed the wheat-based diet and those fed alfalfa hay.
Grubb and Dehority (1975) fed orchardgrass hay to wethers before feeding a diet containing 60% dry-rolled corn once daily at a constant level of intake. Numbers of protozoa were 2-fold greater (Table 1) when the 60% concentrate was fed than during hay feeding. More than 89% of all protozoa were Entodinium spp.
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; Mackie and Gilchrist, 1979; Mackie and Heath, 1979) fed sequential diets containing 10, 24, 44, 60, and 71% concentrate (each diet contained 10% molasses) twice daily to wethers at a constant level of intake. Each diet was fed for 7 d and data for each diet represent the mean of samples on d 1 and 7. The results of statistical tests were not reported by the authors; therefore, concentrations reported were log10 transformed, the SD calculated, and means were considered different if were separated by at least 2 SD. Concentrations of lactate-utilizing bacteria were relatively unchanged until the 60% concentrate diet was fed (Mackie et al., 1978; Table 1
), but lactate-utilizing bacteria increased dramatically when the 71% concentrate diet was fed. Amylolytic bacteria and protozoal concentrations exhibited a more gradual increase as dietary concentrate proportion increased. Bacteria in the genera of Bacteroides and Ruminobacter combined (B. succinogenes and B. amylophilus are currently known as Fibrobacter succinogenes and Ruminobacter amylophilus, respectively) and Butyrivibrio were the predominant amylolytic bacteria present in diets up to 60% dietary concentrate (Table 2), but these genera became less numerous when a 71% concentrate diet was fed for 7 d. The abundance of Lactobacillus when feeding the 71% concentrate suggests that more acidic conditions prevailed (Yokoyama and Johnson, 1988). Lactate-utilizing bacteria of the genera Anaerovibrio and Propionibacterium (Table 3) seemed to replace Veillonella and Selenomonas after the diet contained more than 24% concentrate. Peak ruminal L- and D-lactate concentrations (approximately 5 to 10 mM) generally increased with dietary concentrate. Ruminal L- and D-lactate concentrations peaked by 30 min after feeding and returned to baseline by 2 h across diets. A secondary lactate peak seemed to emerge at approximately 3 h after feeding on the 71% concentrate diet but returned to baseline after approximately 1 h.
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reported that numbers of lactate-utilizing bacteria (Table 1) increased gradually through the 60% concentrate diet followed by a further 16-fold increase when the 71% concentrate diet was fed. Mackie and Gilchrist (1979) indicated that lactate-utilizing bacteria began to increase when the 60% concentrate diet was fed (Table 1). A more gradual increase in amylolytic bacteria was evident; however, a 7-fold increase in the concentration of amylolytic occurred when the 71% concentrate diet was fed. The distribution of bacteria representing various genera (Tables 2 and 3) were similar to the data of Mackie et al. (1978), with the exception of the absence of Lactobacillus until the 71% concentrate diet was fed. Ruminal pH declined and total VFA concentration increased as dietary concentrate increased, whereas total lactate concentration remained below 3 mM. The molar percentage of propionate did not differ as dietary concentrate increased (19.7, 19.7, 19.4, 22.1, and 22.4 ± 1.5%). The molar percentage of acetate was reduced (62.5, 63.5, 61.1, 57.8, and 54.1 ± 3.8%), but butyrate percentage increased (14.4, 14.2, 14.9, 15.5, and 17.4 ± 1.3%) when the 71% concentrate diet was fed compared with diets containing less than 44% concentrate.
Olumeyan et al. (1986) fed 20, 50, and 80% concentrate diets sequentially to steers at a constant level of intake for 14 d each. Amylolytic bacteria concentrations were not influenced by dietary concentrate (Table 1), but lactate-utilizing bacteria increased with dietary concentrate. Moreover, numbers of lactate-utilizing bacteria increased further between 14 and 28 d of feeding the 80% concentrate diet but the percentage of total bacteria that were lactate-users did not change after 14 d. Protozoa counts were greatest on the 50% concentrate diet and intermediate on the 80% concentrate diet; more than 94% of protozoa were Entodinium. Ruminal pH was lowest, and total VFA greatest, for the 80% concentrate diet and intermediate for the 50% concentrate diet. The molar ratios of acetate and propionate were not altered by dietary concentrate, whereas the percentage of butyrate was greatest on the 80% concentrate diet. Hristov et al. (2001) adapted steers to an 80% concentrate diet over 45 d and then made observations every 5 d during a 30-d period of offering ad libitum access to the 80% concentrate diet fed twice daily. After this experimental period concluded, steers were adapted over 20 d to a 92% concentrate diet and observations continued every 5 d during a second 30-d period. Protozoal concentrations were highly dependent on the time of sampling but did not differ between the 80 and 92% concentrate diets during the first 5 d that diets were fed (Table 1); more than 90% of protozoa were Entodinium. However, protozoa concentration averaged across time was greater when cattle were fed the 80% concentrate diet. The concentration of acetate was lower, and butyrate greater, during the first 5 d of feeding the 92% concentrate diet than for the 80% concentrate diet, whereas ruminal L-lactate concentration remained below 2 mM. Franzolin and Dehority (1996) fed steers 0, 50, and 75% concentrate diets based on whole corn for 14 d each. Total protozoa were greatest for the 75% concentrate diet (Table 1), intermediate for the 50% concentrate diet, and lowest for the forage diet.
Leedle et al. (1995) fed heifers successive diets containing approximately 20, 45, 75, and 90% concentrate once daily for 7 d each at 2% of BW. Feed that was not consumed within 2 h was dosed into the rumen in equal portions over three 2-h intervals. Ruminal L-lactate concentration remained below 5 mM, and total organic acid and glucose concentration increased progressively as dietary concentrate increased. Acetate concentration peaked at 75% concentrate, and propionate and butyrate concentrations increased with dietary concentrate. The authors also noted that the frequency of ruminal contractions declined from 1.6 to 0.8/min as dietary concentrate increased.
In the studies reviewed, amylolytic bacteria became more numerous as more dietary concentrate was fed and lactate-utilizing bacteria increased more dramatically when the diet contained more than approximately 60% concentrate and when diet composition was unchanged for 5 to 7 d and culture-based techniques were used. Molecular-based and culture-based enumerations indicate a reduction in Butyrivibrio as more grain is fed, but molecular techniques demonstrate a more prominent role by Selenomonas and Megasphaera. Based on molecular techniques, Selenomonas ruminantium increased dramatically within 3 d of feeding more than 45% concentrate, whereas Megasphaera elsdenii became more numerous by 5 d of feeding more than 60% concentrate and increased to 108 cells/mL within approximately an additional 7 d of feeding a 75% concentrate diet. When intake by individuals was limited and competition between animals at the bunk did not exist, the percentage of lactate-utilizing bacteria changed little after 14 d. The later increase in lactate-utilizing bacteria than for amylolytic bacteria during grain adaptation may relate to a relatively slower growth rate by lactate users. Specific growth rates of the lactate-utilizers Propionibacterium (0.2 to 0.35/h), S. ruminantium (0.5 to 1.0/h), M. elsdenii (0.4 to 0.6/ h), and Anaerovibrio (0.2/h) at optimum pH were lower than the amylolytic bacteria Butyrivibrio (0.7/h) or Streptococcus (2.5/h; Counette and Prins 1981; Therion et al., 1982).
More than 90% of protozoa in the rumen during grain adaptation were Entodinium. Protozoal concentrations increased to a peak of 2- to 4-fold greater when the diet contained approximately 60% concentrate. Numbers were reduced as dietary concentrate surpassed approximately 70% of diet DM, which is likely a function of slow growth (12- to 15-h doubling time; Dehority, 2004) and pH sensitivity (Hungate et al., 1964). It is not clear what role ruminal fungi may play during adaptation, although strains of fungi are capable of contributing to starch digestion (McAllister et al., 1993).
Entodinium spp. play an important role in regulating the rate of starch fermentation by engulfing starch and attached amylolytic bacteria (Hungate, 1966), degrading and fermenting ingested starch more slowly than do bacteria (Abou Akkada and Howard, 1960), and can reduce the rate of ruminal starch fermentation (Mendoza et al., 1993). Defaunating cattle fed an 85% concentrate diet has resulted in lower ruminal pH (5.97 vs. 6.45), a greater concentration of VFA (92.3 vs. 64.8 mM), a greater molar ratio of propionate, and a tendency for greater numbers of amylolytic bacteria than faunated steers (Nagaraja et al., 1992). Data collected from feedlot cattle at slaughter suggest that a range of protozoa abundance occurs in the pen. Up to approximately 10% of cattle may be defaunated and more than 25% of cattle may display counts above 105 cells/mL at a given time (Towne et al., 1990; Hristov et al., 2001), but protozoal numbers are not exclusively related to ruminal pH (Towne et al., 1990; Franzolin and Dehority, 1996). Animals becoming defaunated during the feeding period could be naturally refaunated within a relatively short period (Stern and Hinkson, 1974; Mendoza et al., 1993) with permissive ruminal conditions. Reductions in protozoal populations during the feeding period would be expected to contribute to enhanced growth of amylolytic bacteria and more rapid organic acid production and(or) greater VFA concentrations.
A progressive decline in ruminal pH and a progressive increase in total VFA concentration occurred as dietary concentrate increased in studies reviewed. Ruminal lactate concentrations either exhibited only transient peaks (<10 mM) from 30 min to 2 h after feeding or remained near basal concentrations (<2 mM) in experiments reviewed. In vivo lactate production is tightly coupled to glucose use (Mackie et al., 1984), although less than approximately 15% of VFA produced from medium- to high-concentrate diets originates via lactate (Hungate et al., 1964; Counette and Prins, 1981; Mackie et al., 1984). Lactate use seems to reach a plateau between 60 and 80% concentrate, but has generally been increased up to approximately 5-fold between 0 and 90% concentrate (Kunkle et al., 1976; Byers and Goodall, 1979).
A trend for reciprocal shifts in the molar percentages of acetate (lower) and butyrate (greater) was evident once the diet contained approximately 70% concentrate, whereas the increase in the molar percentage of propionate was less pronounced as dietary concentrate increased. Individual species of rumen bacteria are generally capable of producing several end products (Hungate, 1966), and metabolite production by rumen bacteria at a given time can be influenced by intracellular and extracellular conditions (Russell, 2002). According to Bryant (1979), conditions that promote more rapid glucose flux can lead to limitations in reoxidizing reducing equivalents. The H2 created during this process can be accommodated by methane formation, but the greater flux rates and lower pH with concentrate feeding promote electron disposal by forming propionate and lactate from pyruvate rather than acetate (Russell, 1998). It has been estimated that inhibition of H2 use by methanogenic bacteria by reduced pH itself may account for up to 25% of the change in acetate:propionate (Russell, 1998). The trend noted for butyrate percentage when the diet contains approximately 70% concentrate may be related to the greater numbers of Entodinium (Hungate, 1966), emergence of Megasphaera, and(or) butyrate supply exceeding catabolic capacity by epithelial tissues (Kristensen et al., 2000). Although Butyrivibrio numbers decline as dietary grain increases, strains of Megasphaera evaluated by Marounek et al. (1989) produced predominately butyrate when glucose was supplied and less butyrate relative to propionate when lactate was supplied.
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2 Corresponding author: msbrown{at}mail.wtamu.edu
Received for publication August 12, 2005. Accepted for publication January 9, 2006.
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