Net Absorption and Ruminal Concentrations of Metabolites in nonpregnant Dry Holstein Cows before and after Intraruminal Acetic Acid Infusion1

Net Absorption and Ruminal Concentrations of Metabolites in Nonpregnant Dry Holstein Cows Before and After Intraruminal Acetic Acid Infusion 1 G. B. HUNTINGTON, P. J. REYNOLDS, and H. F. TYRRELL Ruminant Nutrition Laboratory Animal Science Institute Agricultural Research Service US Department of Agriculture Beltsville, MD 20705

not affected by infusion of acetic acid. Net absorptions of ammonia-nitrogen, Llactate, and glucose likewise were not affected. Net absorption of volatile fatty acid and L-lactate was 43% of daily intake of metabolizable energy.

ABSTRACT

Objectives were to define daily patterns of net absorption of various nutrients and to assess effects of intraruminal infusion of acetic acid on concentrations of ruminal fluid and net absorption of various metabolites. These characteristics were measured in three nonpregnant, dry Holstein cows (491 kg) at hourly intervals for 24 h before and after 5 days of intraruminal infusion of acetic acid to provide energy equal to 10% of daily intake of metabolizable energy. Cows were fed a completely mixed, 60% corn silage, 40% grain supplement diet at maintenance intake; daily rations were split into two feedings. Net rates of absorption were greatest after feeding and least during early morning. Net absorption of all metabolites measured was similar for the two daily feeding intervals, indicating daily net absorption could be calculated from either feeding interval. Intraruminal infusion of acetic acid caused increased ruminal and plasma concentrations of acetate, increased net absorption of acetate, and almost a twofold increased loss of urea-nitrogen from plasma to the gut. Sixty-nine percent of acetic acid infused was accounted for by increased net absorption of acetate. Ruminal fluid concentrations and net absorption of other volatile fatty acids were

INTRODUCTION

Received December 9, 1982. 1Mention of a trade name, proprietary product, or specific equipment does not constitute a guarantee or warranty by the US Department of Agriculture and does not imply its approval to the exclusion of other products that may be suitable. 1983 J Dairy Sei 66:1901-1908

1901

Net absorptions of glucose, lactate, or ketone bodies (2, 3, 10, 12, 13, 28), volatile fatty acids (VFA) (3, 5, 11, 15), and nitrogenous compounds (3, 9, 10, 19, 20, 24, 26, 31) have been measured for cattle and sheep. Within constraints of types of diets fed, productive state, and perhaps species, these data provide direct estimates of amounts of metabolites available for maintenance or productive purposes after " c o s t " of the digestive and absorptive process has been deducted. In reports already cited net absorption was measured either for portions of the day or while the animals were fed with timed feeders to distribute feed intake evenly throughout the day. Our first objective was to define daily patterns of net absorption for various metabolites for dairy cows fed maintenance amounts. Some acetate produced by ruminal fermentation is metabolized by gut tissue before entering the blood. Estimates of the proportion of acetate metabolized are 45% in in vitro incubations of rumen epithelium (25) and about 30% in in vivo measurements of sheep (5, 18). lntraruminal infusion of acetic acid at various rates did not affect appreciably apparent digestibility of dry matter (DM) or energy of dairy heifers (23), dairy cows (22), or sheep (1) but did reduce apparent digestibility of gross energy, organic matter, and cellulose for dairy cows fed a 70% hay, 30% concentrate diet (27). In all these infusion studies, animals were

1902

HUNTINGTON ET AL.

fed at or near maintenance. Concentrations of propionate, isobutyrate, and n-valerate, but not n-butyrate, in ruminal fluid were reduced by intraruminal infusion of acetic acid at a rate to provide energy equal to 10% of daily intake of metabolizable energy (ME) (22). Acetate concentration in ruminal fluid tended to be greater with acetic acid infusion. These studies did not include effects of intraruminal infusions on nutrient absorption. Our second objective was to assess effects of intraruminal infusion of acetic acid on ruminal fluid pH, ruminal concentrations of VFA and ammonia-N, and net absorption of various metabolites during 24 h. MATERIALS AND

METHODS

Three nonpregnant dry Holstein cows with permanent ruminal fustulas were used. Cows were about 3 yr old and averaged 491 kg. They were housed indoors in individual stalls and restrained with chains attached to head halters. They were given outdoor exercise daily except in severe weather. They were fed a completely mixed diet of (% DM) 60% corn silage, 40% grain supplement (Table 1) to meet or exceed slightly National Research Council (17) maintenance recommendations (134 kcal ME/kg "Ts daily) in two equal feedings at 0600 to 0700 h and at 1600 to 1700 h. Water was available continuously.

TABLE 1. Diet composition.

Ingredient

% of Dry matter

Corn silage Barley meal Linseed meal Corn gluten meal Dicalcium phosphate Ground limestone Trace mineralized salt Magnesium oxide Sulfur Vitamin A premixa Vitamin D premix a Vitamin E premix a

60.00 20.169 8.000 9.615 1.000 .500 .500 .060 .048 .044 .016 .048

aprovided 4,400, 2,400, and 21 IU/kg of diet of Vitamins A, D, and E, respectively. Journal of Dairy Science Vol. 66, No. 9, 1983

Catheters to blood vessels were installed surgically in the cows and maintained as described by Huntington (10) with the following exceptions: the mesenteric catheter was 1 mm i.d., 1.8 mrn o.d. tygon tubing, the catheter in the portal vein was inserted with a bone chip similar to that described by McGilliard and Thorp (16), and the arterial catheter was inserted into the external circumflex iliac artery. Cows were given at least 2 wk to recover from surgery before experiments began. Cows were given a primed, continuous infusion of para-aminohippuric acid (PAH) for 24 h into the mesenteric vein. Blood samples were collected from catheters in the portal vein and iliac artery at hourly intervals during PAH infusion. Infusion of PAH and procedures for handling blood samples were similar to those described by Huntington (10). Ruminal fluid samples were collected immediately after each collection of blood. Ruminal ingesta was strained through cheesecloth, pH of the fluid was recorded, and 20-ml aliquots were acidified with .05 ml concentrated H2SO4, frozen, and stored. The PAH infusion and sample collection were conducted without intraruminal infusion (control) and after acetic acid (C2) diluted in water was infused intraruminalIy for 6 days by procedures described by Reynolds et al. (22). Infusion of C2 was intended to last 22 h daily to provide time to replenish infusate and exercise the cows; because of irregularities of infusion rate, actual time ranged from 20 to 24 h daily. The amount of C2 infused daily contained energy equal to 10% of each cow's daily ME intake (Table 2). The complete protocol was followed twice with one cow, creating a total of four sets of observations collected over 5mo. Concentrations of ammonia-N in plasma and ruminal fluid and concentrations of urea-N in plasma were determined by automated procedures (Technicon Instruments Corp., Tarrytown, NV); ammonia-N was measured by the hypochlorite method and urea-N by the acetylmonoxime method. Concentrations of VFA were determined by gas-liquid-solid chromatography of ruminal fluid and deproteinized, sublimed plasma (21). Concentration in plasma of glucose (glucose oxidase method) and L-lactate (6) was determined by automated enzymatic procedures. Net absorption of nutrients from

NET METABOLITEABSORPTION IN NONLACTATING COWS

1903

TABLE 2. Live weight, feed intake, and acetic acid infused,a Acetic acid infused Item

No

Live weight, kg Dry matter intake, kg/day Nitrogen intake, g/day Metabolizable energy intake, Meal/day Acetic acid infused, g/day Acetic acid infused, Mcal/day

Yes

493 5.02 139 14.1 0

SE

488 5.02 139 15.5b 404

0

23 .23 6 .7 18

1.4

.1

aFour observations per mean. blncludes energy in acetic acid infused into the rumen.

portal-drained viscera was the mathematical product of portal plasma flow (liters/h) and differences between portal and arterial concentrations in plasma (raM). Main effects for cows and C2 infusion were tested for statistical significance with interaction of cow by C2 infusion as the error term. Effects for the two daily feeding intervals (mroning to evening and evening to morning) were tested against interaction of cow by feeding period in a split-plot model that included cows and infusion as main-plot effects and feeding interval as a split-plot effect. Means for hours within infusion were used to plot diurnal patterns of concentrations, net absorption, and correlations of net absorption with ruminal fluid concentrations.

(P<.05), total VFA (P<.10), and urea-N (P<.10) were greater after morning feeding (0700 to 1700 h) than after evening feeding (1700 to 0600 h). Concentrations in portal plasma of isobutyrate and 3-methylbutyrate (P<.IO) and urea-N (P<.05) also were greater, as were net isobutyrate absorption (P<.10) and

800

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500 0400

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Similar patterns were evident in all variables measured in blood and ruminal fluid. The pattern for portal blood flow (Figure 1) is representative of most variables; there were postfeeding increases in the morning and afternoon and a nadir in the early morning. Patterns for ruminal fluid pH (Figure 2) and net absorption of urea-N (Figure 3) are the inverse of other variables; maximums were evident in the early morning, and minimums were evident postfeeding. Cows consumed feed rapidly. All feed was consumed within 30 min with the exception of one cow during C2 infusion when she immediately ate about 80% of the feed provided, then ate intermittently for several hours. Concentrations in arterial plasma of acetate (P<.05), propionate (P<.10), n-butyrate

1200

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Figure 1. Portal blood flow over 24 h; - - no acetic acid infused, - -- - intraruminal infusion of acetic acid.

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Figure 2. Ruminal fluid pH over 24 h; - - no acetic acid infused, - -- - intramminal infusion of acetic acid. Journal of Dairy Science Vol. 66, No. 9, 1983

1904

HUNTINGTON ET AL.

ACETATE

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Figure 3. Concentrations of urea-nitrogen and acetate in arterial plasma over 24 h; - - no acetic acid infused, ---- intraruminal infusion of acetic acid.

packed cell volume (P<.05). Other than the instances cited, feeding period did not affect concentrations in plasma, differences between portal-arterial concentrations, net absorption, blood flow, or concentrations in ruminal fluid of variables measured. Intraruminal infusion of C2 increased (P<.05) concentrations of acetate and total VFA in arterial plasma, increased differences between portal and arterial plasma concentrations for acetate (P<.05), total VFA (P<.10), and urea-N (P<.10), and decreased differences between portal and arterial concentrations for n-butyrate (P<.10) and ammonia-N (P<.05) (Table 3). Intraruminal C2 infusion increased (P<.10) net absorption of acetate and total VFA and decreased (P<.10) net absorption of urea-N (Table 4). Portal blood flow, concentrations in plasma and net absorption of other nutrients were not affected significantly by C2 infusion, but there were trends toward increased net absorption of propionate, increased (less negative) net absorption of glucose, and decreased net absorption of n-butyrate (Table 4). Energy absorbed as VFA expressed as kcal/ day or as a percentage of ME intake tended to increase in response to intraruminal infusion of C2, but differences were not statistically significant; respective experimental means -+ SE were 5876 + 492 and 40 -+ 3. Energy absorbed as L-lactate was 470 + 17 kcal/day, which was 8% of energy absorbed as VFA or 3.1% of ME intake. Average + SE net absorption of acetate was 803 4-- 76 g/day without C2 infused and 1114 -+ Journal of Dairy Science Vol. 66, No. 9, 1983

167 with C2 infused. The difference was 284 + 96 g/day or 69.8 -+ 21.5% of grams of C2 infused daily. Intraruminal infusion of C2 increased ruminal concentration of acetate (P<.05) and molar percentage of acetate (P<.01) but did not affect ruminal concentrations of ammonia-N, other VFA, or ruminal fluid pH (Table 5). Trends toward increased ruminal concentrations of propionate and decreased ruminal nbutyrate in response to C2 infusion are consistent with trends of net absorption. Ruminal fluid pH tended to decrease in response to C2 infusion, particularly after morning feeding (Figure 2). Some of the samples of ruminal fluid were mishandled inadvertently before VFA determinations, so VFA data in Table 5 are from three cows and include data from one cow that were collected under identical protocol but at a different time from absorption measurements. Intraruminal infusion of C2 caused persistent increases of arterial concentration of acetate in plasma and persistent decreases of ureaurea-N over 24 h (Figure 3)i Identical patterns were evident for concentrations in arterial plasma of total VFA and for concentrations in portal plasma of acetate, urea-N, and total VFA. Ruminal fluid pH was lower 22 of 24 h during infusion of C2 (Figure 2). Similar patterns were not evident for other variables. Correlation of rates of net absorption with concentrations of ruminal fluid was positive (P<.05) for acetate (r=.31), propionate (r=.52), n-butyrate (r=,39), and ammonia-N (r=.82). There was a similar positive correlation for nvalerate (r=.61). Correlations for other individual VFA and total VFA were not significant. Correlations (P<.001) of mole/100 mole of net VFA absorbed with mole/100 mole in ruminal fluid were .73 for acetate, .66 for propionate, and .53 for n-butyrate. DISCUSSION

Similar concentrations of VFA and pH in ruminal fluid and similar rates of net absorption for feeding intervals indicate that total daily net absorption could be obtained from sampling one interval. The intervals were not the same length; more time in the evening-to-morning interval was compensated by m i n i m u m absorption rates during early morning hours, which

NET METABOLITE ABSORPTION IN NONLACTATING COWS

1905

TABLE 3. Concentrations in arterial plasma and differences between portal and arterial concentrations for various metabolites, a Acetic acid infused Item Arterial plasma, mM Acetate b Prop ion ate Isobutyrate n-Butyrate 2-Methylbutyrate 3-Methylbutyrate n-Valer ate Total volatile fatty acids b Urea-N Ammo nia-N Glucose L-Lactate Portal-arterial difference, mM Acetate b Propionate Isobutyrate n-Butyrate c 2oMethylbutyrate 3-Methylbutyrate n-Valerate Total volatile fatty acidsc Urea-Nc Ammonia-Nc Glucose L-Lactate

No

Yes

SE

.070 .002 .0003 .002 .0002 .0OO4 .001

.090 3.57 .42

1.866 .040 .002 .025 .0005 .O010 .005 1.945 5.22 .088 3.66 .47

1.202 ,337 .023 .076 .026 .014 . O11 1.690 -.122 .253 -.08 .12

1.516 .377 .021 .053 .024 .012 .011 2.005 -.194 .231 --.05 .12

.069

1.435 .034 .002 .027 .0007 .0017 .004 1.504 6.27

.071

.35 .007 .14 .04

.029 .001 .006 .002 .001 .001 .084 .017 .006 .01 .01

aFour observations per mean. blnfusion means are different (P<.05). Clnfusion means are different (P<.10).

m a y reflect a circadian r h y t h m o f f e r m e n t a t i v e and absorptive activity or time elapsed since last feeding. Net rates of V F A absorption (Table 4) are similar to rates in dry cows r e p o r t e d by (3, 10) when differences in ME intake are considered. Lactating cows (3) ate 2.7 times as m u c h ME and absorbed V F A 3.8 times as fast as our cows w i t h o u t C2 infusion. Net V F A absorption as a percentage o f ME intake is within the range of percentages based on disappearance of V F A f r o m the r u m e n (29) and similar to percentages based on net absorption of V F A in dry dairy cows (3, 10). Data o f Baird et al. (3) and Wiltr o u t and Satter (30) suggest that at high feed intakes, particularly for lactating cows, n e t absorption of V F A as a percentage of ME in-

take is higher than the 40% in our e x p e r i m e n t . Bergman and Wolff (5) and Pethick et al. (18) f o u n d a b o u t 30% of acetate absorbed by sheep was m e t a b o l i z e d by gut tissue; our data indicate that 30.2% of infused acetic acid was n o t a c c o u n t e d for by net absorption of acetate and was m e t a b o l i z e d ostensibly in the r u m e n or in gut tissue u p o n absorption. Higher correlations between c o n c e n t r a t i o n s in ruminaI fluid and net absorption for p r o p i o n a t e and a m m o nia-N than for acetate and n-butyrate m a y reflect m o r e active m e t a b o l i s m of acetate and n-butyrate by gut tissue. Bergman and Wolff (5) calculated 50% o f p r o p i o n a t e and 85% o f buryrate p r o d u c e d in the r u m e n of sheep did n o t reach portal blood. Consistent elevation in arterial plasma o f Journal of Dairy Science Vol. 66, No. 9, 1983

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HUNTINGTON ET AL.

TABLE 4. Portal blood flow and net absorption of metabolites, a Acetic acid infused Item

No

Yes

SE

Portal blood flow, liters/h

632

652

24

Net absorption, m m o l / h Acetate c Propionate Isobutyrate n-Butyrate 2-Methylbutyrate 3-Methylbutyrate n-Valerate Total volatile fatty acids c Urea-N c Ammonia-N Glucose L-Lactate

577 163 11.2 37~4 12.8 6.9 5.3 813 - 56 121 -- 38 58

774 192 10. 7 28.3 12.0 6.1 5.5 1034 - 100 118 -- 26 62

47 11 .8 4.3 1.7 .9 .7 60 12 2 8 2

aFour observations per mean. bpacked cell volume was 23.8 and 23.1 for no acetic acid and acetic acid infused, respectively. Clnfusion means are different (P<.IO).

TABLE 5. Concentrations of ammonia-nitrogen and volatile fatty acids (VFA) and pH of ruminal fluid. Acetic acid infused Item

No

Yes

pH a Ammonia-N, mM a

6.74 4. 5

6.47 4.5

.10 .4

VFA, mM b Acetate c Propionate Isobutyrate n-Butyrate 2-Methylbutyrate 3-Methylbutyrate n-Valerate Total VFA c

47.7 11.6 .849.1 1.60 .77 .85 72.4

59.5 13.0 .68 6.5 .78 .59 .86 81.9

1.2 1.0 .06 .9 .40 .08 .04 1.1

VFA, mole/100 mole b Acetate d Propionate n-Butyrate

66.0 15.9 12.6

72.6 15.9 7.9

.4 .4 1.2

aData collected at same times as absorption data; four sets of observations. bData for cow C collected at different times from absorption data; three sets of observations. Clnfusion means are different (P<.05). d • Infusion means are different (P<.01). Journal of Dairy Science Vol. 66, No. 9, 1983

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NET METABOLITE ABSORPTION IN NONLACTATINGCOWS

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Figure 4. Net absorption of urea-nitrogen over 24 h; - - no acetic acid infused, ---- intraruminal infusion of acetic acid.

concentrations of acetate (Figure 4) indicates increased net absorption of acetate was superimposed on daily pattern of concentration changes. Rates of acetate turnover probably increased (18), and different rates of acetate turnover were established during 6 days of acetic acid infusion. Net absorption rates of ammonia-N and urea-N and percentage of N intake accounted for by net absorption of ammonia-N (29%) are similar to those from dry cows fed orchardgrass-clover silage (10). Transfer of urea-N from plasma to the gut was 19 g/day or 13.5% of N intake in our cows without infusion of C2; similar rates of urea-N transfer to rumens of steers have been reported (14, 32). We have no clear explanation for the increased transfer of urea-N from plasma to the gut associated with C2 infusion (Table 4). Houpt (8) found that increased osmolarity of rinsed rumen pouches caused more urea-N to transfer to the interior of the pouches. Reynolds et al. (22) found osmolarity of ruminal fluid tended to increase for. cows given intraruminal infusions of C2. Concentrations of urea-N in plasma were consistently lower but retained the daily pattern of concentration changes (Figure 3). Concentrations of ammonia-N in plasma (Table 3) and ruminal fluid (Table 5) were not affected by infusion of C2, which suggests that ammonia-N from urea was converted in the rumen to other nitrogenous compounds, ostensibly microbial protein. These data and trends toward less excretion of urinary N and increased N retention in a study similar to ours (27) suggest that increased transfer of urea-N to the gut may in-

1907

crease retention of N and improve use of dietary N. Rates of net absorption of glucose (Table 4) are similar to rates for dry cows fed orchardgrass-clover silage (10). Glucose metabolized by the gut (negative net absorption) is 17.8% of glucose turnover (214 mmol/h) calculated by the equation of Herbein et al. (7). With no true absorption of glucose from the gut, 17.8% of glucose-dependent metabolism was in the gut; if some glucose was absorbed from the gut, then this would be an underestimate. Bergman (4) reported similar percentages for sheep. Reynolds et al. (22) infused C2 under a protocol similar to ours and found the effect of C2 infusion superimposed on daily patterns of concentrations of acetate in ruminal fluid, which is the same effect we observed. Concentration of acetate in ruminal fluid increased 1.2 times in their experiment (22) and 1.25 times in ours. However, they found concentrations of propionate, isobutyrate, isovalerate, and n-valerate decreased in response to intraruminal infusion of C2, but ours did not (Table 5). Reasons for differences of response may include diet composition; they (22) fed hay as the forage component in forage:concentrate diets 80:20, 50: 50, and 20:80 compared to 60:40 corn silage: grain in our experiments. ACKNOWLEDGMENTS

The authors thank B. Stroud and his staff for help with surgery; D. Deluca, R. Spencer, P. Wilby, and F. Sweeney for care and feeding of the cows; J. Whitt, P. Kellogg, and E. Zetina for their able laboratory work; and P. Marcus, M. Ratte, and C. Graves for their work on preparation of the manuscript. REFERENCES

1 Armstrong, D. G., and K. L. Blaxter. 1957. The utilization of acetic, propionic and butyric acids by fattening sheep. Br. J. Nutr. 11:413. 2 Baird, G. D., M. A. Lomax, H. W. Symonds, and S. R. Shaw. 1980. Net hepatic and splanchnic metabolism of lactate, pyruvate and propionate in dairy cows in vivo in relation to lactation and nutrient supply. Biochem. J. 186:47. 3 Baird, G. D., H. W. Syrnonds, and R. Ash. 1975. Some observations on rnetabolite production and utilization in vivo by the gut and liver of adult dairy cow~ J. Agric. Sci. 85:281. 4 Bergman, E. N. 1975. Production and utilization of rnetabolites by the alimentary tract. Page 292 in Journal of Dairy Science Vol. 66, No. 9, 1983

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9 10

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15

16 17

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Digestion and metabolism in the ruminant. I. W. McDonald and A.C.I. Warner, ed. Univ. New England Publ. Unit, Armidale, New South Wales, Australia. Bergman, E. N., and J. E. Wolff. 1971. Metabolism of volatile fatty acids by liver and portal-drained viscera in sheep. Am. J. Physiol. 221:586. Goodall, S. R., and F. M. Byers. 1978. Automated micro method for enzymatic L(+) and D(--) lactic acid determinations in biological fluids containing cellular extracts. Anal. Biochem. 89: 80. Herbein, J. H., R. W. Van Maanen, A. D. McGilliard, and J. W. Young. 1978. Rumen propionate and blood glucose kinetics in growing cattle fed isoenergetic diet~ J. Nutr. 108:994. Houpt, T. R. 1970. Transfer of urea and ammonia to the tureen. Page 129 in Physiology of digestion and metabolism in the ruminant. A. T. Phillipson, ed. Oriel Press, Newcastle-upon-Tyne, England. Hume, J. D., D. R. Jacobson, and G. E. Mitchell, Jr. 1972. Quantitative studies on amino acid absorption in sheep. J. Nutr. 102:495. Huntington, G. B. 1982. Portal blood flow and net portal absorption of ammonia-nitrogen, ureanitrogen and glucose in nonlactating Holstein cows. J. Dairy Sci. 65:1155. Huntington, G. B., and P. J. Reynolds. 1982. Net volatile fatty acid absorption in nonlactating Holstein cows. J. Dairy Sci. 66:86. Huntington, G. B., R. L. Prior, and R. A. Britton. 1981. Glucose and lactate absorption and metabolic interrelationships in steers changed from low to high concentrate diets. J. Nutr. 111:1164. Katz, M. L., and E. N. Bergman. 1969. Hepatic and portal metabolism of glucose, free fatty acids and ketone bodies in the sheep. Am. J. Physiol. 216: 946. Kennedy, P. M., and L. P. Milligan. 1980. The degradation and utilization of endogenous urea in the gastrointestinal tract of ruminant~ A review. Can. J. Anita. Sci. 60:205. Kurilov, N. V., S. Ya. Shchegolev, and V. N. Korshunov. 1979. Formation and assimilation of VFA with different ratios of coarse and concentrated feeds in the ration fed male calves. Soviet Agric. Sci. 11:53. McGilliard, A. D., and J. W. Thorp. 1971. Catheterization of the mesenteric and portal vein in calves. J. Dairy Sci. 54:129. National Research Council. 1978. Nutrient requirements of domestic animals. No. 3. Page 3 in Nutrient requirements of dairy cattle. 5th rev. ed. Natl. Acad. Sei., Washington, DC. Pethick, D. W., D. B. Lind~y, P. J. Barker, and A. J. Northrop. 1981. Acetate supply and utilization

Journal of Dairy Science Vol. 66, No. 9, 1983

by the tissues of sheep in vivo. Br. J. Nutr. 46:97. 19 Prewitt, L. R., D. R. J~cobson, R. W. Hemken, G. D. Schelling, and R. H. Hatton. 1975. Amino acid absorption by portal-jugular venous differences in sheep fed two maturities of alfalfa hay. J. Anita. Sci. 41:1722. 20 Prior, R. L., G. B. Huntington, and R. A. Britton. 1981. Influence of diet on amino acid absorption in beef cattle and sheep. J. Nutr. 111:2212. 21 Reynolds, P. J., and G. B. Huntington. 1981. Determination of volatile fatty acids in blood plasma by vacuum sublimation and gas-liquid-solid chromatography. J. Anita. Sci. 53(Suppl. 1):425. (Abstr.) 22 Reynolds, P. J., H. F. Tyrrell, and P. W. Moe. 1979. Effect of ruminal infusion of acetic acid onto three hay to concentrate ratios on apparent digestibilities and rumen parameters in cattle. J. Anim. Sci. 48:1491. 23 Rook, J.A.F., C. C. Balch, R. C. Campling, and L. J. Fischer. 1965. The utilization of acetic, propionic and butyric acids by growing heifers. Br. J. Nutr. 17:399. 24 Sniffen, C. J., and D. R. Jacobson. 1975. Net amino acid absorption in steers fed alfalfa hay cut at two stages of maturity. J. Dairy Sci. 58:371. 25 Stevens, C. E. 1970. Fatty acid transport through the rumen epithelium. Page 101 in Physiology of digestion and metabolism in the ruminant. A. T. Phillipson, ed. Oriel Press, Newcastle-upon-Tyne, England. 26 Tagari, H., and E. N. Bergman. 1978. Intestinal disappearance and portal blood appearance of amino acids in sheep. J. Nutr. 108:790. 27 Tyrrell, H. F., P. J. Reynolds, and P. W. Moe. 1979. Effect of diet on partial efficiency of acetate use for body tissue synthesis in mature cattle. J. Anim. Sci. 48:598. 28 Weekes, T.E.C., and A.J.F. Webster. 1975. Metabolism of propionate in the tissues of the sheep gut. Br. J. Nutr. 33:345. 29 Whitelaw, F. G., J. Hyldegaard-Jenscn, R. S. Reid, and M. G. Kay. 1970. Volatile fatty acid production in the rumen of cattle given an all-concentrate diet. Br. J. Nutr. 24:179. 30 Wiltrout, D. W., and L. D. Satter. 1972. Contribution of propionate to glucose synthesis in the lactating and nonlactating cow. J. Dairy Sci. 55:307. 31 Wolff, J. E., E. N. Bergrnan, and H. H. Williams. 1972. Net metabolism of plasma amino acids by liver and portal-drained viscera of fed sheep. Am. J. Physiol. 223:438. 32 Vercoe, J. E. 1969. The transfer of nitrogen from the blood to the rumen in cattle. Australian J. Agric. Res` 20:191.