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Effect of Heated or Unheated Soybean Meal With or Without Niacin on Rumen Protozoa1,2
Effect of Heated or Unheated Soybean Meal With or Without Niacin on Rumen Protozoa 1,2 S. M. DENNIS, M. J. ARAMBEL, E. E. B A R T L E Y , D. O. RIDDELL, and A. D. D A Y T O N Kansas State University Manhattan 66506
ciliate protozoa differ from bacteria in that they cannot synthesize niacin (6, 8, 9) and, therefore, must obtain their niacin from bacteria or feed, we conjectured that heat treating soybean meal might reduce availability of either niacin or tryptophan for bacteria, thus reducing the supply of niacin to protozoa. To test that, we compared the effect of heated or unheated soybean meal, with or without niacin, on protozoal numbers in the rumen.
ABSTRACT
Cattle fed heated soybean meal as a protein supplement had lower rumen protozoal counts than did those fed unheated soybean meal. Adding niacin to unheated soybean meal increased protozoal counts, but the difference was not significant. Adding niacin to heated soybean meal increased holotrich, entodiniomorph, and total protozoal counts. Apparently conventional heating of soybean meal makes niacin unavailable to rumen protozoa.
M A T E R I A L S A N D METHODS
INTRODUCTION
In (12), the effect of nicotinic acid (niacin) was tested in vitro. Synthesis of microbial protein was greater with niacin and soybean meal (SBM) as a nitrogen supplement than with niacin and urea. In a lactation study with fresh cows (13), niacin supplementation increased milk production of cows receiving SBM as a protein supplement but not of those receiving urea. Kung et al. (10) observed a lactation response from niacin of cows in early lactation fed SBM but not of those fed nonprotein nitrogen. Because niacin is involved intimately in energy metabolism, it had been expected to play a significant role in the use of urea by rumen microorganisms. The response to niacin, however, proved to be greater with SBM than with urea. Niacin can be synthesized from tryptophan by bacteria (5). Because rumen
Received December 7, 1981. Contribution 82-173-j, Ruminant Nutrition Group, Department of Animal Sciences and Industry, Kansas Agricultural Experiment Station, Manhattan. 2Supported in part by a grant from Lonza, Inc., Fairlawn, NJ. 1982 J Dairy Sci 65:1643-1646
Four rumen-fistulated Holstein cattle (average body weight 360 kg) were assigned to each of four diets in a 2 x 2 Latin square. Diets contained unheated SBM, unheated SBM plus niacin, heated SBM, and heated SBM plus niacin. Unheated SBM was solvent extracted without heat treatment. Heated SBM, extracted similarly, had been toasted at 90°C for 10 min and was similar to commercial-grade soybean meal. Composition of the concentrate portion of the diet is in Table 1. Cattle were fed daily 1.6 kg of pelleted cdhcentrate and 1.3 kg of wheat straw at 0700, 1500, and 2300 h. Two grams of supplemental niacin was placed in the rumen via rumen fistula at each feeding to achieve an intake of 6 g of niacin per day. Cattle were kept in individual stalls and had free access to water. Following 14-day adaptation, rumen contents were hand-mixed and sampled once prior to and at 2, 4, and 6 h after feeding (0700 h). Samples immediately were strained through four layers of cheesecloth. One milliliter of stained rumen fluid was diluted to 25 ml with glycerol-phosphate buffer (30:70 vol/vol, pH 6.8). One milliliter of the suspension was placed in a counting chamber (11), and protozoa were allowed to settle. Protozoa were counted and identified as described by Dehority (3). All data were analyzed by a two-way analysis of variance and compared by least square means of treatments for significance.
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DENNIS ET AL.
1644
TABLE 1. Chemical composition of concentrate portion of the soybean meal diets. Ingredient
(%)
Corn starch a Dextroseb Soybean mealc Fat Mineral premixd Potassium chloride Choline chloride (50%) Vitamins A, D, Ee
28.16 28.16 34.68 3.00 5.00 .50 .50 +
aBuffalo Starch 3401, Corn Products Co., Englewood Cliffs, NJ. bcerelose Dextrose 2001, Corn Products Co., Englewood Cliffs, NJ. CDiets were formulated to contain one of two sources of soybean meal (SBM). Unheated SBM was solvent extracted without heat treatment; heated SBM, extracted similarly, bad also been toasted similar to commercial-grade SBM. dcontributed by the following in grams/kilogram of diet: Mn, 5.0; Fe, 5.0; Zn, 5.0; Ca, 4.45; Cu, .5; I, .15; Co, .1. eThe diet contained: 8,800 USP units vitamin A as retinyl A palmitate; 1,100 USP units vitamin D as Dactivated plant sterol; and 21 IU vitamin E as d-~tocopheryl acetate/kg of diet.
R ESU LTS
Cattle fed heated SBM had lower (P<.01) protozoal counts (4.59 × 104 protozoa/ml, Table 2) than did those fed unheated SBM (5.96). Adding niacin to unheated SBM increased total protozoal aria entodiniomorph counts, but differences were not significant (P>.05). Adding niacin to heated SBM increased holotrich (P<.05), entodiniomorph (P<.01), and total protozoal counts (P<.01). Supplemental niacin reversed the lower counts resulting from heating. Holotrich counts were greatest 2 h after feeding and then decreased steadily. In cattle fed unheated SBM with niacin and heated SBM without niacin, entodiniomorph numbers were generally highest before feeding, decreased after feeding, probably from dilution, and then increased with time. Such variation in numbers with this type of ration has been observed (4). Numbers of Isotricba, Opbryoscolex, Diplodinium, and Entodiniurn were lower with Journal of Dairy Science Vol. 65, No. 8, 1982
heated than with unheated SBM (Table 3).
Dasytricha was not affected. Niacin added to unheated SBM increased numbers of Charon and Diplodinium. Niacin added to heated SBM increased numbers of Dasytricha, Isotricha, Diplodinium, and Entodinium. DISCUSSION
Riddell et al. (13) and Kung et al. (10) observed that milk production was increased when cows were fed commercial SBM supplemented with niacin. Also, Riddell et al. (12) observed that synthesis of microbial protein in tureen was enhanced both in vivo and in vitro when niacin was added to the diet; they did not separate protozoa from bacteria, however, so it is possible that the increase in synthesis of microbial protein from niacin resulted chiefly from protozoa, because both ammonia and propionic acid concentrations in tureen increased with niacin supplementation. Gains in protozoal numbers increase ammonia and propionic acid concentrations in rumen (1, 2, 7). The decrease in protozoal numbers with heated (commercial) SBM compared with unheated SBM and the increase in protozoal numbers when the heated SBM was supplemented with niacin suggest that conventional heating of SBM makes niacin unavailable to protozoa. Because protozoa are unable to synthesize niacin (6, 8, 9), they must obtain their niacin from feed or from tureen bacteria. Niacin can be synthesized from tryptophan by bacteria (5), but heating SBM may reduce bacterial synthesis of niacin. Yen et al. (15) reported niacin in solvent-extracted SBM (unheated) was 100% available; however, 75% of the available niacin resulted from conversion of tryptophan. If heating SBM reduced availability of tryptophan, then niacin synthesis by rumen bacteria might be reduced. Heating SBM decreases protein availability in the tureen and increases protein bypass (14). Possibly niacin from SBM would be unavailable to protozoa in the same way. The protozoa most affected by heat-treated SBM and niacin supplementation are those that rely on engulfed feed particles (6). Protozoa may provide a model for determining rumen availability of certain nutrients. Hungate (1966) stated " . . . there is probably no test disclosing the approximate nutritional health of a ruminant as easily
TECHNICAL NOTE
1645
TABLE 2. Effect of u nheated or heated soybean meal (SBM) with or w i t h o u t niacin on rumen protozoal counts obtained b etween 0 and 6 h after feeding.
Sampling time
Diet
Entodiniomorphs
Holotrichs
(h)
Total
(× 10S/ml)
Unheated SBM w i t h o u t niacin
0 2 4 6
28.9 39.9 25.9 20.9 28.9 ab
523.8 564.3 565.6 619.3 568.3 c
552.7 604.2 591.5 640.3 597.1 c
Unheated SBM with niacin
0 2 4 6
32.5 40.0 26.1 18.4 29.0 ab
700.8 543.7 581.6 595.7 605.4 c
733.2 582.6 607.7 614.1 634.4 c
Heated SBM w i t h o u t niacin
0 2 4 6
24.9 29.0 19.6 14.0 21.9 a
520.3 396.8 434.7 394.8 436.7 d
545.3 425.8 454.3 408.8 458.6 d
Heated SBM with niacin
0 2 4 6
32.1 50.3 28.1 20.2 32.6 b
684.8 714.9 708.8 672.7 695.3 e
716.9 765.0 736.8 693.0 727.9 e
ab
.
' Means m the same column with different superscripts differ (P<.05).
c'd'eMeans in the same column with different superscripts differ (P<.01).
TABLE 3. Effect of u n h e a t e d or heated soybean meal (SBM) with and w i t h o u t niacin on rumen p r o t o z o a numbers, a Unheated SBM Protozoa
Without niacin
Heated 6BM With niacin
Wi t hout niacin
With niacin
15.3 b 4.0 c 2.5 c 420.4 e 15.5 bf .8 c
22.4 c 6.7 b 3.5 c 671.1 f 23.1cg 1.0 c
(× 103/ml) Dasytricha Isotricha Charon Entodinium Diplodinium Ophryoscolex
• 19.3 bc 6.8 b 1.4 b 526.4 d 39.7 d 2.2 b
20.9 bc 6.7 b 2.9 c 546.7 d 56.2 e 2.5 b
aEach mean is of rumen samples collected at 0, 2, 4, and 6 h. b'CMeans in the same c o l u m n with different superscripts differ (P<.05). d'e'f'gMeans in the same c o l u m n with different superscripts differ (P<.01). Journal of Dairy Science Vol. 65, No. 8, 1982
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DENNIS ET AL.
a n d q u i c k l y as d o e s t h e m i c r o s c o p i c e x a m i n a tions for the protozoa." 8 REFERENCES 1 Abou Akkada, A. R., and K. el-Shazyly. 1964. Effect of absence of ciliate protozoa from the r u m e n on microbial activity and growth of lambs. Appl. Microbiol. 12: 384. 2 Christiansen, W. C., R. Kawashima, and W. Burroughs. 1965. Influence of protozoa u p o n r u m e n acid production and live weight gains in lambs. J. A n i m . Sci. 24:730. 3 Dehority, B. A. 1974. R u m e n ciliate fauna of Alaskan m o o s e (Alces Americana), musk-ox (Ovibos muscbatus) and Dall m o u n t a i n sheep (Ovis dallis). J. Protozool. 21:26. 4 Dennis, S. M., M. J. Arambel, T. G. Nagaraja, E. E. Bartley, and A. D. Dayton. 1982. Effect of protein-free diets on r u m e n protozoa. J. Anim. Sci. (Submitted). 5 Giesecke, D., and H. K. Hendrickx. 1973. Biologic u n d Biochemie der mikrobielle V e r d a u u n g BLV. Verlugsgesellschaft 240. 6 Hungate, R. E. 1966. The tureen and its microbes. Academic Press Inc., New York, NY. 7 Itabashi, H., and M. Kandatsu. 1975. Influence of r u m e n ciliate protozoa on the concentration of
Journal o f Dairy Science Vol. 65, No. 8, 1982
9 10
11
12
i3
14
15
a m m o n i a and volatile fatty acid in connection with the utilization of a m m o n i a in the rumen. Jap. J. Zootech. Sci. 46:409. Jones, A. R. 1974. The ciliates. Hutchinson & Co. Ltd., London. Kidder, G. W. 1967. Chemical zoology I. Protozoa. Academic Press Inc., New York, NY. Kung, L., K. Gubert, and J. T. Huber. 1980. Supplemental niacin for lactating cows fed diets of natural protein or nonprotein nitrogen. J. Dairy Sci. 63:2020. Mishra, B. 1964. Relation of r u m e n mucinolytic bacteria and r u m e n protozoa to bloat in cattle. Ph.D. Dissertation, Kansas State University, Manhattan. Riddell, D. O., E. E. Bartley, and A. D. Dayton. 1980. Effect of nicotinic acid on r u m e n fermentation in vitro and in vivo. J. Dairy Sci. 63:1429. Riddell, D. O., E. E. Bartley, and A. D. Dayton. 1981. Effect of nicotinic acid on microbial protein synthesis in vitro and on dairy cattle growth and milk production. J. Dairy Sci. 64: 782. Tagari, H., I. Ascarelli, and A. Bondi. 1962. The influence of heating on the nutritive value of soybean meal for ruminants. Br. J. Nutr. 16:237. Yen, J. T., A. H. Jensen, and D. H. Baker. 1977. Assessment of the availability of niacin in corn, soybeans and s o y b e a n meal. J. Anim. Sci. 45: 269.
ciliate protozoa differ from bacteria in that they cannot synthesize niacin (6, 8, 9) and, therefore, must obtain their niacin from bacteria or feed, we conjectured that heat treating soybean meal might reduce availability of either niacin or tryptophan for bacteria, thus reducing the supply of niacin to protozoa. To test that, we compared the effect of heated or unheated soybean meal, with or without niacin, on protozoal numbers in the rumen.
ABSTRACT
Cattle fed heated soybean meal as a protein supplement had lower rumen protozoal counts than did those fed unheated soybean meal. Adding niacin to unheated soybean meal increased protozoal counts, but the difference was not significant. Adding niacin to heated soybean meal increased holotrich, entodiniomorph, and total protozoal counts. Apparently conventional heating of soybean meal makes niacin unavailable to rumen protozoa.
M A T E R I A L S A N D METHODS
INTRODUCTION
In (12), the effect of nicotinic acid (niacin) was tested in vitro. Synthesis of microbial protein was greater with niacin and soybean meal (SBM) as a nitrogen supplement than with niacin and urea. In a lactation study with fresh cows (13), niacin supplementation increased milk production of cows receiving SBM as a protein supplement but not of those receiving urea. Kung et al. (10) observed a lactation response from niacin of cows in early lactation fed SBM but not of those fed nonprotein nitrogen. Because niacin is involved intimately in energy metabolism, it had been expected to play a significant role in the use of urea by rumen microorganisms. The response to niacin, however, proved to be greater with SBM than with urea. Niacin can be synthesized from tryptophan by bacteria (5). Because rumen
Received December 7, 1981. Contribution 82-173-j, Ruminant Nutrition Group, Department of Animal Sciences and Industry, Kansas Agricultural Experiment Station, Manhattan. 2Supported in part by a grant from Lonza, Inc., Fairlawn, NJ. 1982 J Dairy Sci 65:1643-1646
Four rumen-fistulated Holstein cattle (average body weight 360 kg) were assigned to each of four diets in a 2 x 2 Latin square. Diets contained unheated SBM, unheated SBM plus niacin, heated SBM, and heated SBM plus niacin. Unheated SBM was solvent extracted without heat treatment. Heated SBM, extracted similarly, had been toasted at 90°C for 10 min and was similar to commercial-grade soybean meal. Composition of the concentrate portion of the diet is in Table 1. Cattle were fed daily 1.6 kg of pelleted cdhcentrate and 1.3 kg of wheat straw at 0700, 1500, and 2300 h. Two grams of supplemental niacin was placed in the rumen via rumen fistula at each feeding to achieve an intake of 6 g of niacin per day. Cattle were kept in individual stalls and had free access to water. Following 14-day adaptation, rumen contents were hand-mixed and sampled once prior to and at 2, 4, and 6 h after feeding (0700 h). Samples immediately were strained through four layers of cheesecloth. One milliliter of stained rumen fluid was diluted to 25 ml with glycerol-phosphate buffer (30:70 vol/vol, pH 6.8). One milliliter of the suspension was placed in a counting chamber (11), and protozoa were allowed to settle. Protozoa were counted and identified as described by Dehority (3). All data were analyzed by a two-way analysis of variance and compared by least square means of treatments for significance.
1643
DENNIS ET AL.
1644
TABLE 1. Chemical composition of concentrate portion of the soybean meal diets. Ingredient
(%)
Corn starch a Dextroseb Soybean mealc Fat Mineral premixd Potassium chloride Choline chloride (50%) Vitamins A, D, Ee
28.16 28.16 34.68 3.00 5.00 .50 .50 +
aBuffalo Starch 3401, Corn Products Co., Englewood Cliffs, NJ. bcerelose Dextrose 2001, Corn Products Co., Englewood Cliffs, NJ. CDiets were formulated to contain one of two sources of soybean meal (SBM). Unheated SBM was solvent extracted without heat treatment; heated SBM, extracted similarly, bad also been toasted similar to commercial-grade SBM. dcontributed by the following in grams/kilogram of diet: Mn, 5.0; Fe, 5.0; Zn, 5.0; Ca, 4.45; Cu, .5; I, .15; Co, .1. eThe diet contained: 8,800 USP units vitamin A as retinyl A palmitate; 1,100 USP units vitamin D as Dactivated plant sterol; and 21 IU vitamin E as d-~tocopheryl acetate/kg of diet.
R ESU LTS
Cattle fed heated SBM had lower (P<.01) protozoal counts (4.59 × 104 protozoa/ml, Table 2) than did those fed unheated SBM (5.96). Adding niacin to unheated SBM increased total protozoal aria entodiniomorph counts, but differences were not significant (P>.05). Adding niacin to heated SBM increased holotrich (P<.05), entodiniomorph (P<.01), and total protozoal counts (P<.01). Supplemental niacin reversed the lower counts resulting from heating. Holotrich counts were greatest 2 h after feeding and then decreased steadily. In cattle fed unheated SBM with niacin and heated SBM without niacin, entodiniomorph numbers were generally highest before feeding, decreased after feeding, probably from dilution, and then increased with time. Such variation in numbers with this type of ration has been observed (4). Numbers of Isotricba, Opbryoscolex, Diplodinium, and Entodiniurn were lower with Journal of Dairy Science Vol. 65, No. 8, 1982
heated than with unheated SBM (Table 3).
Dasytricha was not affected. Niacin added to unheated SBM increased numbers of Charon and Diplodinium. Niacin added to heated SBM increased numbers of Dasytricha, Isotricha, Diplodinium, and Entodinium. DISCUSSION
Riddell et al. (13) and Kung et al. (10) observed that milk production was increased when cows were fed commercial SBM supplemented with niacin. Also, Riddell et al. (12) observed that synthesis of microbial protein in tureen was enhanced both in vivo and in vitro when niacin was added to the diet; they did not separate protozoa from bacteria, however, so it is possible that the increase in synthesis of microbial protein from niacin resulted chiefly from protozoa, because both ammonia and propionic acid concentrations in tureen increased with niacin supplementation. Gains in protozoal numbers increase ammonia and propionic acid concentrations in rumen (1, 2, 7). The decrease in protozoal numbers with heated (commercial) SBM compared with unheated SBM and the increase in protozoal numbers when the heated SBM was supplemented with niacin suggest that conventional heating of SBM makes niacin unavailable to protozoa. Because protozoa are unable to synthesize niacin (6, 8, 9), they must obtain their niacin from feed or from tureen bacteria. Niacin can be synthesized from tryptophan by bacteria (5), but heating SBM may reduce bacterial synthesis of niacin. Yen et al. (15) reported niacin in solvent-extracted SBM (unheated) was 100% available; however, 75% of the available niacin resulted from conversion of tryptophan. If heating SBM reduced availability of tryptophan, then niacin synthesis by rumen bacteria might be reduced. Heating SBM decreases protein availability in the tureen and increases protein bypass (14). Possibly niacin from SBM would be unavailable to protozoa in the same way. The protozoa most affected by heat-treated SBM and niacin supplementation are those that rely on engulfed feed particles (6). Protozoa may provide a model for determining rumen availability of certain nutrients. Hungate (1966) stated " . . . there is probably no test disclosing the approximate nutritional health of a ruminant as easily
TECHNICAL NOTE
1645
TABLE 2. Effect of u nheated or heated soybean meal (SBM) with or w i t h o u t niacin on rumen protozoal counts obtained b etween 0 and 6 h after feeding.
Sampling time
Diet
Entodiniomorphs
Holotrichs
(h)
Total
(× 10S/ml)
Unheated SBM w i t h o u t niacin
0 2 4 6
28.9 39.9 25.9 20.9 28.9 ab
523.8 564.3 565.6 619.3 568.3 c
552.7 604.2 591.5 640.3 597.1 c
Unheated SBM with niacin
0 2 4 6
32.5 40.0 26.1 18.4 29.0 ab
700.8 543.7 581.6 595.7 605.4 c
733.2 582.6 607.7 614.1 634.4 c
Heated SBM w i t h o u t niacin
0 2 4 6
24.9 29.0 19.6 14.0 21.9 a
520.3 396.8 434.7 394.8 436.7 d
545.3 425.8 454.3 408.8 458.6 d
Heated SBM with niacin
0 2 4 6
32.1 50.3 28.1 20.2 32.6 b
684.8 714.9 708.8 672.7 695.3 e
716.9 765.0 736.8 693.0 727.9 e
ab
.
' Means m the same column with different superscripts differ (P<.05).
c'd'eMeans in the same column with different superscripts differ (P<.01).
TABLE 3. Effect of u n h e a t e d or heated soybean meal (SBM) with and w i t h o u t niacin on rumen p r o t o z o a numbers, a Unheated SBM Protozoa
Without niacin
Heated 6BM With niacin
Wi t hout niacin
With niacin
15.3 b 4.0 c 2.5 c 420.4 e 15.5 bf .8 c
22.4 c 6.7 b 3.5 c 671.1 f 23.1cg 1.0 c
(× 103/ml) Dasytricha Isotricha Charon Entodinium Diplodinium Ophryoscolex
• 19.3 bc 6.8 b 1.4 b 526.4 d 39.7 d 2.2 b
20.9 bc 6.7 b 2.9 c 546.7 d 56.2 e 2.5 b
aEach mean is of rumen samples collected at 0, 2, 4, and 6 h. b'CMeans in the same c o l u m n with different superscripts differ (P<.05). d'e'f'gMeans in the same c o l u m n with different superscripts differ (P<.01). Journal of Dairy Science Vol. 65, No. 8, 1982
1646
DENNIS ET AL.
a n d q u i c k l y as d o e s t h e m i c r o s c o p i c e x a m i n a tions for the protozoa." 8 REFERENCES 1 Abou Akkada, A. R., and K. el-Shazyly. 1964. Effect of absence of ciliate protozoa from the r u m e n on microbial activity and growth of lambs. Appl. Microbiol. 12: 384. 2 Christiansen, W. C., R. Kawashima, and W. Burroughs. 1965. Influence of protozoa u p o n r u m e n acid production and live weight gains in lambs. J. A n i m . Sci. 24:730. 3 Dehority, B. A. 1974. R u m e n ciliate fauna of Alaskan m o o s e (Alces Americana), musk-ox (Ovibos muscbatus) and Dall m o u n t a i n sheep (Ovis dallis). J. Protozool. 21:26. 4 Dennis, S. M., M. J. Arambel, T. G. Nagaraja, E. E. Bartley, and A. D. Dayton. 1982. Effect of protein-free diets on r u m e n protozoa. J. Anim. Sci. (Submitted). 5 Giesecke, D., and H. K. Hendrickx. 1973. Biologic u n d Biochemie der mikrobielle V e r d a u u n g BLV. Verlugsgesellschaft 240. 6 Hungate, R. E. 1966. The tureen and its microbes. Academic Press Inc., New York, NY. 7 Itabashi, H., and M. Kandatsu. 1975. Influence of r u m e n ciliate protozoa on the concentration of
Journal o f Dairy Science Vol. 65, No. 8, 1982
9 10
11
12
i3
14
15
a m m o n i a and volatile fatty acid in connection with the utilization of a m m o n i a in the rumen. Jap. J. Zootech. Sci. 46:409. Jones, A. R. 1974. The ciliates. Hutchinson & Co. Ltd., London. Kidder, G. W. 1967. Chemical zoology I. Protozoa. Academic Press Inc., New York, NY. Kung, L., K. Gubert, and J. T. Huber. 1980. Supplemental niacin for lactating cows fed diets of natural protein or nonprotein nitrogen. J. Dairy Sci. 63:2020. Mishra, B. 1964. Relation of r u m e n mucinolytic bacteria and r u m e n protozoa to bloat in cattle. Ph.D. Dissertation, Kansas State University, Manhattan. Riddell, D. O., E. E. Bartley, and A. D. Dayton. 1980. Effect of nicotinic acid on r u m e n fermentation in vitro and in vivo. J. Dairy Sci. 63:1429. Riddell, D. O., E. E. Bartley, and A. D. Dayton. 1981. Effect of nicotinic acid on microbial protein synthesis in vitro and on dairy cattle growth and milk production. J. Dairy Sci. 64: 782. Tagari, H., I. Ascarelli, and A. Bondi. 1962. The influence of heating on the nutritive value of soybean meal for ruminants. Br. J. Nutr. 16:237. Yen, J. T., A. H. Jensen, and D. H. Baker. 1977. Assessment of the availability of niacin in corn, soybeans and s o y b e a n meal. J. Anim. Sci. 45: 269.