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Consumers are becoming increasingly health conscious, and food product choices have expanded. Choices in the dairy case include fluid milk labeled according to production management practices. Such labeling practices may be misunderstood and perceived by consumers to reflect differences in the quality or nutritional content of milk. Our objective was to investigate nutritional differences in specialty labeled milk, specifically to compare the fatty acid (FA) composition of conventional milk with milk labeled as recombinant bST (rbST)-free or organic. The retail milk samples (n=292) obtained from the 48 contiguous states of the United States represented the consumer supply of pasteurized, homogenized milk of 3 milk types: conventionally produced milk with no specialty labeling, milk labeled rbST-free, and milk labeled organic. We found no statistical differences in the FA composition of conventional and rbST-free milk; however, these 2 groups were statistically different from organic milk for several FA. When measuring FA as a percentage of total FA, organic milk was higher in saturated FA (65.9 vs. 62.8%) and lower in monounsaturated FA (26.8 vs. 29.7%) and polyunsaturated FA (4.3 vs. 4.8%) compared with the average of conventional and rbST-free retail milk samples. Likewise, among bioactive FA compared as a percentage of total FA, organic milk was slightly lower in trans 18:1 FA (2.8 vs. 3.1%) and higher in n-3 FA (0.82 vs. 0.50%) and conjugated linoleic acid (0.70 vs. 0.57%). From a public health perspective, the direction for some of these differences would be considered desirable and for others would be considered undesirable; however, without exception, the magnitudes of the differences in milk FA composition among milk label types were minor and of no physiological importance when considering public health or dietary recommendations. Overall, when data from our analysis of FA composition of conventional milk and milk labeled rbST-free or organic were combined with previous analytical comparisons of the quality and composition of these retail milk samples, results established that there were no meaningful differences that would affect public health and that all milks were similar in nutritional quality and wholesomeness., (Copyright 2010 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.)

Very long chain n-3 fatty acids such as eicosapentaenoic acid (EPA; 20:5n-3) are important in human cardiac health and the prevention of chronic diseases, but food sources are limited. Stearidonic acid (SDA; 18:4n-3) is an n-3 fatty acid that humans are able to convert to EPA. In utilizing SDA-enhanced soybean oil (SBO) derived from genetically modified soybeans, our objectives were to examine the potential to increase the n-3 fatty acid content of milk fat and to determine the efficiency of SDA uptake from the digestive tract and transfer to milk fat. Three multiparous, rumen-fistulated Holstein cows were assigned randomly in a 3 x 3 Latin square design to the following treatments: 1) control (no oil infusion); 2) abomasal infusion of SDA-enhanced SBO (SDA-abo); and 3) ruminal infusion of SDA-enhanced SBO (SDA-rum). The SDA-enhanced SBO contained 27.1% SDA, 10.4% alpha-linolenic acid, and 7.2% gamma-linolenic acid. Oil infusions provided 57 g/d of SDA with equal amounts of oil infused into either the rumen or abomasum at 6-h intervals over a 7-d infusion period. Cow numbers were limited and no treatment differences were detected for DMI or milk production (22.9+/-0.5 kg/d and 32.3+/-0.9 kg/d, respectively; least squares means +/- SE), milk protein percentage and yield (3.24+/-0.04% and 1.03+/-0.02 kg/d), or lactose percentage and yield (4.88+/-0.05% and 1.55+/-0.05 kg/d). Treatment also had no effect on milk fat yield (1.36+/-0.03 kg/d), but milk fat percentage was lower for the SDA-rum treatment (4.04+/-0.04% vs. 4.30+/-0.04% for control and 4.41+/-0.05% for SDA-abo). The SDA-abo treatment increased n-3 fatty acids to 3.9% of total milk fatty acids, a value more than 5-fold greater than that for the control. Expressed as a percentage of total milk fatty acids, values (least squares means +/- SE) for the SDA-abo treatment were 1.55+/-0.03% for alpha-linolenic acid (18:3n-3), 1.86+/-0.02 for SDA, 0.23 +/- <0.01 for eicosatetraenoic acid (20:4n-3), and 0.18+/-0.01 for EPA. Transfer efficiency of SDA to milk fat represented 39.3% (range=36.8 to 41.9%) of the abomasally infused SDA and 47.3% (range=45.0 to 49.6%) when the n-3 fatty acids downstream from SDA were included. In contrast, transfer of ruminally infused SDA to milk fat averaged only 1.7% (range=1.3 to 2.1%), indicating extensive rumen biohydrogenation. Overall, results demonstrate the potential to use SDA-enhanced SBO from genetically modified soybeans combined with proper ruminal protection to achieve impressive increases in the milk fat content of SDA and other n-3 fatty acids that are beneficial for human health., (Copyright 2010 American Dairy Science Association. Published by Elsevier Inc. All rights reserved.)

The objective of this study was to determine the effects of altering dry period length in multiparous dairy cows (n = 341) on milk production for a full lactation (294 d). The study used 3 commercial herds in the western United States. Cows producing greater than 8,400 kg of mature-equivalent milk were assigned to treatments 60 d before their due dates. The 4 treatments were 1) 60-d dry period, label use of recombinant bovine somatotropin (rbST; 60d-L); 2) 32-d dry period, label use of rbST (32d-L); 3) 0-d dry period, label use of rbST (0d-L); and 4) 0-d dry period, continuous use of rbST (0d-C). Cows with shortened dry periods produced 625, 1,000, and 1,042 kg of milk during the prepartum period for treatments 2 to 4, respectively. During the postpartum period, cows on the 32d-L treatment produced similar amounts of milk compared with the 60d-L treatment (11,490 vs. 11,968 kg, respectively). However, cows on the 0d-L (10,316 kg) and 0d-C (10,195 kg) treatments produced significantly lower amounts of milk during the postpartum period compared with the 60d-L treatment. Total milk production from the prepartum and postpartum periods was not altered significantly and was 11,974, 12,112, 11,310, and 11,230 kg for treatments 1 to 4, respectively. The concentrations of beta-hydroxybutyrate and nonesterified fatty acids in serum after calving were decreased for cows on the 32d-L, 0d-L, and 0d-C treatments compared with cows on the 60d-L treatment, which may indicate improved metabolic status.

Dry periods of 40 to 60 d have been an industry standard because dry periods <40 d have resulted in reduced milk yields in the subsequent lactation by 10 to 30%. However, recent research has demonstrated no production losses for cows given a 30-d dry period. The current study evaluated milk production effects of shortened or omitted dry periods for cows at mature-equivalent production >12,000 kg of milk and treated with bovine somatotropin (bST). The study used 2 commercial dairies and one university dairy and included 4 treatments. Five multiparous and 5 primiparous cows from each farm were assigned to each treatment: 1) 60-d dry period, label use of bST (60DD); 2) 30-d dry period, label use of bST (30DD); 3) continuous milking, label use of bST (CMLST); and 4) continuous milking with continuous use of bST (CMCST). Per label, bST use started at 57 to 70 d in milk and ended 14 d before drying (60DD and 30DD) or expected calving date (CMLST). In primiparous cows, average milk yields during the first 17 wk of lactation were reduced for cows on treatments 30DD, CMLST, and CMCST vs. the 60DD treatment. (38.3, 35.1, and 37.5 vs. 44.1 +/- 1.3 kg/d, respectively). For multiparous cows, respective milk yields did not differ (46.6, 43.4, 46.5, and 47.7 +/- 2.1 kg/d). Shortened or omitted dry periods may impede mammary growth in primiparous cows, resulting in reduced milk yield in the subsequent lactation. In contrast, a shortened or omitted dry period with either bST protocol did not alter production in multiparous cows treated with bST. Quality aspects of prepartum milk and colostrum require additional characterization. For multiparous cows, milk income generated for short dry periods or for continuous milking might increase their profitability. At 17 wk of the subsequent lactation, estimates of the cumulative net margins of multiparous cows on the 30DD treatment and continuous milking treatments exceeded those of cows on the 60DD treatment by 40 dollars to 60 dollars per cow.

Two trials were conducted to evaluate effects of feeding supplemental fibrolytic enzymes or soluble sugars and malic acid on milk production. In trial 1, 257 cows at four sites were fed a basal diet consisting of no more than 60% of forage DM as corn silage and less than 40% as alfalfa hay. Cows were assigned randomly within site, parity, and two stages of lactation to: 1) control; 2) enzyme A; 3) enzyme B; and 4) soluble sugars and malic acid. There was a 14-d pretreatment and an 84-d treatment period. Enzyme solutions were sprayed on either the forage component or the TMR each day while mixing feed. Trial 2 was similar, except 122 cows at one site in the United Kingdom were fed diets containing forage that was 75% corn silage and 25% grass silage, and all cows began the study between 25 to 31 DIM. Mean milk productions for 233 cows that completed trial 1 were 32.9, 32.5, 32.4, and 32.9 kg/d for control, enzyme A, enzyme B, and soluble sugars and malic acid, respectively. Mean milk productions for 116 cows that completed trial 2 were 28.2, 27.9, 28.8, and 28.4 kg/d, respectively. In vitro analyses of the activities of enzyme solutions indicated that all major cellulose and hemicellulose degrading activities were present; however, the pH optima (approximate pH = 4 to 5) were more acidic, and the temperature optimum (approximately 50 degrees C) was greater than normal pH and temperature in the rumen. If fibrolytic activity in the rumen is a major mechanism of action of supplemental fibrolytic enzymes, it appears that considerable activity of these preparations was lost due to conditions in the rumen. In conclusion, feeding supplemental fibrolytic enzymes or malic acid with soluble sugars had no effect on milk production under the conditions used in this study.

Four multiparous cows with cannulas in the rumen and proximal duodenum were used in early lactation in a 4 x 4 Latin square experiment to investigate the effect of method of application of a fibrolytic enzyme product on digestive processes and milk production. The cows were given ad libitum a total mixed ration (TMR) composed of 57% (dry matter basis) forage (3:1 corn silage:grass silage) and 43% concentrates. The TMR contained (g/kg dry matter): 274 neutral detergent fiber, 295 starch, 180 crude protein. Treatments were TMR alone or TMR with the enzyme product added (2 kg/1000 kg TMR dry matter) either sprayed on the TMR 1 h before the morning feed (TMR-E), sprayed only on the concentrate the day before feeding (Concs-E), or infused into the rumen for 14 h/d (Rumen-E). There was no significant effect on either feed intake or milk yield but both were highest on TMR-E. Rumen digestibility of dry matter, organic matter, and starch was unaffected by the enzyme. Digestibility of NDF was lowest on TMR-E in the rumen but highest postruminally. Total tract digestibility was highest on TMR-E for dry matter, organic matter, and starch but treatment differences were nonsignificant for neutral detergent fiber. Corn silage stover retention time in the rumen was reduced by all enzyme treatments but postruminal transit time was increased so the decline in total tract retention time with enzymes was not significant. It is suggested that the tendency for enzymes to reduce particle retention time in the rumen may, by reducing the time available for fibrolysis to occur, at least partly explain the variability in the reported responses to enzyme treatment.

The health of dairy cows given bovine somatotropin (bST) for one lactation was evaluated in 28 commercial herds located in four regions of the United States. At least six herds were in a region and at least one herd/region contained fewer than 60 cows. Cows (n = 1213) were assigned randomly to control or bST groups and were treated beginning in wk 9 to 10 of lactation and every 14 d until dry-off or d 400 of lactation. Management was according to site practices. Cows were observed for health-related signs by farm personnel daily and by the herd veterinarian biweekly. Average 305-d test-day milk yields were 932 kg greater for bST-treated cows. Pregnancy rates, days open, twinning, cystic ovaries, or abortions were unaffected by treatments. Supplementation of cows with bST had no effect on total mastitis cases, total days of mastitis, duration of mastitis, or the odds ratio of a cow to develop mastitis. Cows supplemented with bST used more medications for health events other than mastitis. This usage was associated primarily with treatments for disorders of the foot and hock. Supplemented cows had a slight increase in foot disorders. There was no effect of supplementation with bST on culling from the herd or removal from study. Overall, the results confirm that label directions for bST are adequate for safe use under field conditions. All clinical signs observed in this study occur normally in dairy herds and were managed in cows supplemented with bST.

Eighty-two lactating Holstein cows in their first, second, or third lactation received either one, three, or five concurrent i.m. injections of a unit dose (0.6 g) of zinc methionyl bovine somatotropin (bST) or five doses of the vehicle. Injections were administered at 14-d intervals from 60 +/- 3 d postpartum until the end of lactation or until necropsy. Thirty-eight cows were continued on the treatment for a 2nd yr. Blood samples were collected at wk -2, -1, 3, and 7 relative to the start of treatment and then every 8 wk (yr 1) or 4 wk (yr 2) thereafter. Untreated cows that were included in a survey of the resident herd were bled at wk 7 or 8, wk 10 or 11, and wk 13 or 14 of lactation and every 4 or 8 wk thereafter. Calves were bled within 72 h of birth and at approximately 5 wk of age. Most parameters associated with erythrocytes were decreased mildly in cows that were treated with bST. However, data remained within generally accepted reference ranges, and changes were not of clinical importance. Decreased hematocrit was not associated with increased hemolysis, hemodilution, or clinical anemia. No morphological lesions related to treatment were noted in the bone marrow or spleen; bST did not affect the incidence of immature cell types. Energy and protein balances did not significantly affect the hematological results of the cows. Calves generally were unaffected by bST treatment of the dam, but heavier calves had higher parameters associated with erythrocyte and lymphocyte counts than did calves with lower body weight. Exogenous bST treatment caused predictable changes in hematological parameters of dairy cows.

Effects of supplemental energy or protein on the milk production response to bST administration were examined in two separate trials. In trial 1, 40 cows were used in a 2 x 2 factorial, completely randomized design to determine the effects of bST and fat supplementation. The study consisted of a 7-d pretreatment period and a 42-d treatment period. Fat was top-dressed at 3.0 Mcal/d of NEL, and bST was administered. Supplemental fat had no effect on milk production, and NEL intakes were unaffected. Administration of bST increased milk production by 7.1 kg/d, and the milk production response was unaffected by supplemental fat. In trial 2, 4 cows were used in four periods with a 2 x 2 factorial arrangement in which water or casein was infused into the abomasum of cows fed for ad libitum intake or at 80% of their requirements. Diets and infusions were initiated simultaneously and continued for 11 d. All cows were given bST during the last 5 d. Infusion of water or casein did not alter the milk production response to bST, but restricted feeding reduced the bST response (3.2 vs. 7.2 kg/d). Concentrations of IGF-I in plasma were increased by bST administration, and the increase was greatest for cows fed for ad libitum intake. The milk production response to bST was not increased by additional energy or protein offered to cows fed well-balanced diets.

To establish the naturally occurring range of insulin-like growth factor-I concentrations in bovine milk, samples from individual cows (n = 409) managed on five Missouri dairy herds were assayed. Parity, stage of lactation, and farm affected milk insulin-like growth factor-I concentration. Milk insulin-like growth factor-I concentration was higher in early lactation than mid and late lactation with concentrations in multiparous cows exceeding those in primiparous cows. Insulin-like growth factor-I concentration was negatively correlated to milk production the day of sample collection (r = -.15) and not correlated to predicted 305-d milk yields. Unprocessed bulk tank milk samples (n = 100) from a commercial processing plant had a mean concentration of insulin-like growth factor-I in milk of 4.32 ng/ml with a range of 1.27 to 8.10 ng/ml. This distribution was similar to the range detected in samples from individual cows, but values were lower than those reported for human milk. Concentration of insulin-like growth factor-I in milk was not altered by pasteurization (at 79 degrees C for 45 s). However, insulin-like growth factor-I was undetectable in milk heated to temperatures (121 degrees C for 5 min) required for infant formula preparation or in commercially available infant formula. These data indicated that insulin-like growth factor-I is a normal but quantitatively variable component of bovine milk that is not destroyed by pasteurization but is undetectable in infant formula. Concentration of insulin-like growth factor-I in bovine milk is lower than concentrations reported for human milk yet similar to those reported for human saliva.(ABSTRACT TRUNCATED AT 250 WORDS)

Multiparous Holstein cows at six universities were utilized to examine effects of ruminally protected methionine and lysine on lactational performance. Three hundred and four cows began the study; 259 cows were included in the production analysis. Following a 21-d standardization period, cows received a basal diet of corn silage and ground corn supplemented with one of five dietary treatments, which were supplements of soybean meal or corn gluten meal, the latter with zero and three combinations of protected methionine and lysine (methionine; methionine and lysine; methionine and double (2x) lysine). Treatment effects were evaluated during early, mid, late, and total lactation (22 to 112, 113 to 224, 225 to 280, and 22 to 280 d postpartum, respectively). On a DM basis, ratios of forage to concentrate (50:50, 60:40, and 70:30) increased, and dietary CP (16.0, 14.5, and 13.0%) decreased during the three periods of lactation. Amount of amino acid supplementation also decreased (15, 12, and 9 g/d methionine; 20, 16, and 12 g/d lysine; and 40, 32, and 24 g/d 2x lysine) with period of lactation. Actual and least squares means for milk, FCM, and milk protein yields were greater for soybean than for corn gluten meal during early, mid, and total lactation. In addition, these variables responded linearly to lysine in early lactation. Response to lysine was quadratic during mid and total lactation for these variables. Differences in nutrient intake explained production responses to protein sources but not to lysine. Serum amino acid responses primarily reflected differences in dietary protein source and rumen-protected amino acid.

Eight pregnant Holstein cows were given weekly injections of 15 g of recombinant bST over a 2-wk period for a total dose of 30 g to determine signs of acute toxicity. Cows were monitored intensively throughout the study, and samples were taken for analyses of hormones, metabolites, chemistries, hematology, and urine analytes. Animal health throughout the study was generally excellent. Mean rectal temperatures were significantly higher in treated cows (38.7 vs. 39.2 degrees C). Least squares means for 3.5% FCM production were 15.9 and 23.0 kg/d, and net energy intakes were 29.4 and 26.9 Mcal/d for control and treated cows, respectively. Somatotropin concentrations reached more than 250 ng/ml on d 10 and remained above 200 ng/ml. Insulin and glucose concentrations were also increased but returned toward baseline values. Free fatty acid concentrations were higher in treated cows, but beta-hydroxybutyrate was not affected. Most hematological measurements were unaffected except for a reduction in erythrocyte number in treated cows and decreases in hematocrit and hemoglobin, but values were within clinically normal ranges. Although cows received in 2 wk a dose that was equivalent to the amount administered during more than 2 yr of continuous use, no signs of acute toxicity to bST were observed.

Eight rumen-fistulated Holstein cows, averaging 77 d postpartum, were used in a replicated 4 X 4 Latin square design with 28-d periods to investigate the effect of ammonium salts of isobutyrate, 2-methylbutyrate, isovalerate, and n-valerate on animal performance and their possible mechanism(s) and site(s) of action. Each cow was fed ad libitum a complete mixed diet of 55% corn silage and 45% concentrate on a dry basis that was supplemented with 1.8 kg of premix daily. Treatments were 1) control, 2) ammonium salts of volatile fatty acids in premix, 3) ammonium salts of volatile fatty acids ruminally infused, or 4) ammonium salts of volatile fatty acids abomasally infused. Mean ruminal fluid pH and concentrations of ammonia and volatile fatty acids for treatment comparisons were not different. Plasma concentrations of isobutyrate, 2-methylbutyrate, and valerate differed among treatments, but there was no significant effect on dry matter intake, milk production, milk composition, or efficiency of feed utilization. Apparent nutrient digestibility; disappearance of dry matter, cellulose, and nitrogen from polyester bags suspended in the rumen; and plasma concentrations of glucose, free fatty acids, and growth hormone also were not significantly affected by treatment.

Four Holstein cows were utilized in a Latin square design with a factorial arrangement of treatments to examine the interaction between effects of dry matter intake (107 vs. 78% of energy requirements) and postruminal infusions (water vs. 395 g/d casein) on lactational performance, utilization of nitrogen and energy, and plasma concentrations of hormones. Yields of milk and milk protein were decreased by feed restriction and increased by casein infusion with no treatment interactions. Restricting feed intake decreased total nitrogen intake by 143 g/d and resulted in smaller quantities of fecal, absorbed, urinary, milk, and retained nitrogen compared with cows fed ad libitum. Casein infusion increased total nitrogen intake (55 g/d), absorbed nitrogen (54 g/d), urinary nitrogen excretion (28 g/d), and milk nitrogen (13 g/d). Casein by dry matter intake interactions were not significant for nitrogen utilization. Restricting feed intake increased plasma growth hormone and decreased concentrations of insulin and triiodothyronine. Glucagon, prolactin, and thyroxine were not affected by intake. Casein infusion did not affect growth hormone, insulin, prolactin, triiodothyronine, or thyroxine. Increased milk and milk protein yields obtained with casein infusion were apparently not mediated through changes in circulating concentrations of these hormones; however, plasma glucagon was increased by casein infusions.

Eight Holstein cows were used to investigate the effects of DM intake and sodium bicarbonate on lactational performance and concentrations of hormones and metabolites in plasma. Cows were fed a diet with or without 1.0% sodium bicarbonate (dry matter basis) in a switchback design. Four cows were fed ad libitum and four cows were fed approximately 80% of their recommended nutrient requirements by restriction of DM intake throughout the three 21-d periods. Supplementing the diet with sodium bicarbonate increased DM intake of cows fed ad libitum. There was a feed intake by sodium bicarbonate interaction for production of 4% FCM. This interaction may be explained by the difference in DM intake of cows fed ad libitum or restricted amounts of feed and supplemented with sodium bicarbonate. Cows fed restricted amounts of feed had lower milk, milk fat, milk protein, milk SNF, and milk energy yields. Restriction of feed intake increased plasma concentrations of somatotropin and nonesterified fatty acids but decreased concentrations of insulin, triidothyronine, thyroxine, glucagon, and prolactin. In contrast, feeding supplemental sodium bicarbonate did not affect concentrations of hormones or metabolites in plasma at either feed intake.

Six rumen cannulated steers were in a replicated 3 X 3 Latin square arrangement to investigate degradability in the rumen of dry matter and crude protein from soybean meal that was untreated or treated with either 8% acetic acid or .3% formaldehyde. Polyester bags containing the soybean meals were suspended in the rumen of each steer and removed after 1, 2, 4, 8, 12, 16, and 24 h to calculate degradation rates. Soybean meals remaining in the polyester bags after 12 h of rumen exposure were collected and referred to as residues. Postruminal digestion was stimulated by incubating 200 mg of soybean meal residues with pepsin and pancreatin. Formaldehyde treatment of soybean meal was more effective than acetic acid treatment in preventing ruminal degradation of dry matter and crude protein under these experimental conditions. However, soybean meal treated with acetic acid and remaining in polyester bags after 12 h of ruminal exposure released a larger quantity of alpha-amino nitrogen during in vitro incubation with pepsin and pancreatin than did residue of soybean meal treated with formaldehyde. Acetic acid may prove to be an effective agent for increasing the supply of absorbable dietary amino acids.

Holstein cows just past peak lactation were used in a 3 x 3 Latin square design to determine the effects of arginine infusion on concentrations of somatotropin and insulin in plasma, milk production, and milk composition. Treatments were: 1) control; 2) arginine injection into jugular vein, and 3) arginine infusion into abomasum. Concentrations of arginine and ornithine in plasma were increased by injection of arginine into the jugular vein compared with the control. The concentration of ornithine in plasma was increased shortly after injection of arginine into the jugular vein, and both arginine and ornithine concentrations in plasma decreased rapidly. Abomasal infusion of arginine significantly increased concentrations of arginine, ornithine, and urea in plasma compared with concentrations in the control treatment. Injection of arginine into the jugular vein increased concentrations of somatotropin and insulin in plasma, but the increase did not persist for more than 30 min. A secondary peak in plasma somatotropin concentration occurred approximately 1 h after the initial peak. Arginine infusion into the abomasum did not alter plasma concentrations of either somatotropin or insulin. Dry matter intake, milk production, and milk composition were not affected by treatments. Lack of changes in milk production and milk composition suggest that acute increases in somatotropin with concomitant increases in insulin are not sufficient to stimulate synthesis of milk and milk components by cows during established lactation.

Four early lactation multiparous Holstein cows were used in a 4 x 4 Latin square to investigate the effects of source of protein (fish meal or soybean meal) and carbohydrate (corn or barley) on ruminal fermentation, flow of nutrients to the small intestine, and animal performance. The treatments, arranged in a 2 x 2 (protein x carbohydrate) factorial were: 1) corn plus soybean meal; 2) corn plus fish meal; 3) barley plus soybean meal; and 4) barley plus fish meal. Dry matter and starch intakes were greater when corn was fed than when barley was fed. Barley-based diets were more extensively degraded in the rumen than corn-based diets and therefore provided more energy for microbial growth. However, passage of amino acids and starch to the duodenum was greater for corn-based diets than barley-based diets, because of the greater intake and lower ruminal degradability of the corn-based diets. Microbial protein constituted a larger portion of the total N and had a greater influence on the pattern and quantity of amino acids that passed to the duodenum than did protein from fish meal or soybean meal, which escaped ruminal degradation. Feeding corn-based diets increased production of milk and milk protein compared with feeding barley-based diets.