Effects of amount of protein and ruminally protected amino acids in the diet of dairy cows fed supplemental fat.

The objectives of this experiment were to investigate the effects of amount of dietary CP and ruminally protected AA supplementation on production of milk and milk components, ruminal fermentation, and nutrient digestibilities by cows fed diets containing high oil corn and tallow. Holstein cows in midlactation producing 22 to 25 kg/d of milk were used in a 5 x 5 Latin square design. Treatments were 1) control (16.8% CP, no added fat); 2) 14.2% CP, no AA; 3) 14.2% CP, with AA; 4) 17.5% CP, no AA; and 5) 17.5% CP, with AA. Diets 2 to 5 contained supplemental fat from high oil corn and tallow. Diets consisted of 33% alfalfa haylage, 17% corn silage, and 50% concentrate DM. Intake of DM was not different among treatments. Dietary fat increased yields of milk, fat, SNF, and total solids and percentages of fat and total solids. Increasing CP from 14.2 to 17.5% did not alter production or composition of milk. Supplemental AA increased yields of 4% FCM, milk fat, milk CP, true protein, and casein protein and percentages of CP, true protein, and casein protein in milk when either 14.2 or 17.5% CP was in the diet. Supplemental fat did not alter ruminal fermentation, but increases in dietary CP increased total VFA concentration in the rumen without affecting proportions of individual VFA. Apparent digestibilities of DM, OM, CP, starch, and energy in the total tract were greater for cows fed the 17.5% CP diets. Addition of AA to the 14.2% CP diet increased apparent digestibilities of DM, OM, ADF, NDF, and energy in the total tract but decreased digestibilities for cows fed the 17.5% CP diets. Feeding AA to midlactation cows in diets containing supplemental fat may alleviate the decrease in milk protein percentage associated with fat supplementation; this response was similar for cows fed diets that contained either 14.2 or 17.5% CP.

Microbial protein synthesis and flows of nitrogen fractions to the duodenum of dairy cows.

Attempts have been made to increase nutrient availability for milk production by increasing feed intake, optimizing ruminal fermentation, and supplementing nutrients to the diet that will escape ruminal degradation. Energy and N are the nutritional factors that most often limit microbial growth and milk production. Ruminal fermentation and flow of microbial and dietary protein to the small intestine are affected by feed intake and by the amount and source of energy and protein in the diet. Feeding protein and carbohydrate that are not degraded in the rumen increases the quantity of dietary protein that passes to the small intestine but may decrease the quantity of microbial protein that is synthesized in the rumen. This often results in only small differences in the total NAN that passes to the small intestine. Because microbial protein supplies a large quantity of total AA that passes to the small intestine, differences in passage of individual AA often are only slight. Additional research with cows consuming large amounts of feed are needed to identify combinations of feed ingredients that synchronize availabilities of energy and N for optimizing ruminal digestion, microbial protein synthesis, nutrient flow to the small intestine, and milk production and composition.

Effects of urea and starch on rumen fermentation, nutrient passage to the duodenum, and performance of cows.

Four midlactation, multiparous Holstein cows fitted with ruminal and duodenal cannulas were used in a 4 x 4 Latin square design to determine the effects of supplementing urea or starch or both to diets containing fish meal on passage of nutrients to the small intestine and performance of lactating cows. The treatments (in a 2 x 2 factorial arrangement) were 1) control and control plus 2) urea, 3) starch, or 4) starch and urea. Supplementing diets with urea did not affect DMI; ruminal, postruminal, or total tract digestibilities of DM, starch, ADF, or NDF; ruminal fluid VFA concentrations or molar percentages; or ruminal fluid or particulate dilution rates. Feeding additional starch depressed DMI but did not alter ruminal or postruminal digestion of OM or VFA concentrations and molar percentages in ruminal fluid. Ruminal fluid ammonia concentration was increased by feeding urea and decreased by feeding additional starch. Passage of nonammonia N, nonammonia nonmicrobial N, or microbial N to the small intestine and efficiency of microbial CP synthesis were not affected significantly by supplying either urea or additional starch. Feeding urea increased passage of methionine to the small intestine, whereas feeding additional starch increased passage of methionine and arginine. Passage of other amino acids to the small intestine was not altered significantly by feeding urea or additional starch. Production of milk and milk protein was increased, but yields of fat and SNF were not altered by feeding diets supplemented with urea. Production of milk and milk fat was not affected, but yields of CP and SNF were decreased when additional starch was fed to cows.