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Department of Animal and Veterinary Science, University of Idaho, Moscow, ID 83844-2330
Corresponding author:
A. N. Hristov; email: ahristov{at}uidaho.edu.
A trial with four ruminally and duodenally cannulated, late-lactation dairy cows was conducted to investigate the effect of dietary carbohydrate (CHO) composition and availability on ruminal ammonia N utilization and transfer into milk protein. Two diets were fed at 8-h intervals in a crossover design. The diets differed in CHO composition: the ruminally fermentable nonstructural carbohydrates (RFSS) diet (barley and molasses) contained a larger proportion of ruminally available CHO in the nonstructural carbohydrate fractions and the ruminally fermentable fiber (RFNDF) diet (corn, beet pulp, and brewer’s grains) contained a larger proportion of CHO in ruminally available fiber. Nitrogen-15 was used to label ruminal ammonia N and consequently microbial and milk N. Fermentation acids, enzyme activities, and microbial protein production in the rumen were not affected by diet. Ruminal ammonia concentration was lowered by RFNDF. Ruminal and total tract digestibility of nutrients did not differ between diets except that apparent ruminal degradability of crude protein was lower for RFNDF compared with RFSS. Partitioning of N losses between urine and feces was also not affected by diet. Milk yield and fat and protein content were not affected by treatment. Average concentration of milk urea N was lower for RFNDF than for RFSS. Proportion of milk protein N originating from ruminal microbial N (based on the areas under the 15N-enrichment curves) was higher for RFNDF than for RFSS. Cumulative recovery of 15N in milk protein was 13% higher for RFNDF than for RFSS indicating enhanced transfer of 15N-ammonia into milk protein with the former diet. The results suggested that, compared to diets containing higher levels of ruminally fermentable starch, diets providing higher concentration of ruminally fermentable fiber may enhance transfer of ruminal ammonia and microbial N into milk protein.
Key Words: dairy cow • carbohydrate • rumen ammonia • milk protein
Abbreviation key: AIA = acid-insoluble ash, ANDF = available NDF, APE = atom percent excess, AUC = area under the curve, MN = microbial nitrogen, MUN = milk urea nitrogen, NSC = nonstructural carbohydrates, NSP = nonstarch polysaccharides, PDA = polysaccharide-degrading enzyme activities, RFNDF = ruminally fermentable fiber, RFSS = ruminally fermentable nonstructural carbohydrates, RS = reducing sugars, TFC = total fermentable carbohydrates
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