Influence of source of rumen-degraded nitrogen on ruminal and whole tract digestion, plasma hormone and metabolite concentrations as well as milk yield and composition in dairy cows

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Abstract
Fifteen multiparous and 10 primiparous cows in early lactation (parturition through 16 wk postpartum inclusive) were fed one of five totally mixed diets. Diets differed in the source of supplemental nitrogen with isonitrogenous substitution of canola meal (a rapidly rumen-degraded true protein source) by Barlein (a rapidly rumen degraded nonprotein nitrogen source). All diets were 40% whole crop oat silage and 60% concentrate on a dry matter basis. Rumen pH, ammonia N and lactate concentrations as well as in sacco rumen degradability of whole diet nitrogen were not influenced by treatment. These results appear to support the assumption that Barlein nitrogen was released in the rumen at a rate similar to that of canola meal protein. Feed intake and body weight change did not differ among treatments. However, milk yield declined linearly as substitution of canola meal with Barlein increased. Milk composition of fat, protein, and lactose were not influenced by treatment although milk urea concentrations increased as substitution of canola meal with Barlein increased. Data collected did not conclusively identify the mechanism for the reduced performance; however, it seems most likely it was the result of reduced rumen microbial growth and escape as substitution of canola meal with Barlein increased, leading to a deficiency in metabolizable protein supply. Key words: NPN, dairy cattle, protein

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