The effect of supplemental protein source on ammonia levels in rumen fluid and blood and intake of alfalfa silage by beef cattle

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Abstract
Growing beef cattle fed legume silage were supplemented with urea, soybean meal, fishmeal or a barley-based control in two experiments. In the first experiment, the concentrates were fed before the silage and in the second the silage and concentrates were mixed at feeding. In exp. 1 there were no significant (P > 0.05) effects of CP supplement on feed intake, weight gains or feed efficiency, although animals fed fishmeal gained the most and those fed urea the least. Rumen fluid ammonia N (RFNH3-N) increased more after feeding urea and soybean meal than the control or fishmeal supplements. Blood urea N (BUN) levels changed in parallel with RFNH3-N levels but the magnitude of change was reduced to about half. Blood ammonia N (BNH3-N) levels showed statistically significant changes only with the urea supplement. Cattle fed urea took longer to consume their supplement than those fed the other supplements. In exp. 2 there was no effect of supplement on gains or feed conversion. Animals fed the control supplement consumed more dry matter than those fed fishmeal (P < 0.05). Cattle fed the control supplement had lower RFNH3-N and BUN than those fed the other supplements (P < 0.01) but differences among the sources of CP were small. The urea supplement resulted in a higher BNH3-N than the control or fishmeal supplements (P < 0.05). There was no apparent effect of treatment on blood glucose level in either experiment. It was concluded that cattle adjust their daily eating pattern to maintain BNH3-N levels within physiological limits; that intake of silage dry matter is limited by level of non-protein N and rumen degradable protein; and that provision of bypass protein can improve efficiency of feed conversion of high-silage diets even when depressing feed intake. Key words: Alfalfa, silage, protein supplement, nitrogen, intake

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