Quantitative studies on nitrogen metabolism in the bovine rumen

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Abstract
1. Monozygous twin cattle with permanent rumen fistulas were used to measure the rate of degradation of casein and ovalbumin. One twin was used as a control and the other had protein administered by fistula about 4.75 h after eating a standard ration.2. Casein was rapidly degraded with a half-life in the range 5.6–21.5 min, with the formation of peptides, free amino acids and finally ammonia. Up to 43% of the casein nitrogen was found as ammonia in the rumen fluid. Most of the amino acids present in casein were found in the free state in the rumen fluid to an extent of less than 7%, but valine, leucine, isoleucine and lysine were present as 25, 27, 21 and 38% respectively of the amounts present in the casein administered.3. When acid-hydrolysed casein was given, ammonia was rapidly formed, the maximum amount found in the rumen being equivalent to 39% of the casein N. The individual amino acids were rapidly broken down except for valine, leucine, isoleucine and lysine of which synthesis in addition to degradation may occur.4. Ovalbumin was degraded slowly with a half-life of 175 min, which was reduced to 132 min by feeding the cattle with ovalbumin for 5 d. Ammonia was produced slowly.5. When added together the two proteins were degraded in the rumen independently at their own rates.6. δ-Amino-n-valeric acid was produced in large quantity when casein or casein hydrolysate was degraded in the rumen.

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