At the present time, knowledge about amino acid utilization by ruminants is lacking. Our lack of knowledge arises primarily because ruminal fermentation intervenes between the diet and the small intestine such that research on the topic is difficult and expensive. From one perspective, our lack of understanding has few practical consequences because the ruminal fermentation ensures that amino acid supply is rarely limiting under most current agricultural practices.

Methionine is the most limiting amino acid in microbial protein for both growing cattle and sheep and, consequently, it has been studied more than the other amino acids. Methionine appears to be used relatively inefficiently for growth by cattle and sheep. In cattle, this has been attributed to its importance as a methyl group donor, whereas in sheep its role as a precursor for cysteine synthesis may be more important. Future work is needed to validate these conclusions.

The study of amino acid utilization by ruminants may yield future rewards as production systems change. Leaner animals with greater capacity for protein deposition may have amino acid requirements that exceed the supply from typical diets. Certainly, the responsiveness of cattle fed grass silages to supplementation with sources of ruminally undegradable protein (Veira et al., 1988; Nelson, 1997) would suggest that amino acid deficiencies are probable when diets contain little true protein. Identification of factors that influence amino acid utilization should improve models to predict amino acid use by ruminants.

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