ARS | Publication request: ARTERIAL AMINO ACID CONCENTRATIONS IN SHEEP CONSUMING FORAGE DIETS

Submitted to: Small Ruminant Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 10, 1997
Publication Date: N/A

Interpretive Summary: The degree to which specific amino acids limit protein synthesis in cattle and sheep might vary among forage-based diets through effects on absorption of specific amino acids, the quantity of energy absorbed and the efficiency with which absorbed energy is used. Dietary inclusion of 40% alfalfa in grass-based diets consumed ad libitum by sheep near maturity resulted in excessive essential amino acid availability relative to potential for use in protein synthesis, with greater coincidence noted for 0 and 20% alfalfa. With growing wethers, increasing feed intake by substituting ryegrass-wheat for bermudagrass or by grinding and pelleting apparently increased potential lysine utilization in protein synthesis relatively more than increases in lysine absorption. Conversely, generally opposite changes were noted for tryptophan, valine, phenylalanine, isoleucine and leucine. Thus, absorbed energy may influence specific amino acids most limiting to protein synthesis, with lysine of greater concern with forages yielding high versus low absorbed energy. This research is important because it may lead to enhanced matching of amino acid absorption for various production classes and growth rates of ruminants, which will decrease cost of production or increase level of performance and lessen nitrogen excretion for improved environmental quality.

Technical Abstract: Inclusion of 40% alfalfa in grass diets of wethers near maturity may have resulted in excessive essential amino acid availability relative to potential for use in protein synthesis, with greater coincidence noted for 0 and 20% alfalfa. In another experiment, growing wethers consumed ad libitum bermudagrass (BG) and ryegrass-wheat (RW) hay in percentages of 0, 33, 67 and 100%. As digestible energy intake decreased with increasing dietary BG, arterial lysine concentration increased linearly. In another experiment, growing wethers consumed ad libitum BG or either chopped or ground chopped grass. Arterial lysine concentration was greater for BG than for RW and was decreased by pelleting. Conversely, concentrations were greater for RW than for BG for tryptophan, valine, phenylalanine, isoleucine and leucine. In addition, grinding and pelleting increased arterial concentrations of valine and phenylaline (P=0.06) and affected isoleucine and leucine concentrations more with RW than with BG. In conclusion, lysine absorption with grass hay diets for which digestible energy intake is relatively high may be inadequate for maximal protein synthesis but sufficient with lower quality grass diets.