ARTERIAL AMINO-ACID-CONCENTRATIONS IN SHEEP CONSUMING FORAGE DIETS

Arterial blood collected from sheep was analyzed for various amino aci ds to evaluate effects of dietary forage characteristics on adequacy o f amino acid availability for protein synthesis. In three experiments with grass diets consumed ad libitum or in limited quantities by nearl y mature wethers, differences in grass source and(or) quality generall y did not markedly affect arterial concentrations of essential amino a cids. In the fourth experiment, wethers (7.5 months of age and 31 +/- 0.8 kg BW) consumed ad libitum bermudagass (Cynodon dactylon; BG) and ryegrass (Lolium multiflorum)-wheat (Triticum aestivum; RW) hay mixtur es in percentages of 0, 33, 67 and 100%. As DE intake decreased with i ncreasing dietary level of BG, arterial lysine concentration increased linearly (P = 0.02; 116, 137, 144 and 157 mu M for 0, 33, 67 and 100% BG, respectively); whereas, concentrations of other essential amino a cids did not differ among treatments. In the fifth experiment, wethers (8.5 months of age and 33 +/- 0.9 kg BW) consumed ad Libitum BG or RW either coarsely chopped or finely ground and pelleted; DE intake was greater for RW vs. BG and for pelleted than for chopped grass. Lysine concentration was greater (P = 0.01) for BG than for RW and was decrea sed (P = 0.03) by pelleting (133, 118, 114 and 78 mu M for chopped BG, pelleted BG, chopped RW and pelleted RW, respectively). Conversely, c oncentrations of tryptophan, valine, phenylalanine, isoleucine and leu cine were greater (P < 0.03) for RW than for BG, and grinding and pell eting increased concentrations of valine (P = 0.07) and phenylalanine (P = 0.06). In conclusion, DE intake with grass diets may influence pa rticular amino acids most limiting to protein synthesis by growing rum inants, with lysine availability being of relatively greater concern w ith forages yielding high vs. low DE intake. (C) 1998 Elsevier Science B.V. All rights reserved.