GROWTH AND ENDOCRINE RESPONSES OF LAMBS FED RUMEN-PROTECTED ORNITHINEAND ARGININE

Twenty Hampshire X Western wether lambs (initial body weight (BW) 29.9 +/- 1.5 kg) were used in a 42-day study to evaluate growth and endocr ine responses to rumen-protected ornithine or arginine. Treatments con sisted of no supplemental amino acid (CON), or supplements that provid ed 0.42 g day(-1) omithine HCl per kg BW (ORN), 0.5 g day(-1) arginine HCl per kg BW (L-ARG) or 0.75 g day(-1) arginine HCl per kg BW (H-ARG ). Amino acids were incorporated into a protective matrix containing g round corn, coconut oil and zein. Serum omithine and urea N concentrat ions were increased (P <0.05) in ORN, L-ARG and H-ARG lambs, while ser um arginine concentrations were increased (P <0.05) in H-ARG lambs onl y. Serum concentrations of total essential amino acids were lower (P < 0.05) in H-ARG lambs compared with L-ARG lambs. Consumption of ORN, L- ARG and H-ARG increased (P <0.05) mean somatotropin (STH) concentratio ns compared with CON, while basal STH concentrations were higher (P <0 .05) in ORN and L-ARG lambs only. Mean and basal STH concentrations we re higher (P <0.05) in ORN lambs compared with L-ARG and H-ARG lambs. Amplitude and frequency of STH pulses were not affected (P >0.05) by t reatment. Serum insulin-like growth factor-I (IGF-I) concentrations we re higher (P <0.05) in ORN, L-ARG and H-ARG lambs compared with CON la mbs. Further increases (P <0.05) in IGF-I concentrations were noted in H-ARG lambs compared with ORN and L-ARG lambs. Performance of lambs w as not improved (P >0.05) by the consumption of rumen-protected omithi ne or arginine compared with CON lambs despite increased STH and IGF-I concentrations. Average daily gain of L-ARG lambs was increased (P <0 .05) compared with H-ARG lambs, resulting in heavier (P <0.05) BW on D ay 42 and improved (P <0.05) feed efficiency. These data provide furth er evidence that increasing post-ruminal supplies of arginine and orni thine may increase circulating STH and IGF-I concentrations in growing lambs. However, performance may not be improved by these amino acid-i nduced responses.