Effect of feed level and supplementary rumen protected lysine and methionine on growth performance, rumen fermentation, blood metabolites and nitrogen balance in growing Tan lambs fed low protein diets

Abstract

Effect of feed level (FL) and supplementary rumen protected lysine (Lys) and methionine (Met) (RPLM) on growth performance, rumen fermentation, blood metabolites, and nitrogen (N) balance was studied in growing lambs fed low protein diets. Thirty-six, 7-month old wether Tan lambs (initial body weight, 35.0 ± 1.66 kg) were assigned into one of six groups (n = 6) in a randomized complete block design. The experiment was a 2 × 3 factorial arrangement of treatments. The lambs were either not supplemented (control) or supplemented (RPLM) with 4.5 g/day rumen protected Lys plus 1.5 g/day rumen protected Met and combined with three FLs (1.0, 0.8 and 0.6 voluntary intake - VI). The basal diet, on a dry matter (DM) basis, consisted of 400 g/kg forage and 600 g/kg concentrate. The average daily gain (ADG) increased linearly (P < 0.05) with increasing FL, and ADG was greater (interaction, P = 0.01) and the DM intake to ADG ratio was lesser (interaction, P = 0.04) in RPLM than control lambs in the 0.8 VI and 1.0 VI FLs, but not in the 0.6 VI FL. Digestibilities of DM, organic matter, crude protein, neutral detergent fiber and acid detergent fiber decreased linearly (P < 0.01) with increasing FL. Concentrations of total volatile fatty acids (VFAs) and butyrate and the acetate:propionate ratio increased linearly with increasing FL (P < 0.05). In addition, concentrations of propionate and iso-VFAs were greater (P < 0.05); whereas, of butyrate and the acetate: propionate ratio were lesser (P < 0.05) in RPLM than control lambs. Daily faecal-N output and N-retention were greater (interaction, P < 0.05) in RPLM than control lambs in the 1.0 VI and 0.8 VI FLs, but not in the 0.6 VI FL. In addition, microbial-N (MN) and the efficiency of microbial protein synthesis increased linearly (P < 0.01) with increasing FL and MN was greater (P = 0.02) in RPLM than in control lambs. Serum glucose concentration increased linearly (P < 0.01) with decreasing FL and was greater (P < 0.01) in RPLM than in control lambs. It was concluded that supplementary RPLM at 1.0 VI and 0.8 VI could improve ADG, feed conversion, and N-balance in growing Tan lambs and could be beneficial for sheep fed a low protein diet.