Effects of Restricted Feeding on Intake, Digestion, Nitrogen Balance and Metabolizable Energy in Small and Large Body Sized Sheep Breeds

Ninety six intact male sheep (12 months old with mean live weight of about 35 kg) were used to assess the effects of restricted feeding on intake, digestion, nitrogen balance and metabolizable energy (ME). The animals were selected from two known Iranian small and large body size breeds: 48 Sangsari (S) and 48 Afshari (A), and were divided into two equal groups: restricted (R) and a control (C). Each group had 48 sheep (24 each breed). The experiment had a duration of 15 and 75 days adaptation and treatment periods, respectively. The animals were individually placed in metabolism cages and fed a diet based on pelleted concentrate mixture consisting of alfalfa, barley grain, cottonseed meal and barley straw. The animals in group C were fed ad libitum, while animals in group R were fed at maintenance level and maintained a relatively constant live weight. During the experiment, the average daily weight gain (ADG) of S and A animals in R group was 0.34 and -0.25 g/d (0.02 and -0.02 $g/kg^{0.75}/d$), respectively. While that of S and A animals in C group was 174.4 and 194.4 g/d (10.16 and 11.48 $g/kg^{0.75}/d$), respectively. Nitrogen (N) was determined by both measured and regression methods. Animals of R group stayed at about zero N balance (0.01 and -0.00 g $N/kg^{0.75}/d$ for S and A animals, respectively). The N retention of animals of both S and A breeds in C group were similar (0.45 and 0.46 g $N/kg^{0.75}/d$, respectively). Digestible organic matter intake (DOMI) and ME requirement for maintenance (MEm) were measured by both constant weight technique and regression method by regressing N balance on DOMI and ME intake on ADG. The measured DOMI during constant weight was 24.61 and 24.27 g $DOMI/kg^{0.75}/d$ and the calculated DOMI from regression equation was 24.24 and 24.22 g $DOMI/kg^{0.75}/d$, for S and A animals, respectively. The measured MEm was 402 and 401 kJ $ME/kg^{0.75}/d$ and the calculated MEm from regression analysis was 398 and 400 kJ $ME/kg^{0.75}/d$ for S and A breeds, respectively. There were no significant differences between both measured and regression techniques. There was no significant difference between S and A breeds for DOMI, N retention, MEm, digestibility and metabolizability values. Digestibility values for OM, GE and CP and metabolizability were significantly (p<0.05) higher in restricted feeding sheep compared with that of sheep fed ad libitum.