Animal Nutrition and Feed Technology

Average and marginal efficiency of utilization of N and energy by breeding ewes from 75 d of gestation was studied by feeding varying levels of concentrate supplement (CS) to guar straw in 1 (EG₁) 2 (EG₂), 3 (EG₃) and 5 (EG₄) multiples of recommended requirement. Control group (CG) was kept alone on guar straw. Guar straw intake was reduced (P<0.01) with higher level of CS in EG₄. Average and marginal efficiency of N retention was higher in EG₁ and energy retention was higher in EG₃. Optimum level of CS was in the range of EG₁ to EG₃, beyond which N and energy retention by ewe was inefficient. No significant difference in the net protein and energy retention by the foetus during subsequent fortnight from 90 d of gestation. Although N and energy retention by ewe was negative in CG, foetal protein and energy retention was not affected. Actual birth weight of the lambs was lesser than the predicted birth weight in CG, EG₁, EG₂ and EG₄ but contrary was true for EG₃ that may be due to higher average and marginal efficiency of N and energy utilization by ewe. Maternal weight loss in CG and EG₄ indicated that both under and over feeding will not yield better lamb weights. In conclusion, threshold level of CS required to breeding ewe was 100 g/d. CS of 300 g/d on fresh basis was optimum for higher marginal efficiency of maternal N and energy retention beyond which there was inefficiency in maternal system.