Energy and nitrogen utilization for body growth in young sheep from two breeds with differing capacities for wool growth

Utilization of energy and nitrogen was assessed by serial slaughter and intake/loss balances, and wool growth was measured, as young sheep grew from c. 25 to 30 kg on either 700 or 1000 g/day of a high-protein diet. Comparisons were made between animals with low (Dorset Horn) and high (Corriedale) propensities to produce wool. Depending on feed intake, fleece-free liveweight gain was 102-215 g/day in the Dorsets and 88-172 g/day in the Corriedales; corresponding growth rates of clean dry wool were c. 5 and 10 g/day, there being little effect of feed intake. Digestibilities of protein and energy were 84 and 74% respectively in both breeds at the lower level of feeding; at the higher level the values were 80 and 71 % in the Dorsets and significantly lower, 77 and 69 %, in the Corriedales. Metabolizable energy was 83 ¦ 0.3 % of digestible energy at the low intake and 86 ¦ 0.5 % at the high intake in both breeds. It averaged 11.7 ¦. 0.07 MJ/kg feed dry matter, being 0.2 MJ/kg more at the low than at the high intake and, in the latter case only, 0.3 MJ/kg more in the Dorsets than in the Corriedales. At the low and high levels of feeding respectively, the fleece gained 0.18-0.16 MJ and 0.93-0.89 g nitrogen daily in the Dorsets compared with 0.31-0.34 MJ and 1.65-1.81 g nitrogen daily in the Corriedales. The corresponding average daily retentions in the body were 1.74-2.46 MJ and 1.80-3.49 g nitrogen in the Dorsets and 1.34-2.08 MJ and 1.25-3.07 g nitrogen in the Corriedales. Thus body growth accounted for 91-94% of the energy retention and 6640% of the nitrogen retention in the Dorsets, but only 81-86 % and 43-63 % respectively in the Corriedales. The composition of weight gain was the same in both breeds and 15-20% of the energy stored in the body was in protein. It is estimated that the marginal efficiency of use of metabolizable energy was 31-34% for body growth and 16-19% for wool growth. It is concluded that, given a high protein diet, animals with a strong propensity to grow wool diverted an appreciable amount of metabolizable energy from body growth to wool production, thereby restricting deposition of both fat and protein.