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

Abstract

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.

The effect of plane of nutrition on the body composition of two breeds of wearier sheep fed a high protein diet

“…The difference in fatness was possibly related to changes in the absorption and utilization of energy and protein with plane of nutrition. The metabolizable energy value of the feed was higher at the lower intake (Graham & Searle, 1982). Studies of digestion under closely similar conditions (Margan et al 1982) showed that a greater proportion of the dietary protein was degraded in the rumen at the low intake than at the high, resulting in the ratio of protein absorbed in the intestines to metabolizable energy being smaller at the low intake.…”

“…Studies of digestion under closely similar conditions (Margan et al 1982) showed that a greater proportion of the dietary protein was degraded in the rumen at the low intake than at the high, resulting in the ratio of protein absorbed in the intestines to metabolizable energy being smaller at the low intake. At both planes of nutrition the diet provided an excess of protein (Graham & Searle, 1982) and this was probably oxidized to provide energy but in a less efficient manner than the other nutrients (Milligan, 1971). Calculation of metabolizable energy available for growth using the observed metabolizable energy intake (Graham & Searle, 1982) and an estimated maintenance requirement (Commonwealth Agricultural Bureaux, 1980) showed that the low plane sheep received slightly more metabolizable energy for growth from 20 to 30 kg than the high plane sheep.…”

“…An opportunity to examine the question further arose in an experiment designed to provide data on the partition of nitrogen and energy between body tissues and wool in two breeds of sheep which differed markedly in wool production: Dorset Horn and Corriedale. This paper describes the effect of breed and diet on body composition in that work; the other aspects are described elsewhere (Graham & Searle, 1982).…”

Nutrient partitioning in Merino rams with different wool growth rates

“…The sheep were shorn at the end of the experiment, and total fleece growth rates calculated for each experimental period from the ratio of re-growth of mid-rib fleece patches and total fleece mass (Graham and Searle, 1982). The sheep were shorn at the end of the experiment, and total fleece growth rates calculated for each experimental period from the ratio of re-growth of mid-rib fleece patches and total fleece mass (Graham and Searle, 1982).…”

“…Much of the above indirect evidence suggests that high producers possess a more efficient metabolism and could thus be expected to exhibit a higher retention of nutrients. Comparing the Dorset Horn and Corriedale breeds, Graham and Searle (1982) found that the Corriedale diverted appreciably more energy to wool growth, and suggested that this may be linked to the incidence of retarded growth rates in lambs. Despite the significant practical implications of distinguishing between these two measures of efficiency, no clear distinction has been made in the available literature on the subject.…”