A study of the protein requirements of the mature breeding ewe

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Abstract
1. An experiment was carried out in which protein utilization in the pregnant ewe was studied using the nitrogen balance technique.2. Eight diets supplying four different intakes of crude protein and two different intakes of energy were each offered to eight individually penned ewes.3. The mean crude protein intakes per day were 7·2, 5·5, 4·1 and 3·0 g/kg W0·73 (where W = body-weight) and the metabolizable energy intakes 134 and 113 kcal/kg W0·73.4. N balances were carried out at 10–12, 14–16 and 18–20 weeks of gestation on five ewes from each treatment.5. The apparent digestibility of both dry matter and crude protein decreased with decreasing protein intake. With the high energy intake, the apparent dry-matter digestibility was increased and the apparent digestibility of crude protein decreased. Stage of gestation had no significant effect on the apparent digestibility of either of these constituents.6. N retention was not affected by the number of foetuses carried. With the higher energy intake and the higher protein intakes, the absolute retention of N was significantly increased at all stages of gestation. N retention increased with advancing pregnancy; the retentions at 10–12, 14–16 and 18–20 weeks of gestation being 0·086, 0·114 and 0·163 g/kg W0·73 per day respectively.7. The efficiency of utilization of apparently digested N was calculated from the regression of retained N as a percentage of apparently digested N against apparently digested N.8. The daily intakes of apparently digested N required for maximum efficiency were 0·551 and 0·620 g/kg W0·73 on the high and low energy intakes respectively. The daily intake for maximum efficiency decreased with advancing pregnancy, the values being 0·623, 0·587 and 0·567 g/kg W0·73 for the 10–12, 14–16 and 18–20 weeks of gestation respectively.9. The levels of N retained at maximum efficiency were 0·235 and 0·202 g/kg W0·73 per day for the high and low energy intakes respectively. The levels of N retained increased during pregnancy from 0·170 g/kg W0·73 per day at 10–12 weeks to 0·286 g/kg W0·73 at 18–20 weeks. The requirements for zero N balance were 0·072 and 0·153 g apparently digested N/kg W0·73 per day for the high and low energy intakes respectively. The requirement for zero N balance decreased from 0·176 g/kg W0·73 per day at 10–12 weeks to 0·071 g/kg W0·73 at 18–20 weeks.10. The results are discussed in relation to other research findings and current recommendations.