Effect of herbage allowance during pregnancy and lactation on feed intake, milk production, body composition and energy utilization of ewes at pasture

Abstract

SUMMARYIn two experiments Dorset ewes were offered low (LP) or high (HP) herbage allowances during pregnancy to induce light (45–50 kg) or heavy (60–65 kg) post-partum body weights, respectively. During the first 6 weeks of lactation three herbage allowances were offered in a 2 x 3 factorial design: 2 (LL), 5 (ML) and 8 (HL) kg D.M./ewe per day. In the first experiment (Expt 1) lambs were removed and ewes machine-milked twice daily. In the second (Expt 2) they were suckled by two lambs; a further group on the ML allowance (MML) was machine-milked. Ewes were slaughtered during early pregnancy, post-partum and after 6 weeks of lactation for determination of body composition. Diet selection and faecal output were measured during lactation.Absolute level of milk production was not affected by pregnancy nutrition when rearing two lambs and producing 2·0–2–7 kg milk/day or at 30–50% lower milk production when machine-milked. When expressed as g/kg W0·75 per day milk production was 14% greater in LP than in HP ewes. Milk fat content was higher at low allowances and was positively related to rate of body fat mobilization. Milk protein concentration was lower in suckled than in machine-milked ewes (40 ν. 47 g/kg) though total milk protein secreted (g/day) was similar. Milk production was 11–13% greater in ewes bearing twin lambs compared with those bearing singles whether machine-milked or suckling two lambs.Mean intakes during the 6 weeks of lactation were 58, 69 and 73 g DOM/kg W0·75 per day, respectively, for LL, ML and HL groups when machine-milked and 51, 75 and 83 g DOM/kg W0·75/day, respectively, for ewes rearing two lambs. LP ewes consumed 13% (Expt 1) and 17% (Expt 2) more feed (g DOM/kg W0·75/day) during lactation than HP ewes, though in absolute terms (kg D.M./day) there was no difference. The difference in OMD between herbage offered (OMD = 0·77) and consumed ranged from 0 when sheep grazed to a residual herbage mass of 910 kg D.M./ha to + 0·04 when residual herbage was < 1600 kg D.M./ha.All sheep, irrespective of pregnancy nutrition, milk production or herbage allowance, were in negative body energy balance during lactation. Mobilization of body fat during lactation averaged 40 g/day for machine-milked and 150 g/day for suckled ewes and was greatest in HPLL groups (92 and 219 g/day in Expts 1 and 2, respectively). In LP groups body protein loss during pregnancy was 10 and 16 g/day in Expts 1 and 2, respectively, and little further net protein loss occurred during lactation. The maximum rate of body protein loss in HP groups was 30 g/day in ewes rearing two lambs on the low herbage allowance.Maintenance energy requirement during lactation averaged 0·238 MJ ME/kg W per day for ewes in both experiments; efficiency of use (Kl, (t)) of total energy available above maintenance for milk synthesis (namely, milk energy/ME intake - estimated energy requirement for maintenance (MEm + mobilized tissue energy) decreased from 0·84 to 0·51 with increasing rate of tissue energy mobilization. There was a positive relationship between Kl(t) and the proportion of mobilized energy derived from body protein.The overall energy requirement of lactating ewes well-fed during pregnancy was similar to previous estimates (Agricultural Research Council, 1980) but that of sheep undernourished during pregnancy was 10–20% greater.