Effects of extended grazing during mid, late or throughout pregnancy, and winter shearing of housed ewes, on ewe and lamb performance

Abstract

A flock of March-lambing ewes was used to evaluate the effects of (i) extended (deferred, winter) grazing of pasture during mid, late or throughout pregnancy, and (ii) winter shearing of ewes housed during mid and late pregnancy, on lamb birth weight and subsequent growth to weaning. Ewes (n = 265) were allocated at random to five treatments for the period from 7 December (~ day 47 of pregnancy) to lambing. The treatments were: housed shorn (HS), housed unshorn (HU), grazing throughout (EG), grazing to 20 January followed by housing (EGH), housed to 20 January followed by grazing (HEG). From 1 March to lambing the HEG and EG ewes were dispersed on the paddocks intended for grazing post lambing. All ewes were offered a concentrate supplement during the final 6 weeks of pregnancy. Housed ewes were offered grass silage while ewes on extended grazing were allocated 1.3 kg herbage dry matter per head per day from swards that had been closed for approximately 10 weeks. Ewes plus lambs (except triplet-rearing ewes which were grazed separately) from all treatments were grazed together post lambing, grouped according to lambing date. For treatments HS, HU, EGH, HEG and EG gestation lengths were 147.0, 145.6, 146.3, 146.6 and 146.9 (s.e. 0.34, P < 0.001) days, lamb birth weights were 4.9, 4.3, 4.4, 4.6 and 5.0 (s.e. 0.10, P < 0.001) kg, and lamb weaning weights were 34.6, 32.1, 33.3, 33.8 and 34.9 (s.e. 0.66, P < 0.001) kg, respectively. Extended grazing in mid and late pregnancy resulted in 35% and 65%, respectively, of the increase in lamb birth weight associated with extended grazing throughout. Treatment effects on lamb birth weight were associated with those on weaning weight (P < 0.01, R2 = 0.93). It is concluded that extended grazing or shearing of housed ewes increased lamb birth weight and subsequent weaning weight. The increased lamb birth weight from deferred grazing in mid pregnancy was probably due to improved protein utilisation from the grazed herbage. Meanwhile, the increased