An experiment was conducted in which 1402 Romney ewes, aged 2 to 5 years, were treated with intravaginal controlled internal drug releasers (CIDR) followed by 1 of 2 pregnant mares serum gonadotrophin (PMSG) preparations (Pregnecol(r), 530 iu v Pharmochem(r), 460 iu) for mating on 1 of 3 dates (December 19, 28 or January 3). Of the 1402 ewes joined with the ram, 71% conceived to the first cycle. Conception rates, proportion of ewes conceiving multiples and foetuses per ewe joined were assessed by ultrasound pregnancy diagnosis at day 50 of pregnancy. Reproductive performance was highest in 2-year old ewes but not influenced by previous reproductive history (wet v wet/dry in the previous spring). All measures of reproductive performance declined at later mating dates compared with those at December 19. Substantial premiums were obtained for early-born lambs, and for their dams when slaughtered at weaning the following September. Out-of-season lambing systems appear to offer economic benefits in areas characterised by good winter and spring growth rates but dry summers.
Proceedings of the New Zealand Society of Animal Production, Volume 48, , 69-70, 1988
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