A low cost system for producing early lambs is desirable both to capitalise on Hawkes Bay's climatic advantage of a mild winter and to minimise the feed deficit that results from a dry summer. A series of trials was undertaken to develop a system for using the "ram" effect to stimulate early oestrus (December/January) in otherwise anoestrous Romney ewes. When Poll Dorset rams (5%) and oestrous Poll Dorset ewes (10%) were joined with Romney ewes in early December, more ewes were mated after 3 cycles than when Poll Dorset rams alone were joined. The effectiveness of social facilitation appeared to vary between years with between 39 and 97% of Romney ewes mated after 3 cycles to an early December joining. The social facilitation effect also appeared to be less marked at a January joining. Romney ewes with a range of lambing rates from May to September and mean lactation periods of 12 weeks were weaned at varying intervals (12. 6 and 0 weeks) prior to ram introduction in early December. There was no difference in rate of onset of oestrus. Mean lambing dates for 90% of the ewes in the 3 groups were 18th, 12th and 10th June, respectively. There was no effect of live weight or body condition on date of first oestrus. Romney ewes were exchanged between Hawkes Bay and Wanganui to examine the effect of genotype and environment on the onset of oestrus. Poll Dorset rams (6%) were introduced on 30th November. By the 27th January 87% of ewes had been mated at the Hawkes Bay site compared with 15% at Wanganui. At each location there was little difference between ewes from the two sources in rate of onset of oestrus.
Proceedings of the New Zealand Society of Animal Production, Volume 49, , 233-236, 1989
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