Groups of Coopworth ewes were mated to Polled Dorset rams at 2 monthly intervals for 12 months following induced ovulation and synchronised oestrus. The ram lambs were made cryptorchid and grown to a target slaughter liveweight of 40 kg. The influence of birthdate on birthweight, pre- and post-weaning growth rate, time from birth to slaughter and carcass leanness was studied. The major influence on lamb performance was the timing of lambing in relation to spring pasture growth and the subsequent summer period, characterised by low pasture growth and quality. When late gestation was during summer lamb birth weight was lower (3.5 and 3.6 kg for January and March born lambs respectively) than for other periods (4.3, 4.4, 4.8 and 4.5 kg for May, July, September and November born lambs respectively). Similarly, pre- and post-weaning growth rates were higher during spring (pre-weaning maximum, 276 g/d for lambs born in July; post-weaning maximum 223 g/d for lambs born in May) and lowest during summer (pre-weaning minimum 150 g/d for lambs born in November, post-weaning minimum 123 g/d for lambs born in September). Age at slaughter was lowest for lambs born in May and July and highest for lambs born in September and November. When corrected to an average liveweight of 42 kg the average age was 230, 208, 176, 175, 245 and 236 days for lambs born in January, March, May, July, September and November respectively. These data suggest that lamb growth to these comparatively high slaughter weights was optimal when lambs were born in May and July. If premiums are offered for out-of-season production of heavy carcasses to supply the chilled meat trade, May appears to be the optimal time for lambing as the average slaughter date for this group was 27.10.87, compared to 12.01.88 for lambs born in July.
Proceedings of the New Zealand Society of Animal Production, Volume 49, , 237-244, 1989
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