Seasonal variation in ovarian and oestrous activity of tropical Menz sheep as affected by plane of nutrition.
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Forty-eight mature Ethiopian Menz ewes were used to investigate the effect of nutrition on oestrous and ovarian activity in tropical sheep. A control group was fed on hay alone and a high nutrition group received an additional 400 g per ewe per d of a concentrate feed providing 263 g crude protein/kg dry matter and 10.5 MJ metabolisable energy/kg dry matter. Half the ewes in each group interacted with harnessed vasectomized rams to detect oestrus, which was also verified by weekly plasma progesterone assays. Ram presence did not depress feed intake or liveweight gain (P > 0.05). The mean percentage of ewes showing oestrus at least once a month was very high (95%) and there was a marked reduction in sexual activity from June to September, the wet season. Only 79% of ewes cycled in August and the number of heats per ewe per month dropped to 1.3 during this wet season in contrast to 1.9 the rest of year. Ewes came into oestrus 21 times (range 18-23) a year with no significant effect of level of nutrition. Mean cycle duration was 17.9 +/- 8.7 d; 22% of cycles were short (< or = 13 d), 56% normal (14-19 d), 11% long (20-26 d), 8% silent or missed (27-40 d) and 3% represented anoestrus (> or = 40 d) with no major difference due to nutrition level. Individual animal progesterone profiles revealed that ewes failing to show oestrus had experienced increased silent ovulations. Forty percent of undetected heats were from the same animals. We concluded that, although Menz ewes are year-round breeders, they experience an apparent reduction in sexual activity from June to September, which appears to be independent of the level of nutrition, but might influence their breeding activity and flock production.