The ratio of light to dark during a day and the absolute periods of light and dark are known to influence reproduction in many species, especially the seasonal breeders. Ewes differ from cattle in that they are anestrous, which means very few come into heat (estrus) between May and July. The decrease in the light-to-dark ratio triggers hormonal change which result in ewes exhibiting estrous cycles. The two months when most ewes show estrus are October and November (temperate zone), and the fewest is between May and July. When ewes first come into estrus in August or September, they do not produce as many eggs (oocytes) and often the eggs are incompetent of normal development. Since there are very few reports of identical twins in sheep (the result of one fertilized egg dividing), two eggs must be ovulated if twins are to be conceived and born. The incidence of twinning is higher in October and November than at other times of the breeding season. There is also a higher occurrence of embryonic death in August and early September than there is later in the fall. October through early November is the ideal time interval to breed for a larger lamb crop.
Ewes normally will complete an estrous cycle every 16 to 17 days until they are bred or enter the anestrous season or period. Ewes are normally receptive to a ram for 24 to 48 hours. Once conception (fertilization) occurs, the ewe will deliver a lamb between 144 to 152 days after mating.
Knowing the pregnancy status of ewes has obvious advantages. The pregnancy rate is the only true measure of flock fertility and is the basis for strategies of nutrition and feeding, utilization of labor, and facilities. Improper management of pregnant ewes often leads to pregnancy toxemia from substandard nutrition, and dystocia and fetal death due to inadequate lambing oversight, especially in bad weather or when adequate shelter is not available.
Induction and synchronization of estrus during anestrus enables some ewes to lamb three times in two years. Producing lambs out-of-season and finishing market lambs in larger and more even groups can improve profits. The induction and synchronization of estrus during seasonal anestrus is practiced by some producers in most of the major sheep-producing nations.
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