The Oestrous Cycle and Its Control in Livestock

In nature it is the general rule that animals breed once annually and parturition occurs in the spring, the time most favourable to the progeny in that they grow up during the period of increasing light and warmth and also at the time when food for the mother is most abundant and adequate lactation is ensured. Under the conditions of feeding and housing provided by domestication the breeding season tends to be lengthened and some of our species, particularly the bovine, may breed at any time during the year; all domesticated animals, however, show a constant tendency to revert to the natural breeding season. For an animal to breed it must be mated and hence must attract the male and be sexually receptive, 'in heat' or oestrus. All domestic species show recurring periods of sexual receptivity, or oestrous cycles, which are associated with the ripening in the ovaries of one or more graafian follicles and culminating in the shedding of one or more ova. If a fertile mating occurs then pregnancy may ensue.