Multiple Pregnancy in Agricultural Animals

the capacity of certain species of agricultural animals to bear several young at a birth. Large animals (horses, cattle) usually bear a single offspring; twins are rare (in mares 0.5 percent, in cows 1–3 percent). There are only a few recorded cases of a mare bearing three or four foal or of a cow bearing three to seven calves. Smaller agricultural animals are more often multiparous. Swine are distinguished by the greatest multiparity: a litter usually contains ten to 12 shoats (sometimes up to 32). Sheep, which are usually uniparous, bear twins 15 to 30 percent of the time. Some breeds of sheep are multiparous; for example, Romanov sheep bear an average of two or three lambs per litter (sometimes eight or nine). Rabbits on the average bear five or six young per litter (up to 18).

Multiparity depends on the number of eggs fertilized in one sexual cycle and is genetically conditioned. It changes with age: multiparity is less common among young and old females. The occurrence of multiple pregnancy decreases if an animal is undernourished, improperly cared for, overworked, moved to a markedly different climatic region, or mated with a closely related individual. Remote hybridization often results in infertility.

When animals for whom uniparity is typical (such as cattle) bear two offspring—one male and one female—the female is infertile (freemartin) 85 percent of the time. This occurs as a result of the concrescence of the vessels of the fetal membranes of the embryos and the suppression of the female’s reproductive system by the male calf s hormones. Multiparous animal species have protective adaptations that prevent such concrescence; thus, the males and females of multiparous litters are fertile and are not inferior to the offspring of uniparous animals in terms of growth and productivity.

In sheep raising, injections of the blood serum of pregnant mares, which contains a gonadotrophic hormone, are sometimes used to induce multiparity. However, a more reliable method is to use the hereditary determination of multiparity and to reinforce this trait by selection and culling and by providing proper nourishment and care to animals with multiple litters. In breeding swine, sheep, and other animals, multiparity is one of the important characters selected.