Animal breeding / by Laurence M. Winters.

  • Winter, Laurence Merriam, 1891-
Date:
1925
Catalogue details

Licence: Attribution-NonCommercial-NoDerivatives 4.0 International (CC BY-NC-ND 4.0)

Credit: Animal breeding / by Laurence M. Winters. Source: Wellcome Collection.

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    242 FERTILITY fertile than their wild sisters. Mere captivity, on the other hand, generally results in a decided loss of fertility and fre¬ quently in complete sterility. Even in his native country and when kept in a good state of nutrition, the elephant seldom breeds in captivity. Lions, tigers, and bears rarelj^ breed in capti\âty. On the other hand, foxes are now more prolific in captivity than in the wild. They, however, are becoming domesticated (Fig. 28), and their environment is more favorable than that usually accorded to lions, tigers, and bears. As a general rule, it may be stated that captivity results in a decrease of fertility and domestication in an increase; this, however, has its exceptions as above mentioned—the elephant. 227. Lactation and Fertility.—Prolonged lactation fre¬ quently, although not necessarily, exerts an inhibitory influ¬ ence on oestrum and hence on fertility. It has been the experience of dairymen that cows being hard pushed for records often fail to breed regularly. It is unusual for a sow to breed until her pigs are Aveaned. Some mares will breed only every two years, undoubtedly being restrained Ьз^ the period of lactation. Beef breeders frequently experi¬ ence difficulty in getting cows with calf when they Ьал'е calves at side; but if the calves are kept in, being allowed to nurse only twice daily, the cows are usually bred with far greater ease. The difference in this case may be due to a lessening of the drain on their systems, and again it may be that with the calves at side their attention is so absorbed that they fail to come in heat. 228. Longevity and Fertility.—Generally, long-lived indi¬ viduals experience a longer i)eriod of fertility than short¬ lived ones. Therefore, anything—such as early breeding, improper feed supply, unfavorable environment, or over¬ work—that impairs the female's constitutional strength and bodily development, in consequence shortens her life and as a result lessens her fertility.