Some Factors Affecting Reproduction

The combined evidence from many sources indicates that a mating has the greatest chance of being successful if it takes place near the time of ovulation. In the various types of farm animals, recommended times of mating where hand mating is practiced are :

Horses—If mated only once, the third day of estrus appears best, on the average. If service can be had more than once, the best practice appears to be to mate on the third day, and on every second day thereafter until the end of estrus.

Cattle—If bred once, mate 12 to 20 hours after onset of estrus. If bred twice, mate immediately after onset of estrus and again 12 to 20 hours later.

Sheep—During the second half of estrus. If feasible, mate about 12 hours after estrus begins and at 12-hour intervals until estrus ends. Estrus lasts about 30 hours, on the average.

Goats—During the second half of estrus. Exact data are not available, but the duration of estrus is similar to that in the ewe.

Swine—Late on first day, or preferably on second day of estrus.

We conducted studies at the Utah Agricultural Experiment Station to determine if giving special feed to range lambs would influence sexual development and reproduction. The first winter is perhaps the most critical time in the development of the range ewe. Up to weaning, the lamb is provided with a reasonably adequate diet in most cases, while with its dam on spring and summer range. When the lamb is weaned and moved to winter range, there is often a decided drop in level and quality of nutrition, and this is accompanied by more severe environmental conditions in other respects.

Results of the study in Utah indicate that when ewe lambs are given special feed during the first winter the reproductive tract develops more fully, as compared with development in ewe lambs maintained on open range. These results, coupled with results of earlier work by A. C. Esplin, M. A. Madsen, and the writer in which larger lamb crops were produced at 2 years of age as a result of lot feeding during the first winter, indicate the desirability of giving special attention to the feeding of ewe lambs in range flocks. How far the rancher can afford to go in giving special attention to his ewe lambs is a problem needing further investigation.

The environment in which animals live may affect the fertility of livestock. An experiment has been conducted cooperatively by the Bureau of Animal Industry and the Florida Agricultural Experiment Station which gives some clear experimental evidence on this point. Thirty pairs of Columbia ewes and two pairs of rams were selected from the Department's flocks at the Dubois station. One member of each pair was retained at Dubois, and the other sent to the North Florida Experiment Station at Quincy. Wool production of the ewes at Quincy was comparable in grease weight and staple length to that of the ewes at Dubois, but the level of fertility as measured by percent of ewes lambing was not as high. This deficiency was especially marked during the first two years the ewes were at Quincy. Columbia ewes descended from those brought from Dubois and raised at Quincy also reproduced at a lower level and had smaller lambs at weaning than Columbia ewes at Dubois, but there was no noticeable reduction in wool production.

Most sheep and goats breed naturally during the fall and early winter months. In some instances, there would be an economic advantage in having part or all of the ewes and does in a flock bred during the spring or summer months. The possibility of stimulating estrus and ovulation in ewes and does during the spring and summer has received much attention in recent years. It has been possible to induce ovulation in a high proportion of the animals by the use of gonadotropic hormones, but induction of estrus in conjunction with ovulation has been quite erratic. No satisfactory explanation of these variable results has been found.

Fertility, as measured by percentage of conceptions, is generally lower in animals in which estrus has been induced or in those force-mated after induction of ovulation, than in animals bred during natural estrus. However, satisfactory fertility has been reported in some cases. Further work is needed to determine the endocrine physiology of normal and experimentally induced estrus in sheep and goats and the specific doses and time sequences that will induce estrus and ovulation, before procedures can be recommended for general use in practice.

THE AUTHOR

Ralph W. Phillips was formerly in charge of genetics investigations in the Bureau of Animal Industry. He took leave during 1943 and early 1944 to help the Governments of China and India with their livestock problems. Dr. Phillips is editor of the Journal of Animal Science and is the author or coauthor of about 100 publications on various phases of animal breeding. He is a graduate of Berea College and took graduate work at the University of Missouri.