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Environmental Sources of Variation in the Gestation Length of the Horse

University of California¹

Abstract

An analysis of the environmental sources of variation in the length of the gestation period of Arabian mares has been made.

Season of breeding is the most important source of variation in gestation length. It accounts for 44 percent of the total variance. Pregnancies resulting from breedings during the period December through May on the average were 10.4 days longer than those resulting from breedings during the period June through November.

Level of nutrition of the mare accounted for five percent of the total variance. Pregnancies of "well fed" mares averaged 4 days shorter than those for mares on a "maintenance ration."

Seasonal variation in gestation length was found to be independent of the level of nutrition of the mare, thus implying that there is some important factor associated with season other than or in addition to level of nutrition of the mare that is operating to produce the observed seasonal variation. The Authors briefly discuss the possibility that this factor may be the seasonal variation in the length of daylight.

The influence of the stallion and other sources of genetic variation in the gestation length of the horse is discussed by the Authors in a companion paper to this (Rollins and Howell, 1951).

¹ Davis, California.