Fertility Level in Artificial Breeding Associated with Season, Hours of Daylight, and the Age of Cattle

Under semi-feral conditions where bulls remain with the cow herd the year around, mating takes place during late spring and summer months.While animals might breed at other times of the year, there is an optimum breeding period when the reproductive powers are at a maximum.Dairy cows under farm conditions may breed the year around, though there is a seasonal peak of calvings in the spring in most areas.In New York State dairymen complain about the difficulty experienced in.getting cows with calf during winter so asto have them freshen in the fall, especially in the case of virgin heifers and old cows.Furthermore, reports have been more frequent recently of artificial breeding organizations experiencing difficulties in collecting semen of satisfactory quality from some bulls during winter.In studies conducted by Salisbury (8) and by Mercier and Salisbury (5), data on 16 of the 40 bulls could not be used for analyses because fertility data were not available for at least 11 months during the year.The fact that of these 16 bulls 5 were young (36.6 ± 20.8 month of age) and ]1 were aged bulls (94.4 ± 14.8 months of age) raised the question of whether the fertility level of bulls of extreme ages might be influenced more readily by seasons than that of bulls of medium age.None of the publications reviewed earlier (6), nor the paper by Trimberger and Davis (11) dealing with artificial breeding, considered the possibility 6f differential responses of animals of varying ages to the cyclic climatic changes which occur throughout the year.The diverse results of published studies concerning the effect of season on reproduction in cattle, whether for natural or artificial breeding, do not enable one to determine if an age differential is involved.The interaction between the age of bulls and cows in determining fertility level, as reported by Tanabe and Salisbury (10), is difficult to interpret directly on physiological grounds.If an ageby-season interaction for fertility level were demonstrated, their results might be Shown to be due to variations in proportion of individuals of the several age classes bred during the different seasons of the year.