Seasonal Variation in Semen Quality of Some Missouri Dairy Bulls

The seasonal nature of sexual activity of many mammals is well recognized, but whether or not the bull has a definite seasonal sexual cycle has not been clearly demonstrated.Morgan and Davis (8) found that the conception rate of Nebraska dairy cattle was lowest in August and September, but months with high conception rate were not seasonal since January and July were both high and December was highest in conception rate.Erb, Wilbur and Hilton (5) reported a seasonal variation in breeding efficiency with spring (May at maximum) giving the best conception rate and late summer (August at minimum) giving the lowest conception rate.The separate effect of the bull and the cow upon fertility are confounded in ordinary analysis of breeding records, however.Few studies have been made of seasonal variations in semen quality.Weatherby, Reece and Bartlett (11) reported monthly averages of semen volume, concentration, and longevity for six bulls in a New Jersey cooperative.Longevity, which is correlated with fertility (10), was generally higher in June and July than in other months, but the differences among the months were very slight.Anderson (1) presented monthly average semen volume and motility observations made on bulls in Kenya which indicated a rather distinct seasonal tendency.Volume and motility were both low from May to August.Erb, Andrews and Hilton (4) made a systematic study of the effect of season upon semen characteristics.Except for a noticeable increase in concentration in the spring, seasons other than summer had little effect upon semen quality.During July, August, and September the motility and semen volume were lowered, the sperm survived for a shorter period, and the percentage of abnormal forms was increased.Statistical analysis showed that semen obtained during the summer months was significantly of lower quality than that of other seasons.Lasley (7) found that beef bulls in Arizona gave higher quality semen as the season progressed from May to September.Volume of semen, percentage of live sperm, and percentage of resistant sperm increased during this period but concentration and percentage of abnormal sperm did not change.The literature in general is not in complete agreement concerning the effects of season upon semen quality of bulls.The explanation of this fact may be that the effect of season is not as great in some localities as in others.