Female and Breed Associated with Seasonal Fertility Variation in Dairy Cattle

Summary Seasonal variation in breeding efficiency was studied, using breeding records of dairy cows inseminated in Arizona with semen collected in three different climatic areas (Palo Alto, California; Columbus, Ohio; and Phoenix, Arizona). Breeding records of cows inseminated outside the state of Arizona at the same time with semen from the same bulls from Palo Alto were used for comparison. Two main seasonal depressions in fertility were found to occur during the year. Pooling data from all breeds (Jerseys, Holsteins, and Guernseys were studied), fertility dropped 3.4 percentage points during the winter and 12.4 percentage points during the hot summer months. These factors point to the female as the major contributor to the summer depression: (1) Seasonal depressions in fertility are comparable for all sources of semen used in Arizona. (2) Cows bred outside of Arizona during the same period and with semen from the same bulls showed a high rate of fertility during the summer months contrasting with the lowered breeding efficiency within the state. Differences in seasonal fertility were found according to breed, also. The Jerseys show no depression during the summer months, whereas both Guernseys and Holsteins show deep declines in fertility. According to the data, the Guernseys have an equivalent low level during the winter, whereas a significant but somewhat lesser depression occurs for the Holsteins as well.