During the past 50 years that AI has been practiced in the U.S. the fertility of virgin heifers has remained relatively constant at approximately 65% first service conception; whereas, the first service conception rate for lactating cows has decreased approximately 33% from 60 to 40%. Conception per insemination is the outcome resulting from a multitude of factors that interact in an intricate fashion. Female fertility, male fertility, environmental factors, and techniques used in AI are the four general categories that determine the ultimate outcome of conception per insemination.
Female fertility refers to any factors directly related to the heifer/cow that may alter her probability of becoming pregnant, including condition of the reproductive tract, nutritional status, changes in body condition from calving to insemination, age, and breed. Male fertility can not be controlled by the dairy producer; however, Estimated Relative Conception Rate or ERCR values calculated by the Dairy Records Management Systems in Raleigh, NC can be used to eliminate low conception rate sires (bulls with ERCR values less than -3).
There is a greater reduction in fertility during the summer for lactating cows than for non-lactating heifers. High milk yield intensifies the effects of heat stress on conception and is related to increased metabolic rates and decreased thermoregulatory ability for cows with high milk yield. When a cow is exposed to temperatures above 75°F her rectal temperature increases 0.03°F for each 2 lbs of fat-corrected milk she produces above 55 lbs per day.
The fourth category is techniques involved with AI and include accuracy of heat detection, timing of insemination, and semen handling and placement in the reproductive tract. What is normal? What should be expected as normal conception rates for dairy cattle? The mean first service conception rate for Virginia DHI herds over the past 12 months was 40 + 13%. The major factor influencing fertility in dairy herds is first calving. Heifers have conception rates close to theoretical optimal values. First calving reduces the conception rate 35 to 50%.
The question is why this tremendous reduction after parturition. Many herds experience at least 50% of cows calving having one or more postpartum disease. The uterine environment changes after the first calving, with 50 to 90% of cows experiencing some level of uterine infection. The conception rate in normal cows and the proportion of normal cows in a herd determine the conception rate ceiling of the overall herd. Factors such as body condition loss, heat detection errors, and less than optimum semen handling and placement, makes it readily apparent why many herds have conception rates less than 25%.
This article was originally published in Virginia Cooperative Extension Newsletter.