Dairy Pipeline: February 2005
Raymond L. Nebel
Dairy Extension Coordinator
Reproductive Management Scientist
Artificial insemination is a powerful tool for genetic improvement that enhances the value of bred heifers. A longer duration of estrus and more mounting activity makes the detection of estrus in heifers easier than in lactating cows, thus heifers are generally more sexually active than cows and easier to catch in estrus. For optimum fertility, heifers should have three or more estrous cycles before the breeding season begins. If the goal is to average 24 months of age at first calving, then on average heifers must be inseminated between 13 and 14 months of age. Therefore, for optimum fertility, heifers should reach puberty by 10 months of age. Puberty in heifers is a function of breed, age, and weight. Most heifers will begin cycling when they reach approximately 60% of the expected mature body weight (550 to 600 lbs for Jersey and 750 to 800 lbs for Holstein heifers). An adequate nutrition and development program is critical for success in a heifer AI program. In general, insemination after a detected estrus is usually associated with higher conception rates compared with fixed-time AI following synchronization of estrus or ovulation. There is no "best" time to observe heifers for standing activity but movement of heifers prior to visual observation appears too enhanced or stimulates standing activity. Insemination should occur within the first 16 hours after the onset of standing activity and usually heifers will still be in "standing" estrus during the actual deposition of semen. While conception rate (number pregnant divided by the number inseminated) is often the focal point of discussion, pregnancy rate (number pregnant divided by number in the group or the number available for insemination) is a more accurate assessment of success. Pregnancy rate accounts for heat detection rate (efficiency) in a non-synchronized group and for synchronization response rate in a synchronized group combined with the conception rate, thus pregnancy rate is the conception rate times the service rate. In a timed or appointment AI program the conception rate and pregnancy rate are equal because all heifers are presented for insemination at the appointed time regardless of estrus. Dairy heifers can be successfully synchronized using CIDR based protocols. In a recent study we published in the Journal of Dairy Science (Peeler, I.D., R.L. Nebel, R.E. Pearson, W.S. Swecker. and A. Garcia. 2004. Pregnancy rates after timed AI of heifers following removal of intravaginal progesterone inserts. J. Dairy Sci. 87:2868-2873). A pregnancy rate of 60.1% was obtained after the first synchronization cycle with timed AI following two different CIDR based protocols. An overall pregnancy rate of 93.3% was obtained after two synchronization cycles. These protocols offer the opportunity of fixed-time AI without estrus detection. Although conception rates are slightly reduced after synchronization, the overall reproductive performance is usually improved because of the insemination of all heifers not just the heifers detected in estrus.