Previous articles entitled “Benefits of Pregnancy Testing in Cow-Calf Herds: Part 1 & 2” discussed the importance of using pregnancy percentage to compare the reproductive efficiency of a cow-calf operation to other similar operations. In order to determine pregnancy percentage, most often producers utilize a veterinarian to stage pregnancies by rectal palpation or ultrasound. Other options for pregnancy diagnosis exist (e.g. blood tests, observing heats, etc.) but these methods do not provide any information other than “pregnant” or “open”. Although useful, pregnancy percentage alone does not adequately describe the efficiency of the breeding season. Using a veterinarian for pregnancy diagnosis also provides an estimate of when the cow was bred within the breeding season. Information on when cows are bred within the breeding season is invaluable to improving the reproductive efficiency and overall productivity of a cow-calf herd.
There are numerous benefits to a calving season where calves are born in a tight time-line, compared to a calving season that is spread over many months. Concentration of human labor resources, decrease in risk of disease (i.e. calf scours), and improved uniformity of calves at time of sale are a few of these benefits. In order to achieve 95% total conception within an approximately 63-day breeding season, 65% of all cows eligible to be bred must conceive in each 21-day interval through the breeding season. It is important to note that if a cow conceives in the first 21-day interval, she is not eligible to be bred in the second 21-day interval. Figure 1 below provides an example distribution of breeding dates across a 63 day breeding season. The breeding season is broken down into 21-day intervals, because this is approximately the length of the estrous cycle for a cow.
In the example below, a total of 32 of 36 cows were pregnant, for a pregnancy percentage of 89%. In the first 21 days, 8 of 36 cows conceived (22%), leaving 28 cows eligible to conceive. In the second-21 days, 11 of 28 cows conceived (39%), leaving 17 cows eligible to conceive. In the third 21 days, 13 of 17 cows conceived (76%), leaving 4 cows open. In this example, neither the overall pregnancy percentage (89%), nor the distribution of conception across the breeding season are adequate. As can be seen, most cows conceived late in the breeding season, and it was only during the last 21-day interval that the target conception mark of 65% was met. This may have been due to poor body condition leading to infertility early in the breeding season, inadequate bull power, or early embryonic loss. Efforts should be made by the producer and their veterinarian to explore potential causes of failure to conceive early in the breeding season, in order to improve the overall reproductive performance of the herd.