More and more producers are taking advantage of opportunities to incorporate targeted, high accuracy genetics using artificial insemination (AI) and estrous synchronization. While some may AI for two heat cycles, a majority will follow one round of AI with natural service bulls for the remainder of the breeding season. This brings up the question of how many bulls are needed for the remainder of the breeding season following an estrous synchronization program. If the AI program is successful, there should be correspondingly fewer females yet to be bred and fewer natural service sires needed However, some are concerned more bulls are needed to cover the subsequent cycle since return heats will be synchronized from the prior treatment.
There are several pieces of research data that shed light on this issue. First, females that return to estrus after a single fixed time AI do not all come back in heat on the same day, rather heats are spread out over 10 to 12 days. This is due to individual animal variation in follicular growth rates and patterns (2 vs 3 waves of follicular growth per cycle). Secondly, most bulls can generally can cover more cows than we think they can. Recommendations for using natural service in conjunction with estrous synchronization are to use bull to cow ratios of 1:15 to 25 in a smaller pasture. Only use bulls that have passed a breeding soundness exam, are agile and active, and are 2 to 4 years of age. Interestingly these ratios are similar to what the 2007-2008 National Animal Health Monitoring Survey reported as the US average for yearling (1:17) and mature bulls (1:23). In the limited data available comparing various bull to cow ratios after synchronization, pregnancy rates for ratios of 1:7 to 51 either did not differ or were only decreased slightly when bull power was stretched to 1:50.
The approach used with the research herd at Hays has been to assume we will get half of the cows pregnant to AI and turn out half the number of bulls for clean-up natural service as we would normally. For example if we had 100 cows in the group and we would normally use 4 bulls for natural service breeding only, then we would turn out two bulls following our AI program. Granted, in the case of smaller pastures or herd sizes, number of bulls may not be reduced since you can’t get ½ of a bull. Producers new to AI and estrous synchronization that lack confidence in the proportion of their cows that may conceive to AI may start with a low estimate for AI pregnancy.
Recently a meta-analysis using data from 33 published reports was conducted looking at final pregnancy rate with various bull to cow ratios after either natural service or estrous synchronization and AI. End of season pregnancy rates were similar for bull to cow ratios of 1:20 to 30 (87.8%) and 1:50 to 60 (89.2%) following estrous synchronization and for ratios of 1:20 – 30 (87.8%) with natural service. You can see that complete report here.
One of the challenges with this question or any related to bull to cow ratios is that a bull can pass a breeding soundness exam but still not sire any calves. At issue could be libido but social dominance is very important as well. In one study when calf sire was determined by DNA testing, five bulls sired 50% of the calves and 10 did not sire any in a single pasture with 27 sires. Other studies support this wide range in progeny numbers per sire in a multi-sire pasture. Parentage tests that are increasingly common in purebred cattle may help us learn more about these differences if the needed additional data can be collected.
Lack of sufficient bull power is expensive so the conservative response is to have a little more bull power than you think you need. In most cases, producers should be able to reduce the number of bulls used after estrous synchronization and AI.