Growing Beef Newsletter
May 2025, Volume 15, Issue 11
Utilizing estrous synchronization in the breeding season
Erika Lundy-Woolfolk, ISU extension beef specialist, and Beth Reynolds, IBC program specialist
Reproductive technologies in the cattle industry have continued to improve in effectiveness and ROI. One example is the growing number of options to synchronize the cow herd’s reproductive cycle. Time and improvements in technology have caused the list of advantages to expand, while reducing the cost and labor expense of utilizing a synch program. Because of the improved cost to reward ratio, more producers are utilizing synchronization protocols with natural service breeding programs.
One of the main advantages of utilizing synchronization technology is that it provides an extra opportunity for females to be bred during the first 21 days of the breeding season. Research has shown that on average, cows that have been exposed to bulls following estrus synchronization will conceive 2 weeks earlier than cows that did not go through synchronization.
By frontloading the breeding season, more calves will be born earlier in the calving season, resulting in older, heavier calves at weaning. Since age is the primary factor affecting weaning weight, this equates to more pounds sold and greater total value on sale day. Other benefits of a tighter calving window include reducing the risk of disease spreading amongst calves, simplifying vaccination schedules and nutritional programs, and streamlining scheduled labor needs since cows are closer in stage of production. The breeding season is really where it all begins, and utilizing estrous synchronization is one opportunity to capitalize on these benefits.
Labor, facilities, and costs are the most obvious constraints to adopting synchronization technologies. NAHMS data from 2020 indicate that in the 2017 calving season, 15.1% of heifers and 5.5% of cows were AI’d and exposed to bulls. Though these numbers have likely increased since, we can still conclude that the percent of females exposed to a synchronization protocol makes up a relatively small portion of the nation's cow herd.
If utilizing a synchronization protocol that will be followed by natural service instead of AI, pairs will still need to be near working facilities at the start of the breeding season. However, fewer injections and trips through the chute are needed, resulting in a less labor intensive process and better cow behavior than those processed through the chute multiple times within a short time period.
While weighing the pros and cons of implementing a synchronization program followed by natural service, keep these considerations in mind:
- Selecting the right protocol. There are a few different protocols that can be utilized, and each has advantages and disadvantages. Options include single injection protocols, protocols that simply involve inserting and removing a CIDR, and feeding MGA to heifers. Utilize your veterinarian, semen company representative, or an area beef specialist to help decide which protocol best fits your facilities, labor availability, and ideal grazing plan.
- Synchronize appropriate candidates. Not all females are good candidates for a synchronization protocol. Late calving females that are less than 45 days postpartum will likely not respond. One advantage of synchronization is jumpstarting females to resume their estrous cycle earlier than they would normally, but keep in mind that this can only be effective if adequate nutrition has been provided. Body condition, plane of nutrition, and days postpartum all need to be adequate before a female can be expected to respond to a synchronization protocol.
- Have adequate and healthy bull power. Since approximately 85% of the U.S. cow herd is serviced by bulls only, bull power is always key in a successful breeding season. All synchronization protocols are designed to bring females into heat within a certain time period, thus more females are ovulating at once, and in a natural service setting, bulls have more cows to cover at one time. Protocols for synchronization followed by natural service result in a less condensed estrous window than protocols utilized for AI, so the increase in mature bull power needed is less than many anticipate, but still an important factor. Bulls in a natural-service synchronization protocol are expected to breed twice as many cows as they would in a non-synchronized herd over a 7 – 10 day period.
Factors like pasture size, bull age, and bull libido play important roles in pregnancy success. In a non-synchronized setting, a good thumb rule to determine appropriate bull to cow ratio is that bull age in months up to 4 years of age at the start of the breeding season indicates how many females he can service. For example, a 15-month-old bull turned out with cows in May is expected to service 15 cows during the 2025 summer breeding season. When utilizing estrus synchronized protocols, research has shown no difference in heifer pregnancy rates when bull to female ratios ranged from 1:16 to 1:25, with an optimal mating ratio of 1:20 or 1:25 for economical purposes.
Yearling bulls tend to mount cows expressing estrus 40% more times than 2-year-old bulls and 60% more than older bulls. While there is no difference in percent of females serviced based on bull age, pregnancy rates of females serviced by yearling bulls tends to be 20% lower than females serviced by mature bulls. Combined, these data suggest that mature bulls are better candidates for synchronized groups, especially with protocols utilizing only natural service.
Finally, it is critical for all bulls to undergo a full breeding soundness exam (BSE) that covers all three components: a general physical exam, inspection of reproductive organs, and semen evaluation. Due to more cows cycling in a tighter window when synchronized, a bull who passes his BSE with flying colors becomes even more important. During the active breeding periods, especially the first cycle, monitor bulls closely so that an injury, disease, or dominance issue doesn’t interfere with breeding success.
See the Iowa Beef Center's estrus synch and multigroup estrus synch planners.
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