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AI technician explains how synching heifers’ heat cycles increases fertility and shortens calving interval

by Wyoming Livestock Roundup

“Heat synchronization and artificial insemination (AI) can be extremely beneficial to any cow/calf operation,” said Harold Miller, rancher and owner of 7 Triangle 7 Cattle Co. located in Akron, Colo. Harold and his wife, Cheryl, have taught over 6,000 students how to successfully AI cattle. 

Inability to predict estrous, or heat cycles, of cattle has made AI impractical in the past. Producers have fought low conception rates and extra labor during past attempts. 

Throughout years of research, several heat synchronization protocols have been developed to help producers attain high conception rates when practicing AI.  

The estrous cycle 

Understanding the estrous cycle is important for producers interested in breeding, especially when trying to achieve maximum conception rates. 

“An estrous cycle is a sequence of events occuring in preparation for mating, conception and pregnancy,” Miller stated. “The average length between cycles is 21 days, although length can range from 17 to 24 days. Heifers or cows outside this range are considered abnormal.” 

Miller explained the three stages of a heat cycle – metestrus, diestrus and proestrus.  

Metestrus starts the cycle by releasing an egg. This phase occurs from day one through day five. 

Diestrus occurs from day five to day 17. This is when the corpus luteum is mature and fully functioning. During this phase, a heifer will release the hormone progesterone. 

Proestrus occurs from day 17 to day 20. During this phase, a heifer releases estrogen and prostaglandin. The corpus luteum begins to regress. 

According to Miller, heifers have longer heat cycles compared to cows. Heat cycles can range from six hours to upwards of 20 hours, but the average is 18 hours for heifers.  

Miller explained, research at Colorado State University regarding synchronized heats compared to natural heat cycles concluded synchronized heats were easier to detect. There was an average of 48 mounts per cow, and standing heat lasted 12 hours on average. Natural heat, however, averaged 22 mounts per cow and only lasted 8 hours. 

“Understanding the estrous cycle is key to heat synchronization and AI,” Miller stated. “When starting the heat synching process, a prostaglandin shot has to be given between days six and day 16.” 

Heat synchronization protocols 

There are several heat synchronization protocols. Producers should take time going over each protocol and carefully consider which works best for their breeding program.  

“I have two heat synchronization protocols I prefer to use when breeding heifers,” Miller explained. “The 14-day Melengestrol Acetate (MGA), an orally active progestin and the controlled internal drug release (CIDR) are my go-to synchronization protocols.” 

Benefits of the MGA protocol include low cost, a high degree of synchrony and high rates of pregnancy. One disadvantage of MGA is every heifer has to consume her amount of MGA every day. 

MGA, when consumed daily, suppresses a heifer’s heat cycle. MGA is fed for 14 days at a rate of 0.5 milligrams per head per day.  

After the 14-day feeding period, cattle are taken off MGA and have a heat cycle around 48 hours later. This is a low fertility heat. Cattle will cycle again, 19 days later. 

The CIDR protocol is used to restart a heifer’s heat cycle. The CIDR is injected into the cow and left for a number of days. The CIDR protocol can be used in addition to hormone shots.  

“The CIDR protocol allows producers to have control over the heat cycle,” Miller said. “The most common way to breed using a CIDR is a 10 day program. On day one, the CIDR is implanted and a shot of gonadotropin-releasing hormone (GnRH) is given.” 

He continued, “A week later, the CIDR is removed and the cow receives a shot of prostaglandin. The cow should cycle two to three days after the CIDR is removed and can then be bred.” 

These protocols are only two of many heat synchronization protocols, noted Miller. Producers should research synch protocols when determining which works best for their operation. 

                  Madi Slaymaker is the editor of the Wyoming Livestock Roundup. Send comments on this article to 

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