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Copyright © 2015
Angus Journal

Control of Estrus in Natural Service

Estrus can be synchronized for natural service as well as AI. Here are some things to keep in mind.

by Lynsey Meharg for Angus Journal

Carl Dahlen

When choosing bulls to breed synchronized females by natural service, Carl Dahlen of NDSU recommended using bulls 2 years old or older that have had previous experience, ensuring they have a satisfactory breeding soundness exam as well as high libido, and stocking at a rate of 1:25.

STAUNTON, Va. (Oct. 15, 2013) —“Bull breeding dominates the U.S. beef industry,” Carl Dahlen, North Dakota State University (NDSU)–Fargo, told those attending the Applied Reproductive Strategies in Beef Cattle (ARSBC) symposium in Staunton, Va., Oct. 15-16. “A vast majority of producers are using bulls for their breeding.”

Dahlen discussed how to improve pregnancy rates in a synchronized natural-breeding program. He considered three main components — the bulls, the cows and the protocols.

Bull factors affecting pregnancy rate, said Dahlen, include age, breeding soundness, libido and stocking rate.

Dahlen shared research showing that while yearling bulls mount and service cows more oftern, older bulls service a greater percentage of the cows in heat and attain a higher pregnancy rate (see Table 1). The end result is a higher overall pregnancy rate.

Dahlen recommended breeding soundness exams be performed yearly for certain attributes, including physical traits, as well as semen motility and morphology. Breeding soundness exams are an indicator of fertility, he said, sharing research that showed bulls rated in satisfactory achieved a 45.6% pregnancy rate, while those rated questionable achieved a 36.5% pregnancy rate — significantly lower.

Table 1: Impact of bull age on breeding success in natural service


Bull age





Mounts, n 207x 120y 85.8y
Services, n 54.5 37.6 40.5
Estrous females serviced, % 69.4 73.8 72.0
Pregnant of serviced, % 39.6x 59.4y 62.2y

Pregnant overall, %




x,yMeans differ P < 0.05.

Source: Carl Dahlen, ARSBC presentation, Staunton, Va., Oct. 15, 2013.

While a breeding soundness exam gives you an indication of a bull's fertility, it tells you nothing about libido, warned Dahlen. “Libido is certainly something that plays a big role in a bull’s ability to go out and service 20-30 cows within a three-day period.”

Though Dahlen said he has found no difference in pregnancy rates between bulls with high libido and bulls with lower libido, the trait is certainly a factor to consider when evaluating bulls before breeding season.

“There were no differences between a bull with a high libido and bull with medium libido, so these are not an indicator of fertility,” said Dahlen, “but a bull certainly has to have the sex drive to get out and breed those cows.”

Dahlen showed research indicating that stocking rates of 1:16 resulted in significantly higher prenancy rates by Day 28 when cows were synchronized (see Table 2). However, using a 1:25 stocking rate achieved the most cost-effective pregnancy rate.

Table 2: Impact of stocking rate on breeding success in natural service


Bull:heifer ratio



- Synchronized -

Bulls per 100 heifers, n 2 2 4 6
Pregnant by Day 6, % 40 38 41 53
Pregnant by Day 28, % 82 77a 83 84b
Day of conception 10a 10a 11a 8b

Pregnant overall, %





a,bMeans within row differ P < 0.05.

Source: Healy et. Al., 1993, per Carl Dahlen, ARSBC presentation, Staunton, Va., Oct. 15, 2013.

When choosing bulls to breed synchronized females, Dahlen recommended using bulls 2 years old or older that have had previous experience, ensuring they have a satisfactory breeding soundness exam as well as high libido, and stocking at a rate of 1:25.

Though the bull is part of the equation, cows need to be in proper breeding condition as well. Breeding females should have a body condition score (BCS) of 5 or higher (see for more information), be 40 days postpartum at the start of synchronization protocol and have a low incidence of calving difficulty.

At the end of the day, evaluating the protocols recommended by the Beef Reproductive Task Force, Dahlen found no difference in pregnancy rate based on which protocol was used, but there was a difference in the date of conception for those cattle treated with a CIDR® as compared to cattle on another controlled treatment. The questions producers need to ask themselves, according to Dahlen, is whether the three-day gain in calving date is worth the extra effort for their particular operation.

Dahlen spoke during Tuesday afternoon's ARSBC session focused on managing factors to improve pregnancy rates. Visit the Newsroom at to listen to his presentation and to view his PowerPoint slides and proceedings paper.

Comprehensive coverage of the symposium is available online at Compiled by the Angus Journal editorial team, the site is made possible through sponsorship by the Beef Reproduction Task Force.

Editor's Note: This article was written under contract or by staff of the Angus Journal. To request reprint permission and guidelines, contact Shauna Rose Hermel, editor, at 816-383-5270.