Natural service synchronization

Farm Forum

According to the 2009 NAHMS survey about 94.2% of all cow and 79.2% of all heifers are not exposed to artificial insemination (A.I.), only being exposed to natural service. The terms A.I. and estrus synchronization are often used hand in hand, however the use of estrus synchronization with natural service (bull breeding) offers many benefits which producers should consider.

Research and history have both proven that calves that are born in the beginning of the calving season have greater weaning weights, yielding higher profits. In addition, females that calve in the earlier in the calving season, will rebred earlier in the subsequent breeding season, and over their life time will be more productive than their later calving counterparts.

The logic behind natural service synchronization is to increase the percentage of females becoming pregnant during the 21 days of the breeding season through estrus synchronization. When considering a natural service synchronization program within a herd, several factors should be carefully evaluated.


Bull power should be taken into consideration, as typical bull to cow ratios will not be adequate when using natural service synchronization since a greater portion of the herd will be cycling at one time. When considering the stocking rate that will be needed several factors pertaining to the bull should be taken into account including: the age of the sire, if the sires have passed a breeding soundness exam, and the libido or the sex drive of the bulls. Bulls that pass a breeding soundness exam, being classified as satisfactory (not deferred) with a high libido, that are experienced breeders 2 years of age or older are the best sire candidates for natural service synchronization herds.

Research from Healy and colleagues in 1993 showed that when heifers were synchronized and bred with natural service there was no differences in the overall pregnancy rates when the bull:heifer ratio was 1:16 or 1:25, however a greater proportion of heifers were bred during the beginning of the breeding season when stocked at a 1:16 ratio. An overall economic evaluation from this trial showed that stocking at a 1:25 ratio was the optimal mating load. Prior to bull turn out producer should take time to analyze their situation to determine a stocking rate taking into account pasture size, terrain, and the sires’ mating abilities.

Heifers and Cows

When determining if cows or heifers are good candidates for natural service synchronization, many of the traditional factors that are evaluated in A.I. with estrus synchronization apply. The days postpartum (days from calving) and body condition, are the two primary factors that should be considered that should be considered for mature cows. In replacement heifers, the percentage of heifers that are pubertal and cycling should be considered, in addition to their body condition.

Estrus Synchronization Protocol

The synchronization protocols that are used typically with A.I. are aimed to tighten the window that cows are ovulating in to allow for timed A.I. through the use of GnRH, however with natural service synchronization, GnRH should not be used as the females may ovulate without showing estrus, the signal for the bulls to inseminate.

Research from Dr. Cliff Lamb in 2008 showed that using a 7-day CIDR protocol with natural service did not result in a difference in overall breeding season pregnancy rates, which agrees with previous findings. However, the average days to conception was shortened for cows that were synchronized prior to bull turn out. This would result in a greater portion of females calving at the beginning of the calving season.

Ultimately the goal of using natural service synchronization within a herd would be to shift the calving distribution of a herd toward the beginning of the season, increasing the percentage of females calving at the beginning of the calving season, and has proven to be an effective tool to do so. The use of natural service synchronization is a management tool that will work well into some operations; however it is not a fit for all herds. While it will increase the amount of herd management needed during the breeding season the pay offs can be well worth the efforts.


· Healy, V.M., G.W. Body, P.H. Gutierrez, R.G. Mortimer, and J.P. Piotrowski. 1993. Investigating optimal bull:heifer ratios required for estrus-synchronized heifers. J. Anim. Sci. 71:291-297.

· Lamb, G.C., C.R. Dahlen, K.A. Vonnahme, G.R. Hansen, J.D. Arseneau, G.A. Perry, R.S. Walker, J. Coement, and J.D. Arthington. 2008. Influence of a CIDR prior to bull breeding on pregnancy rates and subsequent calving distribution. Anim. Reprod. Sci. 108:269-278.

For more information, contact Kalyn Waters at the Winner Regional Extension Center at (605) 842-1267 or contact any SDSU Extension Cow/Calf Field Specialist or Beef Extension Specialist.