How Many Clean-up Bulls are Needed After Estrus Synchronization and Artificial Insemination?

Rick N.  Funston, professor, University of Nebraska, West Central Research and Extension Center, North Platte, Neb.

Hazy R.  Nielson, graduate student

Summary

To evaluate the ideal number of bulls to use following estrus synchronization and artificial insemination (AI), research reporting AI and final pregnancy rates and bull to female ratio in Bos taurus cattle was reviewed and summarized.  Pregnancy rate means were weighted based on the number of females in each study.  Final pregnancy rates for a normal bull to female ratio (1:20 to 30) in a natural service setting were 87.8%.  In comparison, final pregnancy rates following estrus synchronization and AI for a normal, intermediate (1:31 to 49), and half the number of bulls (1:50 to 60) were 87.8, 82.6, and 89.2%, respectively

Introduction

One of the benefits of estrus synchronization and AI is purchasing and maintaining fewer bulls.  However, an idea has been circulating that synchronized females not becoming pregnant to AI will return to estrus at the same time and require the same number of bulls as a natural service pasture would require. 

Larson et al., (Journal of Animal Science, 2009, 87:941– 921) observed cows not conceiving to AI will return to estrus over a 12 day period following a single timed AI.  The most active d had 18% of the herd in estrus, with the remainder of the distribution a bell curve (Figure 1).  Each cow’s estrous cycle is slightly different.  Some cows have two follicular waves during the estrous cycle, while others have three.  This results in a natural variation in cycle length, causing the non- pregnant cows’ return to estrus to vary more than may be anticipated. 

No effect of bull to female ratio or number of females expressing estrus per bull on pregnancy rate was found when comparing bull to heifer ratios ranging from 1:7 to 1:51 in heifers synchronized with SynchroMate B (Theriogenology, 1990, 34(6):1069–1070).  In a comparison of bull to heifer ratios ranging from 1:16 to 1:50 in herds of 100 heifers synchronized with melengestrol acetate (MGA)- PG and immediately exposed to bulls, the optimal bull to heifer ratio for synchronized heifers was 1:25 based on both biological and economic criteria (Journal of Animal Science, 1993, 71:291– 297).  If the optimal bull to heifer ratio in a synchronized natural service setting is 1:25, it can be extrapolated with a hypothetical AI pregnancy rate of 50%, the number of clean- up bulls needed is decreased by 50%. 

A study comparing bull to female ratios following estrus synchronization and AI is needed.  However, considering the breadth of research documenting bull to female ratios, AI pregnancy rates, and final pregnancy rates and the need for this information as soon as possible; the authors have chosen to summarize available data to provide a preliminary answer to this industry- relevant question.

Procedure

Data was collected from published studies reporting AI and final pregnancy rates, and bull to female ratio.  The synchronization protocol utilized, number of females in the herd, and breeding season length were also collected.  The studies collected were limited to those evaluating Bos taurus cattle.  Of the data collected, studies were divided into bull to female ratio groups including Normal- Natural Service (NS, 1:20 to 30 bull to female ratio), and three groups following estrus synchronization and AI; normal (NORM, 1:20 to 30), intermediate (INT, 1:31 to 49), and half (HALF, 1:50 to 60).  A summary of the mean AI and final pregnancy rates, weighted by number of females in each study, are presented.

Results

The weighted means of each bull ratio group are presented in Table 1.  The final pregnancy rate of a normal bull to heifer ratio in a natural service setting was 87.8%.  Pregnancy rate to AI in the NORM was 56.1% and final pregnancy rate was 87.7%.  The INT AI pregnancy rate was 46.5% with a final pregnancy rate of 82.6%.  Pregnancy rate to AI in the HALF was 55.6% and had a final pregnancy rate of 89.2%.  Bulls turned in at half the normal bull to female ratio following estrus synchronization and AI resulted in final pregnancy rates similar to normal bull to female ratio both in a natural service situation and following estrus synchronization and AI.

A consideration to make prior to choosing a bull to female ratio is bull age.   Experienced bulls are more efficient breeders, while yearling bulls are less experienced.   Another consideration is pasture size and terrain; a rugged, multi- windmill pasture may demand more from a bull than a flat single- windmill pasture.   In conclusion, producers utilizing estrus synchronization and AI should keep in mind the similarity between final pregnancy rates when using a 1:25 bull to female ratio and 1:50 bull to female ratio.   Producers need to evaluate the cost difference of purchasing and maintaining twice as many bulls to maintain a 1:25 bull to female ratio following estrus synchronization and AI.