by Sarah Thorson

Training Programs Manager

Genex Cooperative, Inc.

Rising energy costs dominate the news and financial spread-sheets at dairies and companies across the country. Like all other businesses, Genex is committed to providing superior service at a reasonable price, in spite of present and future energy expenses. 
  
Are there opportunities to be more efficient without sacrificing the service to our members and the reproductive performance in their dairies? One opportunity has been - and will continue to be - providing artificial insemination (A.I.) service once-a-day instead of the traditional twice-a-day. 
     
Here's a review of the myths, facts and hard numbers surrounding the once-a-day versus twice-a-day service debate.

A.M./P.M. Rule
In the early 1940s, Trimberger and Davis evaluated conception rates in beef cattle at various periods during estrus. This study, conducted over 60 years ago, became one of the most quoted for the timing of A.I. in cows and heifers. The study included 295 cows and heifers for a total of 489 breedings. The optimum time for A.I. was determined to be six to 12 hours after the onset of detected mounting activity (Table 1). The industry standard of the "a.m./p.m. rule" was developed from the findings of this study.



More Modern Studies
Since Trimberger and Davis' study in 1943, many other researchers have explored the timing of A.I. Interestingly, more recent studies show there is no difference between cows serviced following the a.m./p.m. rule and once daily service (Tables 2 and 3). 
     

Why was there a difference in results from the Trimberger/Davis study and the two more recent studies? That has a lot to do with how the study was performed. The early study designed by Dr. Trimberger was based on data collected under intense heat detection practices. Cows and heifers were observed three times per day for signs of estrus, and individuals detected in estrus were observed every two hours to more accurately determine the end of estrus. Therefore inseminations were performed at accurately determined times during and after estrus. 
     
Other studies were conducted using procedures more similar to today's dairies. Table 3 shows research by Dr. Ray Nebel and others, who studied 166 Pennsylvania dairy farms and over 7,000 first services by Genex A.I. technicians. Half of the herds were bred using the a.m./p.m. rule, and the other half of the herds were bred during a predetermined three-hour period once daily. After three months, the herds switched so each herd used both methods. A management practice survey revealed only 37 percent of the farms observed cows more than twice daily. Researchers concluded when visual heat detection periods were infrequent (less than three times daily), cows could be inseminated soon after detection with resulting conception rates similar to the a.m./p.m. guideline. 

What's the Effect with Synchronization?
Pursley et al. (1998) conducted a study in which A.I. was performed at different times after the second GnRH injection was given following Ovsynch^® protocols. Pursley reported similar conception rates at zero, eight, 16 and 24 hours, with lower conception results at 32 hours (Table 4). We can safely conclude from this study there is a 24-hour window after the onset of estrus to breed a cow with similar conception rates at the different times. 
     
Under normal conditions, a producer observing a cow in heat for the first time wouldn't know if she was in the beginning, middle or end of estrus. Consider if you saw a cow in heat for the first time, but she was actually at the end of estrus (average length of estrus is eight to 12 hours). If you follow the a.m./p.m. rule and wait 12 hours, she would actually be 22 to 24 hours after the onset of estrus before you breed her.

Pursely's study tells us this cow still has a near optimum chance of conceiving. His study also tells us it is better to be early than late when it comes to the timing of A.I. 

In fact, many farms are already successfully practicing the concept of once-a-day service without realizing it - in
the form of Co-synch. During Co-synch the cow is inseminated at the same time GnRH is administered. This compares to a cow inseminated at the very beginning of estrus, as under normal conditions GnRH release and the onset of estrus happen at the same time.