A Reproductive Moment With Mel
Can Breeding Cows
Get More Complicated?
By Mel DeJarnette, reproductive specialist
Maintaining a sound heat-detection program and quality heat-detective personnel can be a never-ending challenge in today’s expanding herds. However, as the accuracy and efficiency of estrus detection declines, the value of incorporating estrus synchronization into the reproductive management program increases proportionately.
It is easy to feel overwhelmed and confused by the variety of estrus-synchronization protocols available. On the other hand, this variety provides extraordinary flexibility in developing tailor-made reproductive management programs. Let’s start with the basics and discuss how and why these protocols were developed. Once you understand them, they may appear less complicated, and perhaps you’ll find the system that best fits your management approach.
THE BASICS
It is possible to maintain good reproductive performance in dairy cows without estrus synchronization, but this requires SOUND HEAT DETECTION.
The foundation of any dairy cow synchronization protocol is prostaglandin F2a (PGF). PGF brings cows into heat by destroying the corpus luteum (CL). The CL is a structure on one ovary of cycling animals that prevents them from coming into heat by producing the hormone, progesterone. By destroying the CL, PGF removes progesterone, allowing the animal to return to estrus.
The use of PGF alone does have limitations:
- PGF is not effective in animals that do not have a CL. This includes animals in the early stages of the estrus cycle (Days 1 to 7) and anestrous animals that have not resumed normal cyclic activity after calving.
- PGF has no effect on follicular waves. Follicles are blister-like structures on the ovary that contain the unfertilized egg. Because follicles continually grow and regress in a wave-like pattern throughout the estrus cycle, there is considerable cow-to-cow variation in the size of the largest (dominant) follicle at the time of PGF injection, consequently variation in the time interval from PGF injection to estrus.
Injecting a group of cows with PGF alone is like firing the starting gun for a race before the runners are lined up. Some still may be in the locker room (anestrous and early cycle cows). Large follicles starting closest to the finish line will come into heat first (24 to 48 hours later), while small follicles have further to go before they are large enough to ovulate (four to six days). That’s why fixed-time A.I. following use of PGF alone seldom produces acceptable results.
However, PGF alone is a very effective management tool if most cows are cycling and the heat-detection program is intense enough to catch animals as they respond.
ADDING GNRH
To improve the synchrony of estrus following PGF, follicles must be assembled at the “starting line” before the race. Gonadotrophin-releasing hormone (GnRH) given seven days before a PGF injection removes most large follicles on the ovaries. Cows then start growing new follicles at the same time—yielding follicles of a similar size when the race begins with a PGF injection seven days later. This results in a more consistent start and synchronous finish to the ovulatory race. GnRH also will induce many anestrous cows to start cycling, and get them out of the locker room and onto the track in time to race.
A GnRH-PGF follicle race may end in several ways. Traditionally, the race continues until all participants cross the finish line. This is analogous to SELECT SYNCH, where animals are inseminated only as they are detected in estrus during the next five days. This system requires close monitoring of the finish line (good heat detection) to ensure that the right trophy goes to the right participant at the right time.
An alternative is to put a time limit on the race (OVSYNCH)—48 hours after the PGF starting gun, the race is declared to be over by giving a second GnRH injection to force ovulation of the newly recruited follicles. All participants received a consolation prize (fixed-time A.I.) 16 to 18 hours later. With the OVSYNCH system, winners and losers are not determined by who finished first or last, but by who happens to crow the finish line just the right time (ovulation coinciding with fixed-time A.I.).
A HYBRID SOLUTION
A third way to end the GnRH-PGF race is using a combination of these two methods, which is called the Hybrid system. The worthy winners (animals that display estrus) are given optimal opportunity to conceive by inseminating to detected estrus within 72 hours following the PGF injection. At 72 hours, most of the winners (cycling animals) will have finished. Rather than waiting for slowpokes that may not finish, conclude the race by giving non-responders a GnRH injection and a consolation prize (fixed-time A.I.).
Select Sires’ field experience suggest the Hybrid protocol optimizes conception rates by allowing most animals to be inseminated at standing estrus, yet gives all animals an opportunity conceive with mass A.I. at 72 hours.
Genetically valuable semen can be used on cows in standing heat, while less expensive semen can be saved for the timed-A.I. services. Higher conception at standing estrus and reduced drug costs may easily offset the necessity for three to four days of estrus detection.
Bear in mind that the success of SELECT SYNCH and the Hybrid protocol are absolutely dependent on good heat detection. If your labor is not up to the challenge, you’d be better served to stick with OVSYNCH.
Although GnRH is 90 percent effective at ovulating or luteinizing the follicles of cows between Day 5 and Day 12 of the estrus cycle, it is only 50 percent effective in cows between Day 13 and Day 17. We may think of these late-cycle animals as “cheaters” – they don’t go back to the starting line. Because of this unfair edge, they may come into heat before the race starts (the day of or before the PGF injection). Therefore, within all GnRH-PGF protocols, heat detection should be extended to 48 hours before PGF injection to catch the cheaters. Although their fertility is normal when bred to detected estrus, they will stand little chance of conception to fixed-time A.I.
MINIMIZING "CHEATERS"
To minimize the number of cheaters, Drs. Bill Thatcher (University of Florida) and Jeff Stevenson (Kansas State) developed a solution they call PRESYNCH. PRESYNCH involves two injections of PGF 14 days apart, with the latter on given 12 days before the first GnRH injection of OVSYNCH. As a result, 80 to 90 percent of the cycling animals will be in a responsive stage of the estrus cycle (Day 7 to Day 9) at the start of OVSYNCH. Preliminary results suggest PRESYNCH improved pregnancy rates to OVSYNCH by 10 to 20 percent (Figures 2 and 3). Although research trials used a 12-day interval between PRESYNCH and OVSYNCH, a 14-day interval would be easier to schedule and should not significantly alter the results.
Thatcher also reported that initiating bST treatment at either the first GnRH injection or the timed insemination of OVSYNCH-treated animals significantly improved pregnancy rates. This positive effect of bST was nearly identical for the two injection times, and was independent of PRESYNCH (Figure 3). Cycling animals treated with PRESYNCH, OVSYNCH and bST had fixed-time A.I. pregnancy rates approaching 60 percent. What bST does physiologically to result in such incredible pregnancy rates is not clearly understood. However, if these preliminary data are reproducible, a revolution in dairy herd reproductive management programs may be imminent.
DECIDING WHAT’S BEST
Yes, synchronizing estrus in cows is getting a little complicated, but it’s working. Can the same be said about your heat-detection program? If the answer is “yes”, then congratulations and keep up the good work. If heat detection is a little subpar, then perhaps PGF or SELECT SYNCH could help. If things are getting in pretty bad shape, try OVSYNCH or the Hybrid system. If your heat-detection program has gone to the dogs, you should try OVSYNCH and you may as well throw in the kitchen sink (PRESYNCH and bST).
Work past the point of thinking of estrus-synchronization protocols as being too many or too complicated. Now that you understand how and why they work, it’s easier to think of these protocols as tools in your reproductive toolbox. Although I often try to fix everything with a hammer, we all know there is a proper tool for every job. Work with your veterinarian or Select Sires sales representative to determine which tool is needed to optimize reproductive performance in your herd.
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