Perception, as the saying goes, is greater than reality.
Sometimes, it seems, a bull-calf streak can go on for years, with heifer calves coming few and far between. But the numbers don"t quite support that version of events. Typically, for every 100 calves born on your dairy, 54 will be bulls and 46 heifers. That ratio fluctuates a bit, but year-in, year-out, that"s what you can expect.
However, mounting evidence indicates that when you change the insemination time for cows enrolled in a synchronization protocol you can get more heifers. Recent research at Michigan State University suggests that you can flip-flop the ratio of 54 bull calves to 46 heifer calves by breeding cows when they receive the last GnRH injection in an estrus-synchronization protocol.
"The results from our current research show that you can gain about 10 percent more female calves with this management change," says Richard Pursley, extension dairy reproductive specialist at Michigan State.
Consult your veterinarian or reproductive specialist to determine if you want to make this change part of your management strategy.
The X factor
Although the exact physiological cause is not yet certain, researchers theorize that one of the reasons for the gender influence stems from sperm survivability. The longer semen is in a cow"s reproductive tract before ovulation, the better the chances of a X-chromosome fertilizing the ovum and producing a heifer calf.
In this scenario, Y-chromosomes may not live as long as X-chromosomes. Thus, more of the Y-chromosomes die before the egg is ready to be fertilized, thus giving the X-chromosomes a better opportunity for fertilization, notes Paul Fricke, University of Wisconsin extension dairy reproductive specialist.
So, by breeding cows shortly after the final GnRH injection, you lengthen the time window for X-chromosomes to predominate.
"You must control the timing of ovulation to achieve any change in the bull:heifer ratio," adds Pursley. That means getting on an estrus-synchronization program and following that program faithfully. The only way you can manipulate the timing of breeding to influence calf gender is to know when ovulation will occur and inseminate at least 28 hours before the projected time of ovulation.
Failure to consistently follow your synchronization protocol means that you do not breed cows within your target window, and that will have a negative impact on gender selection and even more importantly, on overall herd reproduction and profitability.
Conception rate compromise
As with any management change, there is a trade-off with this strategy. It does not result in conception rates as high as those that occur when cows are bred 16 hours after the last injection of GnRH, admits Pursley. In his study looking at a shorter breeding window, conception rates hovered around 29 percent, while those bred later averaged a 39 percent conception rate.
You must evaluate whether this reduced conception rate makes a significant difference to your dairy. Each pregnancy carries an inherent value, regardless of whether it"s a bull calf or a heifer calf.
On the other hand, you can accommodate this change in your breeding strategy without significantly altering its schedule or adding to your investment.
In fact, you can actually cut your labor requirement because breeding takes place two days earlier than it normally would in a synch protocol (generally speaking, depending on the protocol used). You can breed cows at the same time you administer the final GnRH injection; therefore, you don"t have to handle the cows again. It also reduces handling stress on cows.
This breeding strategy also offers opportunities to producers with large numbers of cows that are milked 3X rather than 2X. Because of more-frequent milking schedules, these producers can have a tough time completing a synchronization protocol as intended. "They are limited by milking schedules as to when they can handle and breed cows," says Fricke. "They do sacrifice some fertility (conception rate), but they generally see more heifer calves born."
Not a cure-all
Finally, you must keep in mind that this protocol change results in fairly modest changes to your gender ratio, says Fricke. It will not severely limit the number of bull calves born on your dairy, nor will it wildly increase heifer births.
The concept requires more research. "I"m not sure we can accomplish significant changes to gender ratios with a simple management change," cautions Ray Nebel, extension dairy reproductive specialist at Virginia Tech. Nebel is in the midst of a timed insemination trial that has not yet noted a change in the heifer-calf-to-bull-calf ratio.
And more research is needed on the impact of bulls on determining gender. It has been well documented that the ratio of X-chromosomes to Y-chromosomes in the semen is not always equal, and the ratio can even vary from ejaculate to ejaculate in the same bull.
While not a cure-all, the work performed by Pursley at Michigan State does appear promising. (The study will be submitted to the Journal of Dairy Science this year.) It may provide an opportunity to gain more heifer calves each year. And, at a time when heifer values are on a pretty steep incline, that"s worth looking into.
Countdown to more heifer calves
this strategy to improve your heifer ratio calls for you to change the timing of breeding from 24 hours after the final GnRH injection to breeding immediately after the injection is given. Since ovulation occurs about 28 hours after that injection, semen remains in the reproductive tract longer than if you breed according to the original synch protocol.
In his research, Richard Pursley, Michigan State University extension dairy reproductive specialist, examined breeding cows eight hours before the last GnRH injection (36 hours before ovulation) and 16 hours after it (12 hours before ovulation). However, he suggests breeding at the time of the injection because the results are satisfactory and it better accommodates labor requirements.