Late Breeding and heifer calves

[GLZ]

I was having a discussion with a gentleman I respect a lot as a cattleman this past week.  We got into discussing the timing of AI after detected heats.  Specifically around the thought that breeding later in the window seems to lead to heifer calves come next year. 

Three questions that we didn't know the answers to.  Is it just something lots of people observe to be true, but really is just still your good old 50/50 chance?  Has there been a lot or any for that matter, scientific studies around this?  Finally if it is accurate, what is the reasoning behind it?

 

[JSchroeder]

It’s not a study on the subject exactly but a pretty good study on human nature to believe in that idea…

In the games I run, the sex of offspring is determined by a random number generator that generates a number between 0.00000 and 1.00000.  If that number is above .50000 then it’s a male, otherwise female.  Over the millions of virtual animals born on those sites, the distribution is as close to 50/50 as possible.  Nothing could be more random.

Despite that, I have heard no less than a dozen different theories that people have come up with that they were sure impacted the sex of their animals.  People thought the time of day, the user doing it, the page they went to in order to breed, etc. were all factors in determining the sex of the animals.  Two people on Showcattle.com thought we had programmed the theory you are talking about into the game because they claimed it worked on the site. 

Some of them would even argue with me that I was wrong and they had solid proof of those ‘facts’ when they knew that I’m the one that programmed every single line of code.  I have lost subscribers who insisted I was lying to them when I said it was just a 50/50 shot when they “knew for a fact” that other things influenced it.

Long story short, people will jump through plenty of hoops to believe what they want to believe regarding factors that determine the sex of offspring.

 

[Bawndoh]

I heard that with humans...male sperm cells are faster, but die quicker.  Female sperm cells are slower, and live longer.  Therefore it seemed to depend on what time of the cycle things "got rowdy".  A well known medical doctor said that this, in fact, was true.  However, we as humans would have had to know our own cycles down to a science...and therefore, our cow's as well.  This being said, yeah, it is pretty much just a game of chance.

 

[Cowboy]

I noticed this thread this morning and thought I had better add to it.

After alot of years, and absolutely thousands of cows bred and serviced for ET -- as well as documented facts on sperm morphology -- here is what you will see if you do enough of it.

Male cells swim like crazy, but die off sooner due to lack of energy.

Female cells swim a tick slower, but due to a slower useage of energy -- they live a little longer.

The theory that you breed LATE gets more hfrs is absolutely backwards from reality. The later you breed to the point of being TOO late the higher the chances are of getting a bull - due to the above analogy.

Contrary -- although not always a HUGE difference, if you breed the cow sooner or as early as you can still get her fertilized, the male cells willbe all but done for and give the females a slightly higher percent chance of fertilization.

I have notoriously bred my donors LATE to maximize fertilization of resulting eggs. It works very very well, but I have seen at least a 10-15% difference in bull - hfr ratios due to that timing issue.

Some cows will bend that theory every single time. Some will ovulate early no matter what -- and there goes a bull cell into the egg, others will be late ovulaters -- and give the sperm a higher chance of having female cell there longer. Averages are - as mentioned, real close to 50/50 on the national cow herd. But it can be slightly manipulated if you have the time and desire. It takes a bunch of cows to see the difference however, you'll never see it by doing a few cows a year -- at least not routinely!

Hope that helps every one understand the bottom line. It is not rocket science, more like an ART!!

Terry