So if you ever visit a dairy farm you may notice something a little strange. There may be a male cat or dog, the farmer and some of the workers may be guys but why are there no male cows? Ok so technically cow only refers to fully grown female cattle but you know what I mean. There are baby calves, they're all girls, so the cows are getting pregnant somehow. Cows are not asexual, not any that we've discovered yet anyway, so both sexes are required to make babies. So you search the farm and no matter where you look, aside from maybe a random one less than a week old, you can't find any male cattle anywhere.
I'll give you a hint. They've been in the barn the whole time.
Just not in the shape you'd expect.
They're in straws like these.
Yep that's where the males are, well the semen at least, but you won't find straws just lying around. Semen is stored at extremely cold temperatures (-320 F) in liquid nitrogen. You'll find it in an insulated tank filled with liquid nitrogen.
So why is it that bulls on a dairy farm are found in straws not in the flesh?
A BIG reason is safety. A full grown Holstein, the most common dairy breed in the US, bull can weigh over a ton. Not only are bulls big but the can also be territorial and aggressive. Over the years many farmers have been killed or injured by bulls.
The second and possibly biggest reason is genetic progress. High genetic potential bulls are kept and raised. Their progeny are then analyzed and they are assigned predicted transmitting abilities (PTA) for a whole list of different traits. Catalogs listing these breakdowns are then distributed to farmers who can choose which bulls semen they would like like to purchase to artificially inseminate their cows with. A great example in the impact artificial insemination has had on the US dairy industry is the change in milk production per cow between 1950 and 2000. In 1950 the average milk production per cow was 5,314 lbs per lactation. Fast forward to 2000 and that number had increased to 18,204 lbs. Given there are other variables involved but genetic improvement has played a large role in this increase.
That is why when you visit a dairy farm pretty much all the cattle will be girls like this one.
The Changing Landscape of U.S. Milk Production/SB-978. http://usda.mannlib.cornell.edu/usda/nass/sb/sb978.pdf