Apropos of nothing:
I will admit to a pretty weak segue. Both sentences referred to the perils of live breeding.
I’ve always heard that the Scots and the Kiwis do some good work with sheep.
Mares can be very important. Secretariat was known more for being a sire of quality broodmares, than money winning colts.
The output of a mare is even more limited than that of a stallion. It takes 11 months to make a horse, so a mare might have 15 foals in her, in her life time. By comparison, an in-demand stallion, for example Alydar, even with the live cover rules, sired thousands of horses in his lfetime. So the impact to racing as a whole is somewhat less from even a great mare.
There’s a chapter on pig AI in Mary Roach’s book, Bonk: The Curious Coupling of Science and Sex. The process is very involved and the chapter is hilarious.
Over her stud life, as thoroughbreds don’t allow embryo transfer, a mare will have maybe 10 foals and of those maybe 1/3rd will get to see the track.
A select shuttle stallion will cover 10 times or more each season. At the same conversion rate, after a generation the influence of the stallion far outweighs that of the mares.
If breeding the fastest with the fastest beget the fastest, then horse wouldn’t be the lottery that it is. They are a whole lot better trained, and run on faster tracks, but aren’t necessarily (innately) that much faster.
Among poultry, only Turkeys are artificially inseminated. Chickens, ducks, squab, pheasant, quail: all natural bred.
I’d like to see someone try to artificially inseminate a bee. 
Genetically, each parent contributes 50%. But the foal is constantly with the mare for the first 6 months or more, and so gets much of its temperament & personality from the mare. So for most horses, the dam contributes MORE than the sire. Perhaps not as important for thoroughbreds, where only speed counts, but true for most riding & driving breeds.
But stallions bloodlines are discussed more because of quantity. A mare can produce 15-20 offspring over her whole breeding lifespan. Top stallions might sire that many foals every single year, for 20 years or so. Secretariat produced 680 foals, over a 16 year breeding lifetime – over 42.5 per year average. So a stallion can have a much greater effect on a breed than any mare.
In addition to what everyone else said, a top stallion’s owner is going to want to breed that horse to the best mares available because stallions are judged by their get as well as their own successes. So, if a winning stallion covers a lot of quality mares, but the resulting foals aren’t anything to write home about, then over the years that stallion is going to be less popular and in demand for breeding. And, of course, if he covers mediocre mares, the chances of one of them throwing a winning foal is even less likely.
A responsible stud owner will try to make sure, too, that the mares his stallion covers don’t have the same flaws as the boy does. Let’s face it, every animal has flaws, or at least some areas that are weaker than others. So if a stallion has particularly weak “ankles”, for instance, his owner should look for mares that have at least average ankles, and strong ankles would be even better.
Large sow units are nearly all AI these days. For one thing, it’s much easier to pass on desirable genetic traits using AI. One boar’s semen can be used to breed an entire herd, for one thing. Another big advantage to AI is that the entire breeding herd can have their estrus cycles synchronized, and be bred all at the same time. That way, pigs are all born at the same time, and their growth rate will be much more consistent. I’m sure this is grossly oversimplified, but those are some of the big reasons it’s done. Natural breeding is not feasible for units of 10,000.
A cousin of mine has a herd of beef cattle that takes care of themselves all year round. He just culls it every now and then.
50 years ago, I witnessed an artificial insemination of one of the cows in my uncle’s modest dairy farm.
I’m pretty sure its the norm today.
I recall perusing the stats on the bulls at that time showing milk production of their dams or their offspring. Can’t remember which or both.
A high producing cow is a big edge, and if the bull is worth anything, the owner doesn’t need to transport it around.
I can’t speak for the really big operations, but the ranchers around here (Montana, USA) mostly turn the bulls loose with the cows. There is some AI going on, but it’s not as prevalent as natural breeding.
When I had cattle, I’d just buy a bull calf, let him do his job when he was old enough, and then put him in the freezer.
I always thought steers produced much better meat than bulls.
I also thought that steers produced better meat tham cows, but read somewhere that MacDonalds requires a certain perscentage of cow meat in their burgers which is why their meat just melts away in your mouth.
Oh yeah? So you guys got cows and stuff up there, huh?
We went around the relative scarcity of bulls on most dairy operations, and the use of “retired” milk cows in McDonald’s hamburger a while back:
Yeah, the dairy farmers I grew up around used AI.
Yes, but they are lousy at impregnating the herd! More seriously, even if steers produce better meat than bulls, if a now-useless bull is what you’ve got, I rather suspect that bullburgers are plenty edible.
I remember reading that cattlemen cut the horns off longhorns to make then less mean. That was back in the day, so maybe “horns” was a metaphor (wrong word) for “balls”.
Euphemism. (right word ;))