Yes. A cow that is nursing a calf will come back into heat around 18 to 24 days after she has calved, but should NOT be bred until at least 45 days after she has calved (for reasons why see related question below). Cows that have given birth to a stillborn calf, aborted early or has lost a calf within a week after it is born and dry up (cows will only dry up if you haven't grafted a calf to her) will almost always cycle a bit earlier.
No.
Cows used for breeding only, or cows that are already pregnant.
heheheh
Yes. For beef cows that are suckling their calves for as long as 6 to 10 months, the breeding period starts 45 to 60 days after the calving season has ended, in which the cow will get rebred, but her calf will still be nursing from her.
Yes, breeding cows is considered as animal husbandry.
Some farmers may refer to them as Brood Cows.
Artificial Insemination or AI.
Most likely from watching the cows' calves nursing from the same area.
A male intact bovine that is used for breeding cows and heifers.
Calves that are suckling milk from cows which would be their mothers.
In the first 21 days of what? After they've given birth? Or during the start of breeding season? If it's the latter, the reason is that it's a sign of good fertility in the cow herd if the majority of females are able to cycle and catch (or be successfully bred) within the first 21 to 45 days of the start of breeding season. A shorter breeding season means a shorter calving season, and with a high fertility rate in your herd, that's less of a head-ache (if you are using the proper bulls on your cows!) during calving season because a) it's not going to be dragged out for a third of the year, and b) less labour if more cows can drop calves on their own instead of requiring assistance. A short breeding season also is a great indicator of which cows need to go and which can stay, based on how long it takes them to get back into estrus and get settled by the bull.
Most certainly, especially during breeding season!