In our sheep, we want ewe lambs if at all possible: they are the breeding units and replacement stock for the old ewes as they get too old for bearing. We have not always been lucky in the genetic coin toss, with more years delivering >50% rams than the other way round: 2014 - 2015.
It's a bit more nuanced on the beef and dairy side. One key factor is the conception rate. Cows are pregnant for 9 months and come into estrus about every three weeks. If your bull is shooting blanks and you miss the first chance for getting your cows pregnant it gets the whole cycle out of kilter with the seasons. At least it is obvious - to the bull, if not to you and me - that the first try hasn't 'took' and the farmer has another chance to get a calf in 9 months time.
- If you're a dairy farmer, it's the heifers which have value, they will be your breeding stock in two years time. The bull-calves are a by-product which have to be fed and watered before they can be send off to the factory for hamburger. In the mart steer calves are worth about 2/3rds heifer calves.
- If you're more in it for the beef, then the added testosterone in young males helps pile on the muscle mass.
When you get round to deciding which bull to have a go at your cows, you're better off with data. Breeders Inc. have created an EBI [economic breeding index] which is a score in €€€s supposedly related to the added value which offspring of that bull will accrue. The EBI is based on 7 sub-indices SI : (1) Milk production, (2) Fertility, (3) Calving performance, (4) Beef Carcass (5) Cow Maintenance (6) Cow Management and (7) Health. Health, in turn, is derived from 3 measurable adverse outcomes: lameness; somatic cell count SCC; and mastitis. Obviously the bull himself doesn't have mastitis . . . but his daughters do (or don't); so progeny testing is an important part of the process. You could do this for human semen, but you shouldn't.