We live on flat lands with lots of trees, brush and swampy area in Manitoba. Is there any research that shows whether it's preferable to keep one bull with 30 cows per pasture; or have six bulls in with the 180-head cow herd?
This is a great question. I know of no research that specifically looks at the economics of these two different approaches.
There are advantages to a multi-sire breeding program. First in my mind is the fact that any bull can "go bad" at any time. This may not be obvious when it happens, and he may still be breeding cows. Additionally, if several cows come into heat at one time, multiple bulls should be more effective at servicing and settling them. There is also less need for fencing and working facilities with the multi-sire approach. Larger pastures can be used, or you can rotate through several pastures as forage availability dictates. The multi-sire program is one of my favorite management practices.
Like every good thing, there are some possible drawbacks to consider too. High on the list, injury to bulls and even cows, increases when there are multiple bulls in the herd. You are also don't know the sire of a calf, without DNA testing. And if the "boss" bull is infertile, he may beat back fertile bulls, preventing cows from getting pregnant.
I found one study that looked at multi-sire breeding programs in several large herds. Those researchers found on average bulls sired 15 to 20 calves per breeding season. However, some bulls sired more than 50 calves. And 7.3% of bulls in the program sired no calves. In at least half of the breeding seasons, at least one bull was removed due to injury or poor condition.
All of this tells me that highly prolific bulls will have a disproportional impact on genetic trends in the herd. Are these bulls the genetically superior bulls? Do we want those prolific bulls to determine the direction of the herd?
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