How often do bison go into heat? - January 2003

Dear Gerald:
We have 12 1999 heifers and a 1999 bull. We had 0 calves in spring, so brought in a new bull - a 2000 one in July.  Now last night we had our first calf (Oct 20/02)!!  How often do these gals go into heat??  What will be the easiest way to get them back to calving at the "normal" time of year -- we assume if one calved, there will be more to come in the near future.  Could it have maybe been their young age??  One person told us his bred at 3 years old versus 2.  We are interested in a "professional opinion" since we are new at this and the local guys are mostly new at the bison industry as well.  Thank you.

Answer:

Normally bison are seasonally polyestrous. This means that they go into heat only at a certain time of year called the rut (late July to October).  During this time they have an estrous cycle every 21 days just like a beef cow. At about the middle of October the rut is over and the feales quit cycling. We are not sure what triggers the end of the rut. Most people think that it is a nutritional trigger. When pastures dry up in the fall and winter comes, the decrease in quantity and quality of feed triggers bison cows to stop their estrous cycle.

If we give our cows and heifers supplemental feed during the fall and winter we are removing the trigger to end the rut. We end up having bison cycle throughout the winter and being bred at weird times of the year and calving during the fall and winter. I suspect that this has happened in your case. Will you get more calves this fall and winter? Maybe. If you fed your bison last winter instead of making them winter graze and kept the bull with them all winter, you may very well get more calves in the next few months.

What do you do to get your bison back on track and calving at the right time of the year?

There are 2 things that you can do. Pull your bull out of the herd and pen him separately except for the rut. Take him out now and put him back in next July. This way you can be sure that you get no calves except in the months of April-July. To do this you need the extra pen space and a bull that is content to be by himself and won't jump the fence to get back with the cows.

Another option is to use the theory of restricted feed to stop your cows from cycling at this time of year. Put them on a pasture that is poor for a few weeks or a month to create the nutritional stress necessary to stop estrous. You need to be careful that you don't cause too much nutritional stress. You don't want to create health problems or lose animals through starvation. Try to mimic the natural conditions that bison would have faced 200 years ago on the open prairie.

Gerald Hauer
Bison Production Specialist