Bulls need as much winter care as cows to make sure they will be ready for breeding in the spring.  Dr. John Kastelic from the University of Calgary recommends that if bulls are confined, it is better to feed them in a bunk rather than on the ground to decrease fecal/oral transmission of disease. If you are feeding in the pasture, continue to change feeding sites to clean ground. Adequate nutrition is important to keep these bulls from losing weight in the winter.  Also make sure the feed is free of mold or toxic material as ergot is a fungus on cereal grains that can cause decreased circulation to the extremities, and some molds can affect semen production due to containing estrogen like properties. 

Bulls should always have access to a salt/mineral block since mineral deficiencies are not uncommon.  However, if minerals are a concern, it is better to feed a granular mineral mix as many animals will not be able to get enough mineral off of a block. A feed analysis can tell you which minerals need to be supplemented in your area and by how much.  Travis Olsen from Alberta, Canada, indicates bulls need a lot of room and keeping them in large pastures away from the cows decreases fighting.  Certainly, a couple of windbreaks are needed and a bedded area to lie down to prevent frostbite of the scrotum. If at all possible, it is a good idea to keep yearling and 2-year-old young bulls away from the mature bulls as these decreases fighting and allows you to feed the young bulls extra nutrition as they are still growing at that stage.  As a reminder, have your vet perform a fertility test on every bull every year before breeding season to avoid turning out an infertile bull.