A gomer bull is a standard bull that has had a vasectomy performed on it. Although this type of bull is unable to breed, it is generally used to identify cows that are in heat.
Using a gomer bull for a process referred to as heat detection allows farmers to separate out cows that are ready to breed more quickly and more successfully. These cows then either breed on the farm, are sent elsewhere for breeding or are artificially inseminated. Any bull can be made into a gomer and be useful in the breeding process, despite being unable to actually breed.