You cannot see it in this photograph, but there is an entire field of cows very close to this bull. The problem for this bull is that none of the cows are ready to mate. So, this bull is frustrated.
I’ve watched bulls in the rut for many years now and there is a pattern which repeats itself over and over. A bull will follow a cow, sniffing to see if she is in heat. Typically the cow will trot or run away. This leaves the bull frustrated and almost every single time he will bugle at this exact moment. This is the time to be ready with your camera, which is exactly what I did here in this photograph.
Watching and observing animals is not only enjoyable, it can teach you to be prepared for the best photographic moment!