They look like two mighty mountains clashing together. In an idea world the bush in the foreground wouldnt be there, but when something happens on the spur of the moment you cant exactly control the environment. I like the deep gradient green in the background over the backs of the bison, it compliments the brown of their coat very well. The lighting and textures are good also. Only thing that stands out as being a little unusual are the eyes. The faces are very dark, almost to the point of being in total shadow although I can see some detail, but the eyes are very bright and the whites almost a little too white, if that makes sense. An interesting shot, very clash of the titans.
Bison are a great subject! Togh the composition is good, and the technical details are there, I feel the word, static, when I look at it. I guess that I'd like to see some motion of somesort here... dust from the ground or somesuch.... Nich idea though!