The size would indicate an animal the size of a cow. It is a humerus, the bone between the shoulder and elbow, the shoulder end on the left and elbow on the right of the photo. In cattle there is a slight prominance where the muscles attach which can been seen about one third of the way down the shaft of the bone (on the top surface as you look at the photo). In horse the prominance is much more pronounced. So it is from a species of Cattle. Being from North America there are a number of possibilities and I am not able to narrow it down any further.