Furthermore, their systems are far more talented than a horse's at manufacturing their own amino acids. In comparison, horses absorb most of their nutrients in the small intestine, before the main fermentative section of the digestive system, so the contribution of the gut microbial population is small. As a result, horses have a need for much better quality protein than cattle. Cattle also have the ability to synthesize protein from a number of different sources besides dietary plant proteins. For example, they can process urea, an inexpensive nitrogen source, and turn it into useful amino acids.
Horses can utilize some urea and actually are more resistant to urea toxicity than are ruminants.
Rice explains another of the important differences between bovine and equine digestive systems. The very act of ruminating allows cattle to digest fiber with higher lignin content. A young grass stalk, for example, is low in lignin, while an oak tree has a high lignin content. The same isn't necessarily true for horses, who seem to handle longer or shorter fiber pieces equally well, assuming the horse's teeth are in good shape and the feeds are introduced slowly. Quite apart from the contrasts in their digestive systems, the whole way in which we feed cattle and horses is different, Rice points out.