Introduction
Corn grain, pelleted soybean hulls, and wheat middlings are relatively inexpensive feeds available to cattle feeders. Although the metabolizable energy value of corn is listed as being 50% greater than that of wheat middlings and soyhulls, in practice this may not be the case. Most of the metabolizable calories from corn come from starch. In feedlot diets, fermentation of starch lowers rumen pH and inhibits fiber digestion. Soybean hulls do not contain starch. The calories in soybean hulls are derived from the fermentation of the high quality, readily digestible fiber. While corn starch may depress fermentation of dietary fiber, soybean hulls actually may enhance fiber fermentation. In this case, actual energy value of the diet may be greater than what would be predicted from book values. This phenomenon is called a "positive associative effect". The protein characteristics of corn and these readily digestible fiber sources differ. As a result, the most effective supplemental protein source (true protein versus non-protein N) may differ depending on the energy source being fed. The objectives of this trial were to compare the effects of feeding two carbohydrate types (wheat midds/soy hulls versus corn) and two supplemental protein sources (soybean meal versus urea) on the performance of growing cattle fed low quality grass hay.
Daily gain, dry matter (DM) feed intake, and feed efficiency data were collected. Data were analyzed using GLM procedures of SAS. The model contained effects due to carbohydrate source, protein source, the interaction of these main effects, and the contrast of the low protein negative control versus others. Pen served as the experimental unit.