Effect of heat treatment on ruminal degradation and escape, and intestinal digestibility of cottonseed meal protein.

The effect of heat treatment of ruminal protein degradation and escape was studied using in vitro incubations with autoclaved and commercial cottonseed meal (CSM) samples. Incubations using high ratios of protein to ruminal fluid appeared to overestimate ruminal degradation of CSM protein relative to casein. A biexponential model, assuming CSM contained two protein fractions degraded at two different rates, was used to interpret data from in vitro incubations conducted using ratios of protein to ruminal fluid similar to those expected in vivo. The first fraction was degraded at rates (0.68-1.19/hour) which were 2--3 1/2 times greater than that of casein (0.34/hour). Degradation rates of the second fraction were much slower (0.011-0.093/hour). The effect of heat treatment was to decrease the proportion of the rapidly degraded fraction and to both increase the proportion and to decrease the degradation rate of the more slowly degraded fraction. Estimated ruminal escape increased with each increment of heat treatment. Intestinal protein digestibility (ruminal escape times true digestibility) increased to maximum at 60 minutes autoclaving, and then declined. Estimates of intestinal protein digestibility averaged 30.6 and 50.3% for solvent-extracted and screw-press CSM, respectively. These and previous results suggest heat treatment decreases ruminal degradation partly by blocking reactive sites for microbial proteolytic enzymes and partly by reducing protein solubility.