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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.

Two ruminally cannulated cows were fed a diet containing 65% alfalfa haylage and 35% concentrate every 12 h. During two collection periods, whole ruminal contents were sampled before feeding, when feed not eaten was removed (1 h after initiation of feeding) and 1, 2, 3, 4, 6, 8, 10 and 12 h after removal. At each sampling, fluid-associated microorganisms were obtained by straining whole ruminal contents through eight layers of cheesecloth. A portion of the particle-associated population was obtained by chilling squeezed particles before seven successive extractions with saline solution. Microbial ash concentrations ranged from 9.9 to 16.6% of dry matter, with differences due to population (P less than .01) and to time after feeding for both fluid- (P less than .05) and particle-associated (P less than .01) microorganisms. Within 1 h after initiation of feeding, N decreased (P less than .05) from a pre-feeding concentration of 9.5% of organic matter to 7.7% for fluid-associated microorganisms, and from 9.2 to 6.7% for particle-associated microorganisms. Both populations did not return to pre-feeding concentrations until 6 h after feed removal. Nucleic acid concentrations decreased (P less than .05) in both populations after feeding, and did not return to pre-feeding concentrations until 6 h for fluid-associated and 4 h for particle-associated microorganisms. Nucleic acid-to-N ratios were higher (P less than .01) in the fluid- than particle-associated organisms.(ABSTRACT TRUNCATED AT 250 WORDS)

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