Digestibility of cotton lint fiber and whole oilseeds by ruminal microorganisms

In two series of experiments, rumen microbial digestion of whole oilseeds was measured in sacco (Experiment 1) and of cotton lint, both in sacco and in vitro (Experiment 2). In Experiment 1, two ruminally-fistulated cows were fed diets without or with added fat (500 g d(-1)) in the form of animal-vegetable blend, calcium soaps of tallow fatty acids, or whole oilseeds (cotton, sunflower, soybean). Both cows were fed all the above diets in sequence (3-week periods) and ADF digestibility of ground orchardgrass incubated in sacco for 12, 24, and 48 h, and DM, N, and fatty acid digestibilities of seeds of cotton, sunflower, and soya incubated in sacco for 48 h were measured. There were no diet effects on digestibilities; orchardgrass ADF digestibility (%) was 26.3, 40.0, and 47.3 (SEM = 0.631) at 12, 24, and 48 h, respectively. Digestibility at 48 h of DM, N, and fatty acids for seeds of cotton, sunflower, and soya, respectively, were 8.40, 0.94, and -5.86; 8.25, -5.18, and -16.42; 52.79, 38.65, and 36.60. Digestibilities of all components were higher for soya (P < 0.0001). In Experiment 2, cotton linters were incubated in sacco and in vitro for periods varying from 12 to 120 h. Digestibility at 12 h was essentially zero, even though microbial colonization of fibers was observed microscopically. Digestibility was linear from 12 to 72 h of incubation, indicating that cellulolytic activity was limiting. Soaking fibers 24 h in distilled water prior to incubation increased digestibility (P < 0.0001). Ether extraction prior to incubation improved digestibility and appeared to be additive with welting. Soaking fibers in 4% NaOH for 24 h had a variably positive effect on digestibility. it is postulated that the delay in digestion of cotton fibers after colonization was caused by the highly crystalline structure of cotton fibers, which hydrates slowly. Ether extraction removed a hydrophobic layer which allowed water to penetrate more rapidly. Alkali treatment did not change crystallinity of cotton fibers. The lag in digestion of cotton fibers would increase the pool size of undigested fiber in ruminants fed whole linted cottonseed.