FORESTOMACH AND WHOLE TRACT DIGESTION IN LACTATING DAIRY-COWS FED CANOLA-MEAL TREATED WITH VARIABLE LEVELS OF ACETIC-ACID

Five Holstein cows in midlactation were fed four isonitrogenous (mean 2.76% N) mixed diets containing untreated canola meal or replaced with canola meal treated with 0, 33, 67, or 100% of acetic acid in a 12-wk 4 x 4 Latin square experiment. Diets were 50% barley-based concentrat e, 12% canola meal, 30% whole crop oat silage, and 8% alfalfa silage ( DM basis). Forestomach and whole tract apparent digestibilities of DM, OM, and its components, including fiber and CP, did not differ as pro portions of treated canola meal increased. Rumen pool sizes of OM, NDF , and bacterial OM, as well as total NAN and bacterial N, were not inf luenced by treatment, although bacterial N pool size tended to decline as treated canola meal replaced untreated. This result was consistent with a trend for reduced flow of bacterial N at the duodenum. Duodena l flow of AA protein, and its profile, was not influenced by treatment . Results suggest that low level replacement of untreated canola meal with canola meal treated with acetic acid stimulated microbial growth by providing more sustained delivery of slowly degraded true protein. However, at higher substitution, the rumen microbial pool was reduced progressively, perhaps because of a shortage of readily fermentable N. This hypothesis, although not consistent with all data, seems to be t he most plausible explanation for all observations, although it is cle ar that differences in both digestion and duodenal flow, particularly for protein, attributable to replacement of untreated canola meal by t hat treated with acetic acid were moderate.