Effects of brown midrib 3 mutation in corn silage on productivity of dairycows fed two concentrations of dietary neutral detergent fiber: 3. Digestibility and microbial efficiency

The effects of digestibility of corn silage neutral detergent fiber (NDF) a nd dietary NDF content on ruminal digestion kinetics, site of nutrient dige stion, and microbial N production efficiency were evaluated with eight mult iparous high producing dairy cows in a duplicated 4 x 4 Latin square design with 21-d periods. Experimental diets contained corn silage from a brown m idrib (bm3) hybrid or its isogenic normal control at two concentrations of dietary NDF (29 and 38%). The NDF digestibility estimated by a 30-h in vitr o fermentation was higher for bm3 corn silage by 9.4 units (55.9 vs. 46.5%) . Neither luminal nor total tract NDF digestibility was affected by corn si lage treatment. The bm3 corn silage diet decreased starch digestibility in the rumen and in the total tract, but increased postruminal starch digestib ility compared with control diet. The bm3 corn silage diets increased micro bial N flow to the duodenum and tended to decrease luminal ammonia concentr ation, Microbial efficiency was greater for cows fed bm3 corn silage in spi te of lower ruminal pH. Higher efficiency of microbial nitrogen production might be attributed to faster passage rate of NDF for cows fed bm3 corn sil age compared with those fed control corn silage. Higher in vitro NDF digest ibility might predict enhanced NDF fragility and ease of NDF hydrolysis in vivo. Enhanced in vitro NDF digestibility does not necessarily result in in creased NDF digestibility either in the rumen or in the total tract, but po ssibly increases rate of passage and DMI, improving efficiency of microbial N production.