Effects of corn grain endosperm type and fineness of grind on site of digestion, ruminal digestion kinetics, and flow of nitrogen fractions to the duodenum in lactating dairy cows

摘要

Our objective was to evaluate effects of corn grainendosperm type and fineness of grind on feed intake,feeding behavior, and productive performance of lactatingcows. Eight ruminally and duodenally cannulatedHolstein cows in mid lactation (130 ± 42 d in milk;mean ± standard deviation) were used in a duplicated4 × 4 Latin square design with 21-d periods. A 2 × 2factorial arrangement of treatments was used with maineffects of corn grain endosperm type (floury or vitreous)and fineness of grind (fine or medium). Rationsincluded alfalfa silage, corn treatments, protein supplement,minerals, and vitamins and were formulated tocontain 29% starch, 27% neutral detergent fiber, 18.2%forage neutral detergent fiber, and 18% crude protein.Corn grain treatments supplied 86.2% of dietary starch.Endosperm was 25% vitreous for the floury treatmentand 66% vitreous for the vitreous treatment. The flourytreatment increased rate of starch degradation by 94%(19.2 vs. 9.9%/h) and decreased rate of starch passageby 38% (16.1 vs. 25.8%/h), increasing apparentruminal starch digestibility by 117% (53.7 vs. 24.7%).The floury treatment increased total-tract starch digestibilityby 8% (92.2 vs. 85.1%) despite 37% lowerpostruminal starch digestion for the floury treatmentcompared with vitreous corn (38.4 vs. 60.7% of starchintake). Fine grind size increased apparent ruminalstarch digestibility by 52% (47.2 vs. 31.1%) comparedwith medium grind size by increasing the rate of starchdegradation by 105% (19.5 vs. 9.5%/h) with no effecton rate of starch passage. However, total-tract starch digestibilitywas not affected by fineness of grind becausepostruminal starch digestibility was 37% greater formedium compared with fine grind size (57.2 vs. 41.9%of starch intake). Endosperm type did not affect flowof nitrogen (N) fractions to the duodenum or microbialN efficiency, whereas fine grind size increased duodenalflow of nonammonia N by increasing duodenal flow ofmicrobial N by 22% compared with medium grind size(438 vs. 359 g/d) but did not affect apparent total-tractN digestibility. No interactions were detected for anymeasure of starch digestion, ruminal N metabolism, orflow of N fractions to the duodenum. Endosperm typegreatly affected ruminal and total-tract starch digestibilityindependent of the fineness of grind of corn grainwith no effects on flow of N fractions.