Factors Associated with Variation in Milk Fat Depression Resulting from High Grain Diets Fed to Dairy Cows

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Abstract
Thirty lactating Holstein cows were used in a crossover design with 4-wk periods to establish the relationship between chewing activity, molar proportions of ruminal volatile fatty acids, and milk fat depression resulting from feeding high grain-low forage diets. Milk production and dry matter intake were not affected by treatment. When cows were fed the high grain-low forage diet, milk fat decreased from 3.39 to 2.87%, ruminal acetate plus butyrate:propionate ratio decreased from 2.7 to 2.0, and total chewing time decreased from 659 min/24 h to 472 min/24 h. Percentage change in milk fat test between treatments was correlated to percentage change in the ruminal molar proportion of acetate, propionate, acetate plus butyrate:propionate ratio, and percentage change in total chewing time per kilogram neutral detergent fiber intake (n = 30, r = .38, -.50, .53, and -.30, respectively). More correlation coefficients were significant and correlations were higher if only cows that proceeded from control to high grain-low forage treatment were considered. As milk fat depression was increased by high grain feeding, magnitude of depression in chewing activity was decreased. Within treatment, time spent chewing increased as milk fat percentage decreased.