Milk Fat Depression in Dairy Cows: Role of Silage Particle Size

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Abstract
The effect of particle size of alfalfa silage on lactating cow metabolism was investigated with reference to milk fat depression. Eighteen cows in a crossover design were fed total mixed rations containing either fine (2.0 mm), medium (2.6 mm), or coarse (3.1 mm) ground alfalfa silage as 55% of dietary DM. Intake of DM and NDF was not influenced by particle size of the ration. Actual milk production also was unaffected, but milk fat percentage decreased from 3.8% for cows fed the coarse ration to 3.0% for cows fed the fine ration. This decline lowered 4% FCM yield and efficiency of production for cows fed the fine ration compared with cows fed medium or coarse rations. Cows fed the coarse ration had longer rumination and total chewing times. Ruminal pH and acetate:propionate ratio decreased when cows consumed the fine ration. Plasma glucose and serum insulin increased with decreasing particle size of silage. Results indicated that reduced size of silage particles altered chewing behavior, ruminal fermentation, glucose metabolism, and decreased milk fat secretion.