Milk Fat Depression in Dairy Cows: Role of Particle Size of Alfalfa Hay

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Abstract
Two tirals were conducted to examine the role of particle size of alfalfa hay in modifying milk fat secretion in dairy cows. In trial 2, four ruminally fistulated cows in a crossover design were fed total mixed rations (29% NDF) containingeither fine (1.0 mm), medium (1.5 mm), or coarse (2.1 mm) ground alfalfa hay as 55% of dietaryDM. In tiral 2, nine cows in a switchback design consumed rations (32% NDF) containing fine (.9 mm) or coarse (2.3 mm) ground alfalfa hay as 55% of dietary DM. Daily intake of NDF and DM was not influenced by particle size of hay. Actual milk production was unaffected by particle size, but milk fat percentage on fine ration was .84 and .78 that on coarse in trials 1 and 2, respectively, resulting in reduced 4% FCM and apparent efficiency of production. Cows fed fine rations ruminated less and spent less time ruminating per kilogram of NDF intake. In both trials, ruminal pH decreased and urminal propionate, plasma glucose, and serum insulin concentrations incrased when cows consumedthe fine ration. Blood levels of total cholesterol and glycero, and adipose glycerol-P dehydrogenase activity were not significantly affected by particle size of hay, although trends indicated increased adipose lipogenic activity when cows were fed the fine ration. These results suggest that reduced particle size of hay produces metabolic changes predictedby teh glucogenic theory for milk fat depression.