Etiology of milk fat depression in dairy cows fed forages reduced in particle size

A series of three trials were conducted to examine the role of forage particle size in depressing milk fat secretion in dairy cows. In all three trials, alfalfa silage or hay was fed as 55% of dietary dry matter in a total mixed ration to emphasize forage effects upon the animal. In trial 1, seventeen cows were fed diets containing either fine (2.6mm), medium (2.8mm), or coarse (3.0mm) chopped alfalfa silage in a crossover design. In trial 2, four ruminally cannulated cows consumed diets containing either fine (.9mm), medium (1.5mm), or coarse (2.1mm) chopped alfalfa hay. In trial 3, nine cows in a switchback design were fed diets containing fine (.8mm) or coarse (2.3mm) chopped alfalfa hay. Daily intakes of dry matter and neutral detergent fiber were not influenced by particle size of silage or hay. Actual milk production was unaffected, while milk fat percentage declined by approximately 20% for cows consuming fine compared to coarse chopped forage. The depression in milk fat secretion reduced 4% fat-corrected milk yield and apparent efficiency of production in all cows consuming finely ground forage. Cows fed fine silage and hay had less rumination and total chewing times. Changes in the diurnal pattern of rumination activity with respect to forage particle size were evident. For both silage and hay, ruminal pH decreased and propionate concentration increased with decreasing particle size of forage. An increase in ruminal escape rate of fiber particles was associated with milk fat depression for silage, but not for hay, of similarly low NDF level (NDF = 40-45%). Plasma glucose and serum insulin increased proportionally with decreasing particle size of silage or hay. Blood total cholesterol and glycerol levels, and adipocyte glycerol-P dehydrogenase activity were not affected significantly by particle size of hay although numerical trends were in the direction predicted by the putative lipogenic action of insulin in ruminants. Results indicated that decreased particle size of forage decreased rumination time, elevated ruminal propionate concentration, blood glucose and insulin levels, and decreased milk fat secretion. These results support the glucogenic theory for milk fat depression