Effects of Varying Energy and Roughage in Rations for Lactating Cows on Rumen Volatile Fatty Acids and Milk Composition

Experimental rations containing three levels of roughage, 0.75, 1.50, and 2.50 kg per 100 kg of body weight, were fed at 90, 100, and 120% of National Research Council requirements for digestible energy to 27 Holstein cows throughout lactation.Molar concentrations of total volatile fatty acids in rumen liquid were higher, and pH was lower early in lactation than in late lactation.Energy in the ration did not affect the molar proportions of acetate, propionate, n-butyrate, and u~valerate, but the proportion of iso-butyrate decreased and iso-valerate increased with increased energy.An increase in the proportion of roughage in the ration was associated with a significant increase in the molar proportion of acetate and a decrease in the proportion of propionate.Percentage milk fat was significantly higher when cows were fed rations with high proportions of roughage; percentages of milk protein and solids-not-fat were significantly higher when the cows were fed rations with the higher energy.The higher energy in the ration was also associated with significant increases in the 8:0, 10:0, 12:0, and 14:0 acids, whereas the 18:0, 18:1, and 18:3 fatty acids were decreased significantly.In general, there was a greater incorporation of long-chain fatty acids into milk fat during the first half of lactation, associated with loss in body weight.