Some effects of unsaturated oils given to dairy cows with rations of different roughage content

Abstract

SummaryThree experiments were conducted to determine the effect of feeding polyunsaturated fish oils to dairy cows receiving rations of high, medium or low roughage content. In the rumen, the decreases in the proportion of acetic acid and increases in propionic acid induced by the oils became greater as the amount of oil given was increased but the magnitude of the response to any dose depended upon the composition of the basal diet. The effects on volatile fatty acids (VFA) proportions of small amounts of the oils (125–150 ml/day) were greatest with the low-roughage rations. With large doses of oil (375–450 ml/day) the responses were variable but it is concluded that, in general, changes in VFA proportions are least with low-roughage rations.The fat content of milk was more sensitive to dietary oil supplementation than were the rumen VFA proportions. When the unsaturated oils were given there was a decrease in milk fat percentage and an increase in the proportion of unsaturated fatty acids in the fat; there was also increased incorporation of fatty acids with more than 18 carbon atoms in the milk fat. The metabolism in the rumen tended to become adapted to the feeding of 150 ml/day of oil, the VFA pattern returning during the second and third week of supplementation to that observed before the addition of oil.