Energy utilization and milk fat responses to rapeseed oil when fed to lactating dairy cows receiving different dietary forage to concentrate ratio

We evaluated energy and N utilization, performance, and milk fatty acid (FA) profile using grass silage-based diets when rapeseed oil (RO) was included in high-or low-forage diets. Four multiparous Nordic Red cows averaging 101 +/- 16 days in milk at the beginning of the study were randomly assigned to a 4 x 4 Latin square design with a 2 x 2 factorial arrangement of treat-ments. Each 21-d period consisted of a 14-d diet adaptation period and 7-d collection period. Cows were fed the following diets comprised total mixed rations based on grass silage with forage to concentrate (FC) ratio of 35:65 and 65:35 containing 0 or 50 g/kg of RO. Significant FC x RO interactions were observed for milk yield, milk protein and lactose yields, milk fat concentration, and milk proportions of trans-11 18:1, trans-10 18:1, trans-10, cis-12 18:2, and saturated FA. Feeding low-forage diet was effective in increasing milk yield compared with the high-forage diet, and the RO supplementation increased it further (P <= 0.01). A similar pattern was observed for the yields of milk protein and lactose. Supplementing the low-forage diet with RO reduced milk fat concentration by 19% relative to other diets without affecting milk fat yield. The proportion of N intake lost as urine decreased (P <= 0.05) with the RO supplementation of low-forage diet without affecting energy and N balances. Nutrient intakes were greater (P <= 0.01) in cows fed low-forage diet, whereas RO decreased (P < 0.05) protein, starch, and fiber intakes. Methane production, expressed as a proportion of energy intake, decreased with low-forage compared with high-forage diets and this variable declined similarly by RO supplementation of both diets (P < 0.01). The milk proportions of trans-10 18:1 and trans-10, cis-12 CLA increased (P <= 0.01) by RO supplementation of the low-forage but not high-forage diet. However, RO supplementation of both high-and low-forage diets increased (P < 0.01) total trans FA and decreased saturated FA proportions, even though the changes were more profound in low-forage diet (P <= 0.01). In addition, RO increased (P < 0.01) cis monounsaturated FA in milk for both high-and low-forage diets. Overall, the low-forage diets had lower methane emissions and RO increased partitioning of N towards milk secretion (P <= 0.01) without influencing energy or N balances. According to the results, RO supplementation did not compromise intake of nutrients with low-forage diets con-taining 150 g/kg starch, and oil could be preferentially used to improve milk production and milk fat quality accompanied by a reduction in methane energy loss.