Effect of dietary antioxidant and increasing corn oil inclusion on milk fat yield and fatty acid composition in dairy cattle.

The objective of this study was to examine the effect of a dietary synthetic antioxidant on feed intake, yields of milk and milk components and milk fatty acids (FA), in combination with increasing concentrations of dietary corn oil to provide increasing rumen unsaturated fatty acid load (RUFAL) challenges. Twenty-six Holstein cows (177 ± 57 d in milk; mean ± standard deviation) were assigned to treatment in a randomized complete block design. Treatments were a control diet (CON; n=13 cows) or the same diet supplemented with a synthetic antioxidant (AOX; 6.1g/d; dry blend of ethoxyquin and propyl gallate, Novus International Inc., St. Charles, MO; n=13 cows). In period 1 (21 d), no supplemental corn oil was fed; in periods 2, 3, and 4 (14 d each), corn oil was supplemented at 0.7, 1.4, and 2.8% of the diet [dry matter (DM) basis] to incrementally increase RUFAL. For all variables measured, no significant interactions were detected between treatment and period, indicating no differences between the CON and AOX treatments at all levels of oil inclusion. Intake of DM was lower for AOX compared with CON but AOX had no effect on milk yield or milk fat concentration and yield. Milk protein yield and feed efficiency (energy-corrected milk/DM intake) tended to be greater for AOX compared with CON. Increasing dietary corn oil concentration (RUFAL) decreased DM intake, milk yield, milk fat concentration and yield, and feed efficiency. The AOX treatment increased the concentration and yield of 16-carbon milk FA, with no effect on de novo (<16 carbon) or preformed (>16 carbon) milk FA. Milk FA concentration of trans-10 C18:1, trans-10,cis-12 C18:2, and trans-9,cis-11 C18:2 were unaffected by AOX but increased with increasing RUFAL. In conclusion, supplementation with AOX did not overcome the dietary-induced milk fat depression caused by increased RUFAL.