Source of supplemental dietary fat interacts with relative proportion of forage source in Holstein dairy cows: Production responses, milk fat composition, and rumen fermentation.

• The effects of alfalfa hay to corn silage (AH: CS) ratio (70:30 vs. 30:70) in high-concentrate diets containing two sources of fat supplements (calcium salts of palm fatty acids vs. C16:0 enriched palm fat) on production responses and milk fat secretion of dairy cows were evaluated. • The source of fat supplement interacted with AH: CS ratio to affect milk fatty acid profile and protein yield but not milk production of dairy cows. The effects of alfalfa hay to corn silage (AH:CS) ratio in high-concentrate diets containing 2 sources of fat supplements on production responses and milk fat secretion of dairy cows were evaluated using 8 lactating multiparous Holstein cows in a duplicated Latin square with a 2 × 2 factorial arrangement of diets with 21-d experimental periods. Diets comprised AH:CS ratios of 70:30 or 30:70 supplemented with either calcium salts of palm fatty acids (FA; CaS-FA, 3.5% of dietary dry matter (DM)) or C16:0 enriched palm fat (Palm-FA; 3.0 % of dietary DM). Dry matter intake (DMI) was greater (1.74 kg/d) in cows fed high compared with low AH:CS ratio. Similarly, the high AH:CS ratio had greater production of milk yield, 3.5% fat-corrected milk, and energy-corrected milk compared with low AH:CS ratio whereas fat sources did not differ in any of these parameters with the exception of greater milk yield by CaS-FA compared with Palm-FA. Milk protein yield was the greatest in the high AH:CS ratio + CaS-FA diet. Milk protein content was greater for diets containing the high AH:CS ratio and also for Palm-FA vs. CaS-FA diets. Milk fat yield was not affected by the treatments, but milk fat content was greater for Palm-FA diets. Feed efficiency (milk yield/DMI) was the lowest for the high AH:CS ratio + Palm-FA diet. The molar proportion of propionate was greater for diets containing high AH: CS ratio, but otherwise volatile FA were not affected by treatments. The high AH:CS ratio had greater de novo synthesized FA (< C16:0) in milk fat without affecting mixed-origin FA (C16:0 plus cis -9 C16:1) or preformed FA (> C16) compared with Palm-FA. CaS-FA had greater proportions and yields of de novo synthesized and mixed-origin FA, and lower preformed FA including many C18 biohydrogenation intermediates such as trans -10 C18:1 and trans -10, cis -12 C18:2 which are due to greater intake of n -6 C18:2 indicating altered ruminal biohydrogenation. CaS-FA also had greater unsaturated FA in milk fat compared with Palm-FA. Overall, the source of fat supplement interacted with AH:CS ratio affecting milk FA profile and protein yield but not milk production of dairy cows. [ABSTRACT FROM AUTHOR]