LAUSR

Abstract Effects of grain type and dietary oil supplement on milk fat depression and milk fatty acid (FA) composition of dairy cows were evaluated using eight multiparous Holstein cows (77±22.1 days in milk; mean±SD) in a replicated 4×4 Latin square design with a 2×2 factorial arrangement of diets. Experimental diets contained either ground barley or ground corn supplemented with either fish oil or soybean oil at 2% of dietary dry matter (DM). Experimental periods were 25 d, with the final 7 d used for sample and data collection. Dry matter intake tended ( P =0.09) to be greater for barley- vs. corn-based diets (23.2 vs. 22.3 kg/d), but was reduced for the fish oil compared to soybean oil supplemented diets (21.1 vs. 24.3 kg/d; P P cis- 9 C16:1) but decreased the concentration of preformed FA ( P trans -11 C18:1 ( P =0.03) and cis -9, trans -11 C18:2 ( P =0.01) which was a reflection of greater intake of cis -9, cis -12 C18:2 as substrate for rumen biohydrogenation. Severity of MFD was greater for fish oil than for soybean oil which was evidenced by the increased concentration and yield of biohydrogenation intermediates (especially trans- 10 C18:1) associated with MFD ( r =–0.61; n =32) in milk fat. However, fish oil increased concentration of both trans -11 C18:1 ( P =0.001) and cis- 9, trans- 11 C18:2 ( P P P de novo synthesized- and preformed-FA as compared with soybean oil supplemented diets.