The present study evaluated the effects of incorporating polyunsaturated fatty acid (PUFA)- and rumen undegradable protein (RUP)-enriched feeds on ruminal fermentation, methane production, milk yield and milk FA profiles in dairy cows. Experiment 1 was carried out on 12 multiparous HF dairy cows in mid-lactation (106 ± 18.6 days in milk, 13.1 ± 1.67 kg/d of milk). Environmental conditions throughout the experiment were 34.5 ± 1.9 8°C, 58.0 ± 2.65% relative humidity and 85.7 ± 2.71 temperature–humidity index. Cows (n = 6/group) were randomly allocated to a control diet (Ctrl) consisting of corn silage and concentrate (F:C 60:40) containing 27.2% RUP in the crude protein (CP) or a treatment diet (Trt) consisting of 58.3% corn silage, 29.6% concentrate, 3.24% roasted soybean meal, 5.82% corn dried distiller grains with solubles and 3% added oil, which contained linseed, sunflower and tuna crude oil (1:1:1, wt/wt). The Trt diet contained 35.0% RUP in the CP. Experiment 2 was conducted in vitro using ruminal fluid collected from three Holstein Friesian dairy cows. The experimental design and diets were the same as in Exp. 1. Dry matter intake (DMI) tended to be lower (P = 0.06) in the Trt group. No difference in milk yield was detected, but the lower
(P < 0.01) milk fat was measured in the Trt group (2.82% vs. 3.81%). Feed efficiency (milk/DMI) tended to be greater (P = 0.06) in the Trt cows (1.08 vs. 0.88). Lower 18:0 (5.73% vs. 8.29%), but greater concentrations of c9,t11 CLA (1.88% vs. 0.34%), c9,c12 18:2 (1.55% and 0.69%) 18:3n-3 (0.30% vs. 0.12%) and 22:6n-3 (0.17% vs. 0%) were
detected in milk fat of cows fed Trt (P < 0.01). Atherogenicity and thrombogenic indices were lower (P < 0.01) with Trt. Concentrations of ruminal total volatile fatty acids and methane, which was assessed by production of volatile fatty acids, were 10.2% and 11.7% lower P < 0.01) in the Trt group. In conclusion, incorporating PUFA- and RUPhigh feeds in lactating cow diets in the tropics causes a reduction in concentrations of ruminal VFA, methane production and milk fat while improving aspects of milk quality without affecting milk production.