ENERGY SUPPLEMENTATION OF HIGH FORAGE DIETS FOR MID-LACTATION DAIRY COWS

Alfalfa silage-based rations containing corn grain, oats or tallow as the primary supplemental energy source were fed to a total of 48 mid-lactation cows in two 56-d experiments. Corn, oats or tallow were mixed with the alfalfa silage and fed ad libitum. A corn-barley-soybean meal-based concentrate was fed twice daily proportional to milk production. Overall rations varied from 55 to 70% alfalfa silage on a dry matter basis. Feed intake was lower for tallow-supplemented cows, whereas milk yield and milk proportions of fat and lactose were similar for cows in all groups. Tallow-supplemented cows produced milk with a lower protein proportion and protein yield was also lower. Tallow-supplemented cows produced milk with major shifts in milk fatty acids from most short-chain fatty acids (<C16:1) to primarily C18:1 as well as C18:0 and C20:0. Parameters of rumen fermentation were similar among all groups and did not suggest a ruminal mechanism for reduced feed intake in spite of the high level of added fat (8.6 and 6.2% of dietary dry matter in exp. 1 and exp. 2, respectively). Isoenergetic substitution of oats for corn had no negative effects on animal performance in these high-forage diets. However, addition of tallow to alfalfa silage at these levels depressed overall animal performance. Key words: Tallow, oats, corn, milk fatty acids, dairy cows