DIETARY SOYBEANS EXTRUDED AT DIFFERENT TEMPERATURES - MILK-COMPOSITION AND IN-SITU FATTY-ACID REACTIONS

Twenty-four multiparous Holstein cows, 4 of which were ruminally fistu lated, were assigned to one of four diets containing full-fat soybeans , either raw or extruded at 120, 130, or 140 degrees C. Our hypothesis was that the extrusion of full-fat soybeans, as well as the extrusion temperature, would affect the bypass of fatty acids in the rumen and, thus, would modify the fatty acid profile of milk fat. Total mixed di ets containing 23.7% soybeans (percentage of DM) were fed for 8 wk. Mi lk yield was lower, and the proportion of milk CP was higher, for cows fed raw soybeans than for cows fed extruded soybeans. Compared with r aw soybeans, extruded soybeans increased the concentration of Delta-11 -trans-C-18:1 from 2.72 to 11.41% in milk fat but had no effect on yie ld or percentage of milk fat. Polyunsaturated fatty acids of raw soybe ans disappeared more rapidly than did those of extruded soybeans from bags incubated in the rumen of fistulated cows. However, more Delta-11 -trans-C-18:1 and C-18:0 appeared in bags containing extruded soybeans than in bags containing raw soybeans. Extrusion of full-fat soybeans influenced the metabolism of fatty acids in the rumen and the fatty ac id profile of milk fat, but the temperature of extrusion had only mino r effects on these parameters.