Evaluation of a new high protein variety of soybeans as a source of protein and energy for dairy cows

20 Holstein cows in midlactation were used in a Latin-square design to evaluate the nutritional quality of a high-protein soyabean (CP 45%) given raw or roasted. Treatments were: (1) control (soyabean meal); (2) conventional soyabean (Maple Isle), raw; (3) conventional soyabean, roasted; (4) high-protein soyabean (AC Proteus), raw; and (5) high-protein soyabean, roasted. Diets were formulated to be isonitrogenous and consisted of approximately 10% whole-crop barley silage, 15% maize silage, 19% lucerne silage, 31% rolled barley, 17% cracked maize and from 6.5 to 8.6% of the appropriate protein source on a DM basis. Replacement of conventional soyabean with the high-protein soyabean increased milk and milk component yields. All soyabean treatments lowered milk protein percentage vs. soyabean meal although milk protein yield was only reduced for the raw Maple Isle soyabean treatment. Milk fat percentage was reduced for the roasted AC Proteus soyabean treatment vs. soyabean meal and both Maple Isle soyabean treatments, although total milk fat yield did not differ between treatments. Heat treatment by roasting tended to affect total milk yield positively for both types of soyabeans, but only the increase for Maple Isle was significant. Milk from cows given full-fat soyabeans had more long-chain fatty acids than milk from cows given soyabean meal. Roasting the soyabeans further increased the amounts of long-chain fatty acids. The new high-protein soyabean, AC Proteus, appears to be an excellent source of supplemental protein and energy for lactating dairy cows..