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

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