Dairy Lactation Experiment
Twelve intact (four primiparous, eight multiparous)
and four ruminally fistulated, multiparous Holstein dairy cows were assigned to one of four treatments in
a replicated 4 × 4 Latin square design with 3-wk periods.
Cows averaged 162 ± 10 d in milk when they were
assigned to treatments. Cows were not injected with
bovine somatotropin during this experiment. Dietary
treatments were arranged as a 2 × 2 factorial of corn
hybrid (early- vs later-maturing) and Bt trait. The diets
contained (DM basis) 10% alfalfa silage, 40% of the
test corn silage, 28% rolled corn grain, and 22% of a
concentrate mixture composed of soybean meal, blood
meal, tallow, minerals, and vitamins. The diets were
designed so that the corn grain and corn silage were
from the same hybrid for each diet to maximize any
possible effect of the Bt trait on animal response. The
ingredient composition of the experimental diets is
shown in Table 4. All diets were formulated to contain
approximately 17.5% CP and to meet the metabolizable
protein requirement as predicted by the Cornell Net
Carbohydrate and Protein Model (1994). The four diets
were fed as total mixed rations twice daily in amounts
to ensure a minimum of 10% refusals.