Effect of rumen-undegradable protein and energy on growth and feed efficiency of growing Holstein heifers.

Thirty-two Holstein heifers were used to determine the effect of energy and rumen-undegradable protein (RUP) on growth and feed efficiency. Treatment diets were in a 2 x 2 factorial consisting of two energy concentrations (supporting 0.6 or 0.9 kg of average daily gain) and two percentages of RUP (30 or 50% of crude protein). Isonitrogenous diets based on corn silage, alfalfa silage, orchardgrass hay, corn, soybean meal, blood meal, and minerals were fed. Treatment diets were fed until 385 d of age, constituting phase 1 of the trial. During phase 2 (until calving), heifers were housed together and fed a common diet for 0.7 kg of average daily gain. Dry matter intake (kilograms per day) during phase 1 was 4.46, 5.42, 7.38, and 5.95 for heifers fed low energy and low RUP, low energy and high RUP, high energy and low RUP, and high energy and high RUP diets, respectively. Average daily gain (kilograms) during phase 1 was 0.62, 0.74, 1.01, and 0.96, respectively. Dry matter intake and gain were higher for heifers receiving high energy diets. Dry matter efficiency was higher for heifers fed high energy and high RUP diets. Apparent total digestible nutrient efficiency was higher for heifers fed high RUP diets. Average daily gain during phase 2 was 0.53, 0.43, 0.33, and 0.50 kg, respectively. Overall average daily gain was not affected by energy or RUP levels during phase 1 because the slow growth of heifers during phase 1 was compensated by faster growth during phase 2.