INFLUENCE OF UNDEGRADABILITY OF PROTEIN IN THE DIET ON INTAKE, DAILY GAIN, FEED-EFFICIENCY, AND BODY-COMPOSITION OF HOLSTEIN HEIFERS

Thirty-two Holstein heifers with body weights (BW) between 213 and 231 kg were randomly assigned to one of four treatments for the 50-d tria l. Treatments consisted of four percentages of rumen-undegradable prot ein (RUP) (31, 43, 50, and 55% of total N) at 100% of National Researc h Council recommendations for total digestible nutrients and crude pro tein. Total mixed diets composed of corn silage, ground barley straw, soybean meal, blood meal, urea, and minerals were formulated for a mea n daily BW gain of 0.60 kg. Ration RUP percentage was varied by shifti ng protein sources. Mean dry matter intake (grams per kilogram of BW0. 75) was 97.6, 84.4, 77.8, and 73.5 for 31% RUP (soybean meal), 43% RUP (blood and soybean meal), 50% RUP (blood meal with urea), and 55% RUP (blood meal) treatments, respectively. Daily gain was 0.84, 0.89, 0.9 1, and 0.96 kg/d, respectively. Intake of digestible energy (megacalor ies per kilogram of BW0.75 per day) was 0.28, 0.24, 0.22, and 0.21, re spectively, and feed efficiency (megacalories of digestible energy per kilogram of BW gain) was 20.6, 16.1, 15.2, and 13.3, respectively. Dr y matter intake (grams per kilogram of BW0.75), digestible energy inta ke, feed efficiency, daily BW gain, and hip height differed with respe ct to treatment. There were no differences in growth, wither height, o r heart girth because of treatments. Changes in percentage of empty bo dy fat as estimated by urea space procedures was 6.73, 4.67, 6.67, and 7.32, respectively, and did not differ with respect to treatments. Th ese results indicate that increasing the RUP percentage in the diets o f growing heifers improves feed efficiency and increases BW gain.