The response of altering the ratio of dietary protein to energy on growth,feed efficiency, and mammary development in rapidly growing prepubertal heifers

This study was conducted to evaluate the effects of increasing the ratio of dietary protein to energy above National Research Council recommendations on average daily gain, feed efficiency, structural growth, and indirect mea surements of mammary growth. Forty-five Holstein heifers were randomly assi gned to either a low, medium, or high ratio of dietary crude protein (CP) t o metabolizable energy of 46:1, 54:1, and 61:1 g/Mcal, respectively. The ra tio of dietary protein to energy was altered by adjusting the concentration of CP with a similar amount of energy across all diets. Heifers were indiv idually fed and began the treatment period at 200 kg of body weight and 28 wk of age. During the 3-wk adaptation period and 20-wk treatment period, al l heifers were fed for a daily dry matter intake (DMI) of 2.45% of body wei ght. Body weight was monitored for two consecutive days each week and was u sed to adjust the dry matter offered on a weekly basis. The high versus low ratio of dietary protein to energy increased feed efficiency 6%, which res ulted in larger heifers that were subsequently fed 3% more DMI over the cou rse of the trial. The increased feed efficiency and DMI increased average d aily gain by 9% for the high versus low ratio of dietary protein to energy. For the high versus low ratio of dietary protein to energy, hip width, hip height, wither height, and heart girth growth was increased 13, 16, 18, an d 12%, respectively. The heifers fed the high ratio of dietary protein to e nergy had a lower rate of increased body condition score compared with the heifers fed the low ratio of dietary protein to energy. Teat length growth was used as an indirect measurement of mammary ductal development and was i ncreased by 35 to 38% for heifers on the high versus low ratio of dietary p rotein to energy. Feeding dietary ratios of protein to energy above NRC rec ommendations improved feed efficiency and increased average daily gain, str uctural growth, and mammary development while decreasing body condition sco res in heifers between 28 and 48 wk of age.