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 measurements of mammary growth. Forty-five Holstein heifers were randomly assigned to either a low, medium, or high ratio of dietary crude protein (CP) to metabolizable energy of 46:1, 54:1, and 61:1 g/Mcal, respectively. The ratio of dietary protein to energy was altered by adjusting the concentration of CP with a similar amount of energy across all diets. Heifers were individually 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, all heifers were fed for a daily dry matter intake (DMI) of 2.45% of body weight. Body weight was monitored for two consecutive days each week and was used 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 resulted in larger heifers that were subsequently fed 3% more DMI over the course of the trial. The increased feed efficiency and DMI increased average daily 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, and 12%, respectively. The heifers fed the high ratio of dietary protein to energy 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 increased by 35 to 38% for heifers on the high versus low ratio of dietary protein to energy. Feeding dietary ratios of protein to energy above NRC recommendations improved feed efficiency and increased average daily gain, structural growth, and mammary development while decreasing body condition scores in heifers between 28 and 48 wk of age.
All Science Journal Classification (ASJC) codes
- Food Science
- Animal Science and Zoology