The effects of accelerated growth rates and estrogen implants in prepubertal Holstein heifers on growth, feed efficiency, and blood parameters

Sixty-eight Holstein heifers were used to determine the effects of accelera ted growth rates by increased nutrient intake and estrogen implants on feed efficiency, structural growth, and blood parameters in heifers between 19 and 39 wk of age. At the beginning of the treatment period, the heifers wer e assigned to one of four treatment groups by using a randomized complete b lock design in a 2 x 2 factorial arrangement. The treatments were standard growth rate (700 g/d), accelerated growth rate (1000 g/d), standard growth rate with an estradiol implant, and accelerated growth rate with an estradi ol implant. All heifers received the same diet, but dry matter intake was a djusted weekly to achieve the target rate of gain. Accelerating heifer grow th rates from 705 to 1007 g/d improved feed efficiency 5.1%, increased the rate of withers height, heart girth, sind hip width growth 12, 27, and 27%, respectively, and body condition scores 0.25 points. Estradiol implants im proved feed efficiency 2.4% and decreased the rate of withers height 6% and heart girth growth 3.5%. Increased nutrient intake and average daily gain depressed mean plasma growth hormone and urea nitrogen content 17 and 7%, r espectively, while elevating insulinlike growth factor-1 levels by 10%. Est radiol implants increased mean plasma growth hormone content by 29% and ins ulin-like growth factor-1 levels by 17%, but decreased urea nitrogen conten t by 11%. Feeding prepubertal heifers for accelerated growth rates increase d structural growth with a small increase in body condition, whereas estrad iol implants improved feed efficiency and decreased the growth rate of with ers height and heart girth without affecting the rate of hip width growth.