EFFECT OF RATE AND TIME OF GAIN ON DEVELOPING HEIFERS AND THE EFFECT OF ENERGY INTAKE ON LH LEVELS

Three experiments were conducted during three consecutive years to compare rates and time of gain of weaned heifer calves on their growth, reproductive development and future calf production. Three groups of heifers were fed one of the following treatments from 30 days after weaning until breeding: (1) no gain the first half of the development period followed by .91 kg/head/day gain the last half; (2) .45 kg/head/day gain the entire period; and (3) .91 kg/head/day gain the first half of the development period and no gain the last half. By the end of the feeding trials there were no differences (P \u3e .05) in weight, wither height or heart girth circumference. There were no differences (P \u3e .05) in age at puberty, conception rate or calf production the following year. Two trials were conducted with yearling heifers to evaluate the effect of increased levels of dietary energy on weight change serum LH levels, estrous activity, conception rates and services/conception. In Trial 1, three groups of eight heifers each were fed for 68 days a daily ration of 18.2 kg corn silage and one of three supplements: (1) .45 kg of 40% crude protein (CP), (2) .91 kg of 20% CP and (3) 1.82 kg of 10% CP. Juglar blood was collected at 6 hour intervals beginning at estimated 72 hours before estrus and continuing for 24 hours after estrus. Length of estrous cycle (18.7, 19.4 and 19.7 days) and daily weight gains (.48, .49 and .62 kg) were not different (P \u3e .05) for treatments 1, 2 and 3, respectively. Mean peak serum LH levels of the preovulatory surge were 19.0, 23.6 and 31.1 ng/ml for treatments 1, 2 and 3, respectively. Heifers fed Supplement 3 had a higher (P \u3c .10) LH peak than those fed Supplements 1 or 2. Heifers fed Supplements 1 and 3 had higher (P \u3c .005) LH levels at 6 hours before estrus than those fed Supplement 2. Three groups of 18 yearling heifers each on pasture were use in Trial 2. There were no differences (P \u3e .05) in weight gain during the summer or conception rate resulting from feeding no supplement, .91 kg of 20% CP or 1.82 kg of 10% CP supplement for 21 days prior to and continuing through the first 21 days of a 42 day breeding season