The Effect of Dietary Source of Energy on Age and Weight at Puberty in Beef Heifers

This experiment was conducted to establish the effects of source of dietary energy on age and weight at puberty of beef heifers. Forty-four Angus and Simmental-sired -crossbred heifers (mean age = 202 days, mean wt = 249 kg) were allotted to either a high forage (HF; 80% alfalfa-brome hay and 20% corn) or a high concentrate (HC; 12% alfalfa-brome hay, 80% corn, and 8% supplement) diet in Experiment 1 for 172 days. Feed intake was adjusted according to body weight (measured every 28 days) to insure equal gains between treatments. Heifers were considered pubertal when progesterone concentrations were ≥ 1 ng/ml for two consecutive weeks or they were observed in estrus during the 40-day breeding period that followed the 172-day treatment period. Serum collected on day 1, 95, and 172 was assayed for glucose (GLC) colorimetrically and insulin {INS) by radioimmunoassay. Age at puberty (354.0 vs 352.6 days), weight at puberty {331.1 vs 305.1 kg), and pregnancy rate (60.9 vs 65.0%) were similar for HF and HC treatment groups. Serum GLC and INS were not affected by diet. Glucose was correlated with age on day 1 (r = -.44, P, P\u3c.01) and day 172 (r=-.35, P\u3c.05). Serum GLC was higher on day 1 for heifers that were pubertal by day 116 than nonpubertal heifers (91.3 vs 79.0 mg/dl, respectively; P\u3c.05). The HC diet had no beneficial or detrimental effect on development of replacement heifers