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Feeding a high energy diet to heifers weaned at 2 months of age hastens sexual maturation and results in precocious puberty (first ovulation before 300 days of age). Two experiments were designed to study metabolic, reproductive and molecular responses that lead to precocious puberty and in the first experiment, impact of composition of high energy diet on these responses in beef heifers. In the first experiment the objective was to test the influence of feeding either a high energy diets with a high starch content (46.4 % starch, n = 10) or low starch content (14.48 % starch, n = 12) and control diet (19.03 % starch, n = 11) weaned at 76 ± 1.1 d of age and beginning the diets at 98 ± 1.1 d of age on age at puberty and the impact of these diets on metabolic and reproductive hormones. Precocious puberty was induced in 42 % (5/12) and 60 % (6/10) of heifers in the low and high starch diets, respectively and none in the control diet. The high energy diets induced increased IGF-I concentrations as compared to the control diet that contained lesser energy, and this response was only marginally influenced by composition of the high energy diets. Across the two high energy diets, heifers tha