Interaction of dietary energy source and body weight gain during the juvenile period on metabolic endocrine status and age at puberty in beef heifers
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2017 American Society of Animal Science. All rights reserved. Using a previously established model for nutritional acceleration of puberty, beef heifers (n = 48; 1/2 Angus 1/4 Hereford 1/4 Brahman) were used in a replicated 2 2 factorial design to examine the effects of diet type (high forage [HF] vs. high concentrate [HC]) and rate of BW gain (low gain [LG], 0.45 kg/d, vs. high gain [HG], 0.91 kg/d) on key metabolic hormones and age at puberty. After weaning at 14 1 wk of age, heifers were assigned randomly to be fed HC-HG, HC-LG, HF-HG, or HF-LG (n = 12/ group) beginning at 4 mo of age for 14 wk. Heifers were then switched to a common growth diet until puberty. Average daily gain was greater (P < 0.04) during the dietary treatment phase in HG heifers (0.81 0.06 kg/d) than in LG heifers (0.43 0.06 kg/d), and there was no diet type rate of gain interaction. Puberty was achieved at a younger age (54.5 1.8 wk) in both HG groups than in LG groups (60.2 1.9 wk; P < 0.04), but dietary energy source (HC vs. HF) did not influence this variable. Moreover, mean BW at puberty did not differ by diet type or rate of gain dur- ing the dietary treatment phase. Nonetheless, heifers fed HC-HG exhibited a striking increase (P < 0.0001) in serum leptin beginning at 26 1 wk of age and remained elevated (P < 0.01) throughout the remainder of the experimental feeding phase compared to all other treatments. However, serum leptin in HC-HG dropped precipitously when heifers were switched to the common growth diet and did not differ from that of other groups thereafter. Overall mean concentrations of serum glucose were greater (P < 0.006) in HG heifers than in LG during the dietary treatment phase, with serum insulin also greater (P < 0.04) in HG than in LG only during weeks 20, 22, and 30. Mean serum IGF-1 was not affected by dietary type or rate of BW gain. We speculate that failure of the marked increase in serum leptin observed in HC-HG heifers during the dietary treatment phase to further accelerate puberty compared to HF-HG occurred because of its abrupt decline at the onset of the common growth phase, thus attenuating the temporal cue for activation of the reproductive neuroendocrine system.
