Metabolic, endocrine, and reproductive responses of beef heifers submitted to different growth strategies during the lactation and rearing periods.

The effects of different feeding strategies (0.7 kg/d target ADG [LO] and 1.0 kg/d target ADG [HI] during the lactation period (LACT; 0-6 mo) and the rearing period (REAR; 6-15 mo; HI-HI, HI-LO, LO-HI, and LO-LO treatments) on the growth and reproductive parameters of beef heifers bred by fixed-time AI at 15 mo were analyzed. Animal weights were recorded weekly (from birth to 18 mo), and size measures were recorded at 6 and 15 mo. Heifers were bled to determine the onset of puberty and the metabolic and endocrine (IGF-I and leptin) status. During lactation, calves in the high lactation treatment (LactHI) had greater weight ( < 0.001), weight gain ( < 0.001), and body size ( < 0.001) than calves in the low lactation treatment (LactLO). The greater energy balance of LactHI heifers at weaning was reflected in greater concentrations of plasma glucose ( < 0.001), urea ( < 0.001), and IGF-I ( < 0.001); plasma levels of NEFA were lower ( < 0.001). During REAR, LactLO heifers had a greater growth rate than did LactHI heifers ( < 0.001), partially overcoming the lower gains during lactation. The differences in size measurements registered at weaning were also compensated, with the exception of LO-LO heifers. The IGF-I profile was highly correlated with animal performance traits and metabolic profiles, providing a useful indicator of growth, nutritional, and metabolic status at key points in development. By contrast, the function of leptin as an indicator of growth and reproductive development of heifers was less clear. All treatments had similar weights at puberty onset (55.9% mature BW), although LactLO ( < 0.01) and the low rearing treatment (RearLO; < 0.001) heifers were older than the others. The animals with greater glucose and IGF-I levels at weaning and greater cholesterol concentrations during REAR reached puberty earlier. The fertility rate (86%) was similar among treatments. The heifers in the high rearing treatment (RearHI) required more AI services to become pregnant and were older at conception ( < 0.05). The age of conception was positively correlated with glucose ( = 0.57, < 0.01) and cholesterol ( = 0.68, < 0.001) at 9 mo. Our results show that a 0.7 kg/d gain from birth allowed the first breeding at 15 mo, 6 mo earlier than usual for these conditions, without any negative effect on heifer reproductive performance.