Genetics Is the Essential Factor for the Precocious Puberty in Nellore Heifers

There is a paradigm that Nellore heifers' reproductive life starts at ≥ 24 mo of age due to nutritional conditions of low-quality pasture. This study aimed to determine the genetics and nutrition effects on puberty attainment in Nellore (Bos indicus) heifers. Fifty-eight weaned heifers (174 ± 6 kg initial BW, and 8 ± 1 mo of age) were assigned to 28 feedlot pens. Heifers were born from 4 sires: 2 were precocious (P – negative expected progeny difference (EPD) to age at first calving; n = 33) and 2 non-precocious (NP - positive EPD to age at first calving; n = 25). Heifers of each EPD were randomly assigned to 2 nutritional strategies: high ADG (HG; 0.7 kg) or low ADG (LG; 0.3 kg), resulting in 4 treatments: heifers from P sires were submitted to either HG (PHG; n = 17) or LG (PLG; n = 16), and heifers from NP sires were submitted to either HG (NPHG; n = 12), or LG (NPLG; n = 13). The HG heifers were fed with 75% concentrate diet, whereas the LG heifers were given 93% of forage in their diet. Blood samples were collected at 9, 14, 18, 24, and 28 mo of age for plasma IGF-I determination. Transrectal ultrasonography and progesterone concentration were assessed weekly to determine puberty onset. The proportion of heifers that attained puberty at 18, 26 and 30 mo of age, and BW at puberty were assessed by GLIMMIX procedure using binomial, and normal option, respectively. The MIXED procedure, with repeated measure analysis, was used to assess the interaction between treatment and time in IGF-I concentration. There was a treatment effect (P < 0.01) in the % of heifers that attained puberty by 18 (PHG: 62%, PLG: 0%, NPHG: 0%, and NPLG: 0%), 24 (PHG: 100%, PLG: 6%, NPHG: 54%, and NPLG: 0%) or 36 (PHG: 100%, PLG: 100%, NPHG: 100%, and NPLG: 38%) mo of age. The BW at puberty was 360^ab, 340^a, 468^c, and 390^b ± 15 kg for PHG, PLG, NPHG, and NPLG, respectively. Plasma IGF-I concentrations were higher in P heifers than NP cohorts in all ages and in the same ADG (P < 0.01; treatment × age interaction). In conclusion, EPD of age at first calving was the essential factor for puberty onset at 18 mo of age in Nellore heifers, but ADG was a limiting factor at puberty.