164 Different supplementation strategies during the rearing phase and their effects on the body weight and the response to puberty induction protocol in Nelore heifers

Heifers require special attention due to their high energy and nutritional demands, and the backgrounding phase often coincides with the dry season for several months. During this period, forage productivity and nutritional composition decline. Weight gain can be influenced by both the quality and availability of pastures, as well as the sexual precocity of the future breeding stock. In this regard, this research aims to evaluate the impact of supplementation during backgrounding on body weight (BW), age at puberty, and response to a puberty induction protocol. Sixty-four weaned Nellore heifers (~8 months old, 270 kg) were assigned to a completely randomized design and subjected to four treatments based on the level of supplementation during the rearing phase: T1 – Low (Mineral - 0.03 g/kg of BW), T2 – Marginal (0.3 g/kg of BW), T3 – Medium (0.6 g/kg of BW), and T4 – High (0.9 g/kg of BW). The phase lasted 7 months, during which the live weight of the animals was evaluated at the beginning, middle, and end of the phase. Reproductive evaluations were conducted at 12 and 14 months to assess puberty status based on the presence or absence of a Corpus Luteum (CL). Following the final evaluation, a progesterone (D-0) and estradiol (D-14) based hormonal protocol was applied for puberty induction. After 24 days, an analysis of the protocol response was performed. According to Table 1, at the first weighing, the four groups had the same weight. By the second weighing, T4 had a significantly higher weight than T1 (p = 0.04). At the final weighing, treatment significantly influenced body weight, with T1 being lower than all other groups (p < 0.001) and T4 showing the highest average weight compared to T2 (p = 0.003). At the first reproductive evaluation, none of the heifers had reached puberty. By the second evaluation, three animals (two from T3 and one from T4) were pubertal, with no significant difference between treatments (p > 0.05). Following the puberty induction protocol, 46 out of 64 heifers (71.9%) reached puberty, with no significant differences between treatments (p > 0.05). However, a tendency was observed between T1 and T2 (p = 0.0652). Body weight data were analyzed using the ‘lm’ function in R, considering treatment as the sole fixed effect in the model. For the presence of the Corpus Luteum (CL), the ‘glm’ function was used for a logistic model (binary trait). Supplementation had a direct impact on body weight; however, no significant differences were found between treatments regarding age at puberty or response to the puberty induction protocol. A high level of supplementation resulted in greater body weight gain during the backgrounding phase.