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Improving livestock production through nutrition and breeding can increase efficiency and has the potential to mitigate methane (CH 4) emissions. Additionally, supplementing beef cattle in the rainy season balances the dietary protein:energy (P:E) ratio, which can increase animal performance and reduce energy losses from CH 4 production. Therefore, the objective of this study was to evaluate the effect of supplementation strategy (SS) and genetic group (GG) on the intake, digestibility, performance, and enteric CH 4 emissions of growing beef bulls grazing tropical grass during the rainy season. One hundred sixty-two growing beef bulls averaging (mean ± SD) 10 ± 2 months old and 262 ± 31 kg of initial body weight (BW) were distributed, according to their BW, in a randomized complete block design in a 2 × 3 factorial arrangement. Factors included (1) two SSs (mineral supplementation at 0.3 g/kg of BW per day and a corn-based supplementation at 3 g/kg of BW per day) and (2) three GGs (Nellore [NN], ½Senepol½Nellore [SN], and ½Angus½Nellore [AN]). Animals were allocated in 12 paddocks composed of Urochloa brizantha (A. Rich.) Stapf. cv. Xaraés for 99 days during the rainy season. Regardless of the GG, the intakes of total DM, supplement DM, OM, CP, aNDFom, EE, and NFC were increased in animals supplemented with a corn-based supplement. The SN bulls had a greater digestibility of DM, OM, and CP, and animals supplemented with a corn-based supplement had greater CP and EE digestibility. There was an interaction between GG and SS for NFC digestibility, which was decreased in AN animals fed a corn-based supplement. However, the corn-based supplementation improved the animal's performance and carcass characteristics as demonstrated by the increase of final BW (kg), ADG (kg), REA (cm2), and FT (mm). Moreover, NN animals fed a corn-based supplement showed an increase in ADG (kg). An interaction between SS and GG was observed for GPH (kg/ha) and CaG (kg), with the greatest values observed in NN and SN animals supplemented with a corn-based supplement. Enteric CH 4 emissions (g/d, g/kg of DMI, and g/kg of dOM) were lower in animals fed a corn-based supplement. A decrease in CH 4 emissions (g/d) was observed in SN compared to NN animals. In addition, there was an interaction between SS and GG for CH 4 emissions (g/kg of CaG), with the lowest values for NN and SN animals supplemented with a corn-based supplement. Taken together, our results demonstrate that corn-based supplementation is an effective nutritional strategy for use in the rainy season, especially for NN and SN genetic groups, to improve animal's performance and carcass characteristics and to decrease enteric CH 4 emissions, per unit of product, of growing beef bulls grazing tropical grass. • Corn-based supplementation in the rainy season improved animal performance and carcass characteristics of grazing cattle. • Corn-based supplementation in the rainy season decreased CH 4 emissions, per unit of product, of grazing cattle. • Crossbreeding can be used for reducing CH 4 emissions (g/d) of grazing cattle. • Body weight and carcass weight were greater in Nellore and ½Senepol½Nellore animals supplement with corn-based. [ABSTRACT FROM AUTHOR]

• Protein-energy supplementation to grazing beef bulls was effective to maximize ruminal fermentation during the rainy season. • Senepol animals showed a lower acetate:propionate ratio; and when fed protein-energy supplement, a greater relative abundance of prevotella sp., S. bovis and S. ruminantium was observed. • Nellore animals had an increase in the relative abundance of total Archaea, F. succinogenes (when protein-energy supplemented) and R. albus (when mineral supplemented). The objective of this study was to evaluate the effects of mineral or protein-energy supplementation and animal genetic groups on rumen fermentation, digestibility and microorganisms of young beef bulls grazing tropical grass during the rainy season. Twelve (nn = 4 per genetic group) ruminally cannulated beef bulls [16 ± 2-months-old and 411 ± 34 kg initial body weight (BW)] were used in a cross-over design, with 4 periods, in a 2 × 3 factorial arrangement. Factors included: 1) two supplementation strategies (SS; mineral supplement [30 g/100 kg BW per day] and protein-energy supplement [300 g/100 kg BW per day]); and 2) three genetic groups (GG; Nellore [NL], ½Angus½Nellore [AN] and Senepol [SN]). Animals were continuously allocated in twelve paddocks composed of Urochloa brizantha (A. Rich.) Stapf. cv. Xaraés for 126 days during the rainy season. Supplementing animals of all GG with protein-energy supplement resulted in an increase of the intake of DM, OM, CP, EE, NFC and ME, and apparent total-tract digestibility of CP and EE. Additionally, animals fed protein-energy supplement showed highest rumen NH 3 N concentration, microbial-N synthesis, and retained-N. However, a decrease in rumen pH and acetate:propionate ratio was observed when animals were fed protein-energy supplement. Except for Ruminococcus flavefaciens , the relative abundance of total Archaea and Prevotella. spp. were increased when animals where fed protein-energy supplement. Moreover, a GG effect was observed on the relative abundance of total Archaea, Prevotella.spp. and Ruminococcus flavefaciens , which were highest in NL, SN and AN animals, respectively. SN animals fed protein-energy supplement showed an increase of Streptococcus bovis and Selenomonas ruminantium relative abundance, while Fibrobacter succinogenes and Ruminococcus albus were highest in NL animals supplement mineral and protein-energy supplement, respectively. In conclusion, protein-energy supplementation to beef bulls grazing high-quality tropical grass is an effective strategy to increase rumen fermentation during the rainy season. In addition, it modulated rumen microorganism leading to an increase in the relative abundance of Prevotella. spp., Streptococcus bovis and Selenomonas ruminatium , and a decrease in acetate:propionate ratio in Senepol animals. [ABSTRACT FROM AUTHOR]