Abstract

The objective of this study was to evaluate the effect of seasons under a tropical climate on forage quality, as well the effect of an Urochloa brizantha cv. Marandu grazing system on enteric methane (CH₄) emissions from Nellore cattle in the Southeast region of Brazil. Sixteen Nellore steers (18 months old and initial weight 318.0 ± 116.59 kg of LW; final weight 469 ± 98.50 kg of LW) were used for a trial period of 10 months, with four collection periods in winter (August), spring (December), summer (February) and autumn (May). Each collection period consisted of 28 days, corresponding to the representative month of each season where the last six days were designed for methane data collection. Animals were randomly distributed within 16 experimental plots, distributed in four random blocks over four trial periods. CH₄ emissions were determined using the sulphur hexafluoride (SF₆) tracer gas technique measured by gas chromatography and fluxes of CH₄ calculated. The forage quality was characterized by higher CP and IVDMD and lower lignin contents in spring, differing specially from winter forage. Average CH₄ emissions were between 102.49 and 220.91 g d^-1 (37.4 to 80.6 kg ani^-1 yr^-1); 16.89 and 30.20 g kg^-1 DMI; 1.35 and 2.90 Mcal ani^-1 d^-1; 0.18 and 0.57 g kg^-1 ADG^-1 and 5.05 and 8.76% of GE. Emissions in terms of CO₂ equivalents were between 4.68 and 14.22 g CO₂-eq^-1 g^-1 ADG. Variations in CH₄ emissions were related to seasonal effect on the forage quality and variations in dry matter intake.

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