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Resumo -O objetivo deste trabalho foi quantificar a taxa de emissão de metano (CH 4 ) pela técnica do gás traçador, hexafluoreto de enxofre (SF 6 ), em bovinos leiteiros a pasto em condições tropicais brasileiras. As medições foram realizadas na estação das chuvas, com adequada oferta de forragem, em animais da raça Holandesa e Mestiça Leiteira Brasileira em pastagem de capim-tobiatã (Panicum maximum Jacq. cv. Tobiatã) adubada, com vacas em lactação, vacas secas e novilhas, e em pastagem de capim-braquiária (Brachiaria decumbens Stapf.) não adubada com novilhas. As concentrações de CH 4 e SF 6 foram determinadas por cromatografia gasosa. A emissão de CH 4 pelas vacas em lactação foi de 13,8 a 16,8 g/hora, pelas vacas secas de 11,6 a 12,3 g/hora, pelas novilhas em pastagem adubada de 9,5 g/hora, e pelas novilhas em pastagem sem adubo de 7,6 a 8,3 g/hora ou 66 a 72 kg/animal/ano. A emissão de CH 4 por matéria seca digestiva ingerida foi de 42 a 69 g/kg em vacas em lactação, de 46 a 56 g/kg em vacas secas, 45 a 58 g/kg em novilhas ingerindo pasto adubado e 58 a 62 g/kg em novilhas em pastagem sem adubo. A emissão de CH 4 por bovinos leiteiros ingerindo gramíneas tropicais é superior à emissão por bovinos ingerindo gramíneas de clima temperado.Termos para indexação: traçador interno, hexafluoreto de enxofre, metano ruminal, gás de efeito estufa. Dairy cattle enteric methane measured in Brazilian tropical conditionsAbstract -The objective of this work was to quantify methane (CH 4 ) emission using the sulfur hexafluoride (SF 6 ) tracer technique, by dairy cattle on pasture in Brazilian tropical field conditions. Measurements were performed in the rainy season, with Holstein and Holstein x Zebu crossbred, from lactating and dry cows and heifers grazing fertilized Tobiatã grass, and heifers grazing unfertilized Brachiaria grass. Methane and SF 6 concentrations were determined by gas chromatograph. Methane emissions by lactating cows varied from 13.8 to 16.8 g/hour, by dry cows from 11.6 to 12.3 g/hour, by heifers grazing fertilized grass was 9.5 g/hour and by heifers grazing unfertilized grass varied from 7.6 to 8.3 g/hour or 66 to 72 kg/head/year. Methane emission per digestive dry matter intake (DMDI) varied from 42 to 69 g/kg DMDI for lactating cows, 46 to 56 g/kg for dry cows, 45 to 58 g/kg for heifers grazing fertilized grass and 58 to 62 g/kg for heifers in unfertilized grass pasture. The CH 4 emission measured on dairy cattle feeding tropical grasses was higher than that observed for temperate climate conditions. Index terms: internal tracer, sulfur hexafluoride, ruminal methane, greenhouse gas.(1) Embrapa Pecuária Sudeste, Caixa Postal 339, CEP 13560-970 São Carlos, SP. E-mail: odo@cppse.embrapa.br (2) Embrapa Meio Ambiente, Caixa Postal 69, CEP 13820-000 Jaguariúna, SP. E-mail: rosa@cnpma.embrapa.br, magda@cnpma.embrapa.br (3) Universidade Estadual Paulista, Fac. de Ciências Agrárias e Veterinárias, Via de acesso Professor Paulo Donato Castelanne, km 5, s/n o , CEP 14884-900 Jaboticabal, SP. E-mail: pedreira@f...

Ruminal gases, particularly methane, generated during the fermentative process in rumen, represent a partial loss of feed energy and are also pointed to as an important factors in greenhouse effect. This study aimed at quantifying methane (CH4) emission rates from lactating and dry cows and heifers, 24 month-old in average, on pasture under Southeast Brazil tropical conditions, using the tracer gas technique, sulphur hexafluoride (SF6), four animals per category, distributed in four blocks. Measurements were performed in February and June, 2002, with Holstein and Brazilian Dairy Crossbred (Holstein ¾ x Gir (Zebu) ¼), maintained on fertilized Tanzania-grass (Panicum maximum Jacq. cv. Tanzania) and fertilized Brachiaria-grass (Brachiaria decumbens cv. Basilisk) pastures. Heifers of both breeds were maintained on unfertilized Brachiaria-grass to simulate conditions of extensive cattle farming systems. CH4 and SF6 levels were measured with gas chromatography. Differences in CH4 emissions were measured (p < 0.05) for genetical groups. Holstein produced more methane (299.3 g day-1) than the Crossbred (264.2 g day-1). Lactating cows produced more methane (353.8 g day-1) than dry cows (268.8 g day-1) and heifers (222.6 g day-1). Holstein, with greater milk production potential, produced less CH4 (p < 0.05) per unit of dry matter intake (19.1 g kg-1) than the Crossbred (22.0 g kg-1). Methane emission by heifers grazing fertilized pasture (intensive system) was 222.6 g day-1, greater (p < 0.05) than that of heifers on unfertilized pasture (179.2 g day-1). Methane emission varied as function of animal category and management intensity of production system.

<p>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 <em>Urochloa brizantha </em>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.</p>