Effect of feeding forage characteristic of wet- or dry-season tropical C4 grass in northern Australia, on methane production, intake and rumen outflow rates in Bos indicus steers L. A. Perry A,B,D , R. Al Jassim A , J. B. Gaughan A and N. W. Tomkins B,C A School of Agriculture and Food Sciences, The University of Queensland Gatton Campus, Gatton, Qld 4343, Australia. B CSIRO, Australian Tropical Science and Innovation Precinct, Building 145, James Cook University, James Cook Drive, Douglas, Qld 4812, Australia. C Meat and Livestock Australia, Level 2, 527 Gregory Terrace, Fortitude Valley, Qld 4006, Australia. D Corresponding author. Email: l.perry1@uq.edu.au Abstract. Methane production (MP) from Bos indicus steers fed Chloris gayana hay characteristic of the ‘dry season’ (LQH), and a fresh Urochloa mosambicensis grass (PAS) or a C. gayana hay (HQH) characteristic of the ‘wet’ season was determined. A longitudinal feeding trial incorporated a 42-day covariate period (P1) in which Brahman steers (total n = 12) were fed ad libitum LQH (g/kg DM: crude protein (CP) 25; acid detergent fibre expressed exclusive of residual ash (ADFom) 487; DM digestibility (DMD) 380) followed by a 42-day treatment period where steers (n = 4 in each) were randomly assigned to PAS, HQH or remained on LQH (control). The diet composition in P2 was HQH (g/kg DM: CP 88; ADFom 376; DMD 590), PAS (g/kg DM: CP 90; ADFom 324; DMD 630) and LQH (g/kg DM: CP 31; ADFom 461; DMD 410). For each period, on Days 35–41, individual dry-matter intakes (DMI), rumen fermentation parameters and both fluid and particulate fractional rumen outflow rates were measured. On Days 41 and 42, MP was determined using open-circuit respiration chambers. There were diet effects on MP, DMI, volatile fatty acids, and ammonia-N. Both PAS- and HQH-fed steers had greater MP (g/day; P < 0.05) and DMI (P < 0.05) than did those fed LQH, but a lower MP per kilogram DMI digested. The use of predictive equations compared with measured data confirmed prior observations that MP from tropical grasses in the northern Australian rangelands may be overestimated using the current equations for greenhouse gas accounting. Additional keywords: cattle, forage quality, greenhouse gas, northern rangelands, respiration chamber. Received 19 June 2015, accepted 1 July 2016, published online 18 October 2016 Introduction Enteric methane production from ruminant livestock is a significant source of anthropogenic methane emissions for the agriculture sector. Agriculture contributes 16% of Australian greenhouse gas (GHG) emissions, with enteric methane being the largest contributor (87.9 Mt carbon dioxide equivalent (CO 2 - eq); 67%; Department of the Environment 2014). In comparison with CO 2 , methane has 28 times the global warming potential (IPCC 2014). This high global warming potential, combined with a relatively short atmospheric lifespan (12.5 years) compared to CO 2 , makes methane a candidate for GHG mitigation (IPCC 2014). Methane, produced by methanogenic archaea in the rumen, provides a hydrogen sink for re-oxidation of reducing equivalents, NADH and NADPH, essential for the continued anaerobic fermentation of plant material (Johnson and Johnson 1995). Methane production also represents loss of up to 12% of the gross-energy intake (GEI) in ruminants (Johnson and Johnson 1995). Methane mitigation has the dual benefit of redirecting methane energy into animal production, and reducing the anthropogenic contribution of agriculture to GHG emissions. Forage diets are characterised by higher methane production (MP), as a consequence of fermentation of hemicellulose and cellulose rather than neutral detergent soluble carbohydrates in grain diets (Moe and Tyrrell 1979; Johnson and Johnson 1995) and a shift in rumen fermentation pathways from propionate to acetate (Bergman 1990; Johnson and Johnson 1995). Approximately 10% more methane is produced by cattle grazing C4 than those grazing C3 grasses (Archimède et al. 2011) associated with forage composition. Tropical C4 grasses have greater internal lignification and cell wall content (cellulose and hemicellulose), resulting in longer retention times in the rumen and characteristically low digestibility (Minson 1990) compared with C3 species. Methane production from forage-fed cattle is improved when C4 grasses are replaced by C3 grasses (Ulyatt et al. 2002), legumes (C3) are included in the diet (Beauchemin et al. 2008; Soltan et al. 2013), CSIRO PUBLISHING Animal Production Science http://dx.doi.org/10.1071/AN15314 Journal compilation Ó CSIRO 2016 www.publish.csiro.au/journals/an