Methane emissions by beef cattle consuming hay of varying quality in the dry forest ecosystem of Costa Rica

In livestock production systems, methane (CH4) is produced and released during the digestive process, representing a loss of energy that can be as high as 12% of total intake. In Costa Rica there are not actual in vivo measurements of methane produced from enteric fermentation in the livestock sector. This research represents the first effort to quantify the CH4 emitted by growing beef steers fed three different diets during the dry season in the Dry Tropics ecosystem of Costa Rica, using the SF6 tracer technique. Three diets were evaluated, all of them offered at libitum: 1) Good quality hay of transvala (Digitaria decumbens; GOOD). 2) Low quality Brachiaria tanner hay (POOR). 3) Low quality hay (B. tanner) plus a supplement of 1 kg/d of sugar cane molasses mixed with 46 g/d of urea (POOR+MU). Nine Brahman steers (329±38 kg of body weight) were utilized in a triplicated 3×3 Latin square for a total of 9 replicates/treatment. Variables measured were in vivo CH4 emissions, feed intake, and apparent total tract digestibility of nutrients using indigestible neutral detergent fiber (NDF) as an internal indigestible marker. Dry matter intake (DMI) was greater in GOOD (7.9 kg/d) compared to the remaining two diets (3.6 and 4.2 kg/d for POOR and POOR+MU, respectively). Enteric CH4 emission (g/d) was similar (P>0.05) for POOR (110.4) and POOR+MU (125.8) but lower (P<0.0001) than that of GOOD (181.5); when the methane emitted was reported as g of CH4/kg of DMI, greatest (P<0.0001) emissions were detected with POOR (31.0) and POOR+MU (29.8), and lesser in diet 1 (23.0±1.9). Estimated methane yield (Ym) for GOOD (6.9) was similar to that suggested by the Intergovernmental Panel on Climate Change (IPCC); however, greater values than those reported by the IPCC were obtained for POOR (9.3) and POOR+MU (9.0). In conclusion, CH4 emitted by growing Brahman steers fed hay of varying quality was closely related to daily DMI. Furthermore, when CH4 emission was expressed per unit of DMI, poor quality hay increased emissions intensity, regardless of supplementation with urea and molasses. Supplementing poor quality B. tanner hay with urea and molasses increased hay digestibility but did not alter methane emissions. Feeding good quality D. decumbens hay decreased CH4 emissions (in g/kg of DMI) by 30% relative to those by steers receiving poor quality B. tanner hay during the dry season in Costa Rica.