Seasonal Effect on Feed Intake and Methane Emissions of Cow–Calf Systems on Native Grassland with Variable Herbage Allowance

SIMPLE SUMMARY: Improving grazing management provides the opportunity of limiting methane emissions from beef cattle systems, and consequently offers economic along with environmental benefits. The aim of this study was to measure methane emissions and herbage intake, in order to estimate the methane yield from beef cows grazing on native grasslands at different herbage allowances. The trial, that it is part of a long-term experiment, consisted in two treatments of herbage allowance, with forty pregnant heifers. Methane emissions and intake were estimated for three 17-day periods during autumn, winter and spring. Methane emissions and organic matter intake did not differ between herbage allowance treatments, which resulted in similar methane yield. However, all variables were significantly affected by the period, with a marked increase in spring, except for methane yield expressed as a proportion of Gross Energy intake. Results show that methane emissions and intake were significantly affected by the season of the year, but not by the level of herbage allowance used in this study. These are the first data obtained on methane emissions in pregnant heifers in native grassland for Uruguay. ABSTRACT: The aim of this study was to measure methane emissions (CH(4)) and herbage intake, and, on the basis of these results, obtain the methane yield (MY, methane yield as g CH(4)/kg dry matter intake (DMI) and Ym, methane yield as a percentage of Gross Energy intake), from beef cows grazing on native grasslands. We used forty pregnant heifers, with two treatments of herbage allowance (HA) adjusted seasonally (8 and 5 kg dry matter (DM)/kg cattle live weight (LW), on average), during autumn, winter and spring. Methane emissions (207 g CH(4)/d), organic matter intake (OMI, 7.7 kg organic matter (OM)/d), MY (23.6 g CH(4)/kg DMI) and Ym (7.4%), were similar between treatments. On the other hand, all variables had a marked increase in spring (10.8 kg OM/d and 312 g CH(4)/d), except for Ym. The methane emission factor from Intergovernmental Panel on Climate Change (IPCC) Tier 2 estimated with these results was 78 kg CH(4)/head/year. The results show that methane emissions and intake were influenced by the season, but not by the HA analyzed in this study. This information for cow–calf systems in native grasslands in Uruguay can be used in National greenhouse gases (GHG) inventories, representing a relevant contribution to global GHG inventories.