Performance and methane emissions by beef heifer grazing in temperate pastures and in integrated crop-livestock systems: The effect of shade and nitrogen fertilization

Methane (CH4) emissions from enteric fermentation by cattle are the main source of greenhouse gases in livestock systems, but scarce information is available on the effect of production scenarios on CH4 emissions. The objective of this study was to assess animal performance and CH4 emissions by beef heifers grazing in a mixed Lolium multiflorum × Avena strigosa pasture in two different types of integrated systems, crop-livestock only (CL) or crop-livestock-tree (CLT), and with two nitrogen (N) supply levels (90 and 180 kg N ha−1, N90 and N180, respectively). The study was performed during the winter season (i.e., stocking season) over three years (2012-2014). Pasture shading by 6- to 8-year-old trees in the CLT system led to a reduction in the total annual herbage accumulation of approximately 2.6 Mg dry matter ha−1 (−51%) and a reduction in winter carrying capacity of 0.5 stock units ha−1 in comparison to the CL system. Average daily gain (ADG) was reduced by 32% in CLT compared to CL. Average CH4 emissions were 163 ± 9.12 g CH4 day−1 in heifers with an initial average live weight (LW) of approximately 250 kg. Emissions of CH4 per kg of LW did not differ between treatments (P > 0.05) with a mean of 0.58 ± 0.030 g kg−1. However, there were significant differences between treatments and among years when CH4 was expressed in g kg ADG−1 and kg ha−1 day−1. Per unit area, CH4 production ranged between 0.51 ± 0.05 (CLT N180) and 0.86 ± 0.12 kg ha−1 day−1 (CL N180). Possible strategies to reduce losses in animal production in the CLT systems are discussed, as well as the potential of C sequestration by woody biomass of eucalyptus trees to mitigate CH4 emissions.