Effect of sward management on the emissions of CH4 and N2O from faeces of sheep grazing Italian ryegrass pastures

Abstract

Rotatinuous stocking (RN) management is based on animal ingestive behaviour responses, where optimal pre- and post-grazing sward heights are defined to increase nutrient intake per unit of grazing time. We hypothesized that the optimal sward structure and consequently, a high herbage nutritive value in RN treatment results in a greater faecal nitrogen (N) excretion by sheep and consequently, a greater faecal greenhouse gas (GHG) emissions compared with the traditional rotational stocking (RT) management, which is based on a maximum herbage accumulation and harvest. Therefore, the aim of this study was to evaluate the effect of two grazing management strategies (RN and RT) on the amount of dry matter (DM) faecal excretion, faecal N excretion and faecal GHG (CH4 and N2O) emissions from growing sheep grazing Italian ryegrass pastures. In order to evaluate faecal production and N excretion per animal and per ha, a first experiment (1) was carried out: RT - pre and post-grazing sward heights of 25 and 5 cm, respectively and, RN - pre and post-grazing sward heights of 18 and 11 cm, respectively. A second experiment (2) was carried out to measure the CH4 and N2O fluxes from faeces, using the static chamber method. Daily DM faecal and N excretion per animal were higher (P < 0.001) in RN compared with RT treatment. However, when considered daily DM faecal and N excretion per ha, results were lower (P < 0.001) for the RN than the RT treatment. CH4 and N2O emissions from faeces were higher (P < 0.001) in RN compared with RT treatment, both per animal and per hectare. In conclusion, our study showed that the RN grazing management, based on animal behaviour, resulted in a higher daily N excretion per animal and higher CH4 and N2O emissions from faeces of sheep grazing Italian ryegrass pastures. This study contributes to improve GHG national inventories for the subtropical Brazilian climatic conditions, where estimations from CH4 and N2O emissions factors for faeces from growing sheep grazing Italian ryegrass are markedly lower than the values reported by IPCC Default Tier 1.