Greenhouse gases measurement time reduction in Portable Accumulation Chambers with grazing sheep evaluated morning and afternoon

Portable Accumulation Chambers (PAC) represent an important tool for quantifying greenhouse gases (GHG) in small ruminants. The objectives of the study were to evaluate to reduce the PAC time evaluation in sheep without compromising the GHG, methane (CH₄) and carbon dioxide (CO₂) measurements and to evaluate the PAC methodology, considering the variability of gaseous emissions in the day. Four times the animals spent in the PAC (10, 20, 30 and 40 minutes) and two collection times during the day (morning and afternoon) were tested for two consecutive days. The sheep used in the study came from a field experiment where they were subjected to different grazing feeding systems (only Italian ryegrass, Italian ryegrass in a mixture of legumes (Persian clover (Trifolium resupinatum L.) and White clover (Trifolium repens L.)) pasture and lambs grazing Italian ryegrass and consuming an energetic supplement (RyeG + Sup)). The gas samples collected at PAC were analyzed for CH₄ and CO₂ concentration on a gas chromatograph. There was no interaction in CH₄ and CO₂ measurements between times, shifts and grazing feeding systems (p > 0.05). CH₄ emission between times did not differ (p > 0.05), while CO₂ emissions were higher in the first 10 minutes of measurement and lower in the remaining times, fitting the non-linear regression model, with a reduction until 27.7 minutes. In relation to shifts, there was a difference for CH₄ (p < 0.01), and CO₂ (p = 0.01) with the highest emissions of CH₄ (21.7 ± 4.3 g/day) and CO₂ (1282 ± 332 g/day) found in the afternoon. The results of this study indicate that 27.7 minutes of sheep staying in the PAC is enough to evaluate CH₄ and CO₂ emissions and to improve the estimate of the gas emissions, evaluations should be carried out in the morning and afternoon.