Farm-dependent effects of slurry discharge frequency on organic matter transformation and methane emissions in pig slurry

Frequent slurry discharge from pig houses is widely implemented to mitigate methane (CH 4 ) emissions, but its effectiveness varies across production systems, and the underlying mechanisms remain insufficiently understood. This study investigated how slurry discharge frequency affects organic matter transformation and CH 4 production. Slurry from weekly and 4–5 week discharge treatments in two growing finishing pig farms was incubated with corresponding faeces and urine at 10, 17.5, and 25 °C for 90 days. Results showed that the effect of discharge frequency on CH 4 production differed between farms. In slurry from farm A, weekly discharge was associated with 18–73% lower CH 4 production, particularly at higher temperatures, together with lower organic matter degradation rates and slower VFA turnover. In slurry from farm B, the effect was limited, associated with higher organic matter degradability and sustained VFA supply. These conditions may have sustained methanogenesis under both discharge treatments, thereby weakening the effect of discharge frequency on CH 4 production. Despite limitations in reproducing practical slurry conditions, weekly discharge could also reduce in-house CH 4 emissions in systems like farm B by reducing slurry volume. However, rapid CH 4 production in the weekly discharge treatment indicates limited suppression of methanogenesis, highlighting the need for additional mitigation measures. These findings indicate that farm-specific differences in organic matter degradation rates may influence the effect of slurry discharge on CH 4 production and should be considered in process-based CH 4 models. Science4Impact Statement. Frequent slurry discharge from pig houses is a common approach for CH 4 mitigation, but its effectiveness varies substantially across farms. This study reveals that differences in organic matter degradation and VFA dynamics were associated with contrasting CH 4 production responses to discharge frequency. Rapid organic matter degradation and sustained VFA accumulation may weaken the effect of discharge frequency on CH 4 production, indicating that such systems may benefit from additional mitigation strategies. These findings highlight the importance of considering variation in slurry degradation when evaluating slurry management strategies and developing process-based CH 4 models.