Transitioning to low chemical nitrogen perennial ryegrass white clover pastures on wetland grazing dairy systems

Context Reductions in chemical nitrogen fertiliser applications in agricultural systems within the European Union will have a significant role in reducing greenhouse gas emissions related to agriculture. Aims The current study investigates the transition from old permanent perennial ryegrass (PR) swards (PR-old) to newly established PR (PR-new) receiving high levels of chemical nitrogen (N) application or PR white clover swards (PRWC) receiving lower levels of chemical N application on wetland soils in the border, midland and western region of Ireland over 3 years (2021–2023, inclusive). Methods The experimental site, which consisted of old PR swards, was transitioned to either new PR swards receiving up to 250 kg N/ha per annum or PRWC swards receiving up to 125 kg N/ha annually. Key results In the year of establishment, PR-new and PRWC-new swards has a significantly reduced total pasture DM yield (8925 and 8561 kg DM/ha) compared to older PR swards (14,182 kg DM/ha) while PRWC oversown (PRWC-over) swards were intermediate (11,330 kg DM/ha). In subsequent years, PR-new, PRWC-new and PRWC-over swards achieved increased DM yield (14,891, 15,642 and 15,218 kg DM/ha) compared to older swards. Within PRWC, white clover contents increased from 0 g/kg DM in early 2021 to 250 and 190 g/kg DM in 2022 and 2023, respectively. Consequently, significant reductions in chemical N fertiliser applications were achieved, from 229 and 200 kg N/ha for PR-old and PR-new swards, respectively, and 124 and 84 kg N/ha for PRWC-over and PRWC-new swards, respectively. The PRWC system tended (P < 0.08) to have increased milk and milk fat plus protein yield (5197 and 473 kg/cow, respectively) compared to the PR (5092 and 461 kg/cow, respectively) during the 3 year study period. Conclusion These results highlight the potential for PRWC systems to increase pasture production and milk production, while reducing chemical N applications in comparison to PR only systems on a wetland soil. Implications The extent of sward renewal and initial reduction in pasture production may result in shortages in winter feed production during the transition to low chemical N PRWC systems.