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

Helena Walsh , Donal Patton, Luc Delaby, JohnJoe Collins, Karina Pierce, Brendan Horan

Abstract

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 to 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 (8,925 and 8,561 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, WC 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<0.085) to have increased milk and milk fat plus protein yield (5,197 and 473 kg/cow, respectively) compared to the PR (5,092 and 461 kg/cow, respectively) per cow 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 resulted in substantial shortages in winter feed production during the transition to low chemical N PRWC systems.

AN24284  Accepted 28 February 2025

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