Productivity of pasture-based systems with lower chemical and feed n inputs on a wet mineral soil

To meet national and European Union (EU) climate targets will require pasture-based dairy systems to reduce reliance on purchased inputs while sustaining output. Evidence to guide such reductions on wet mineral soils remains underexplored, as the majority of Irish research has been undertaken on freely draining soils in southern location. This study quantifies system performance under reduced chemical and concentrate feed nitrogen (N) on such soils. The objective of this study was to evaluate the impact of lowering farm-gate N inputs (i.e., N imported through the farm gate) through reductions in chemical fertilizer and concentrate feed N on the performance of a pasture-based dairy system on a wet mineral soil. A 2 × 2 factorial design was used to establish 4 experimental farmlets of 13 ha, consisting of 2 sward systems (SS), and 2 concentrate feed levels (CF). The SS were (1) perennial ryegrass (PRG; Lolium perenne L.) monoculture swards fertilized with 225 kg chemical N ha^-1 (PR) and (2) PR white clover (WC; Trifolium repens (PRWC)) swards fertilized with 125 kg chemical N ha^-1. The CF were (1) a high concentrate level (HC; 1,250 kg DM cow^-1) and (2) a medium concentrate level (MC; 750 kg DM cow^-1). The farm-gate N inputs for the 4 farmlets were 300, 260, 200 and 160 kg ha^-1 for PRHC, PRMC, PRWC-HC and PRWC-MC, respectively. Each farmlet was stocked at 2.7 cows ha^-1 and was managed independently over 2 grazing seasons (2023 and 2024). Cows were randomly allocated to each system before calving in spring 2023 and remained in their assigned treatment for the duration of the study. The PRWC SS achieved increased total pasture production (+1,062 kg DM ha^-1; P < 0.05) compared with PR swards. Milk and milk solids (MS) production per hectare did not differ between SS. Although PRWC required less chemical fertilizer N application (-77 kg ha^-1) resulting in reduced farm-gate N balance (-83 kg N ha^-1), the inclusion of biological N fixation resulted in greater total farm N balance (256 kg N ha^-1) and lower N use efficiency (NUE; 23%) for PRWC compared with PR (229 kg N ha^-1 and 27%, respectively). In contrast, HC achieved a greater (P < 0.05) milk yield (+1,301 kg ha^-1) and tended (P = 0.07) to have a greater MS yield (+101 kg ha^-1) compared with MC, and required less pasture silage supplementation during lactation, thereby resulting in similar farm-gate (187 kg ha^-1) and total (242 kg ha^-1) N balance. These results indicate that integrating WC into PR swards, combined with medium concentrate supplementation levels can improve the efficiency of pasture based dairy systems on wet mineral soils by improving pasture productivity, reducing requirements for chemical N inputs and maintaining milk production per hectare with minimal impact on total N balance and NUE.