Effect of sward species diversity combined with a reduction in nitrogen fertilizer on the performances of spring-calving grazing dairy cows

The objective of this study was to evaluate the effect of sward diversification combined with a reduction of chemical nitrogen (N) fertilizer on the performance of spring-calving grazing dairy cows within a farm systems experiment. Three farmlets were created: a monoculture of perennial ryegrass (Lolium perenne L.; PRG) fertilized with 250 kg N/ha (PRG-250N), a PRG-white clover (Trifolium repens L; WC) sward fertilized with 125 kg N/ha (PRGWC-125N), and a multispecies sward (MSS) comprising grasses, legumes, and herbs, also fertilized with 125 kg N/ha (MSS-125N). Each farmlet had its own herd of dairy cows on a total area of 18.7 ha divided into 20 paddocks. Each herd comprised pure Holstein-Friesian (HF) and HF-Jersey crossbred (JFX) animals, and cows were randomly assigned through the 3 treatments. For 3 years (2021 to 2023), the performances of both swards (grass yield, botanical composition, nutritive value) and grazing animals (milk production and composition, BW, and BCS) were recorded. We found no significant differences in pasture production or sward nutritive value between sward systems, and grazing season length was also similar (264 d). On average over the 3 years, PRGWC-125N contained 150 g/kg DM of legumes, and MSS-125N contained 160 g/kg DM legumes, 130 g/kg DM plantain, and 40 g/kg DM chicory. Both individual cow milk and fat plus protein (milk solids) yield were lower for PRG-250N (5,018 and 452 kg, respectively), intermediate for PRGWC-125N (5,139 and 463 kg, respectively), and highest for MSS-125N (5,297 and 476 kg, respectively) whereas milk and milk solids production per hectare from grazing were similar during the study period (11,523 and 1,016 kg/ha, respectively). Breed also had a significant effect, with the JFX having lower milk yield but higher fat and protein concentration compared with HF. This resulted in higher milk solids production per kilogram of BW for the JFX compared with HF (0.96 and 0.87 kg milk solids/kg BW, respectively). The results of this study highlight the possibility for more diverse pastures to reduce chemical N fertilizer input requirements and maintain pasture productivity while increasing animal performance within pasture-based spring-calving systems.