The impact of sward species diversity and chemical nitrogen fertiliser application on the performance of grazing dairy systems: a multi-year study comparing a grass monoculture, grass-legume mixture, and multispecies sward

The shift towards more intensive grazing systems in temperate regions has led to the predominance of perennial ryegrass (Lolium perenne L., PRG) monoculture swards due to its high nutrient quality, and responsiveness to frequent defoliation and chemical fertilisation. Recently, PRG-white clover (Trifolium repens L., WC) and multispecies swards (MSS), composing of grasses, legumes and forage herbs such as chicory (Cichorium intybus L.) and plantain (Plantago lanceolata L.) have gained interest as they can improve animal performance and offer environmental benefits, although much of the recent research has been within short term or component evaluations. Therefore, the objective of this thesis was to evaluate the impact of sward species diversity and chemical nitrogen (N) fertiliser application on the sward and animal performances of grazing dairy systems in a multi-year farm systems study. Three sward types were evaluated: a PRG monoculture receiving 250 kg N/ha (PRG-250N), a PRGWC receiving 125 kg N/ha (PRGWC-125N) and a MSS receiving 125 kg N/ha (MSS-125N). In Chapter 3 no difference was observed in annual herbage yield between sward types (13,015 kg DM/ha) despite a substantial difference in chemical N application. The nutritive values of the three sward types were also similar, however the organic matter digestibility (OMD) of MSS-125N was lower (788 g/kg) compared to both PRG-250N and PRGWC-125N (801 and 799 g/kg, respectively). In chapter 4, significant differences in milk solids (MS) production were observed between swards with PRG-250N (452 kg/cow) lowest, MSS-125N (475 kg/cow) greatest and PRGWC-125N (463 kg/cow) intermediate, in each case due to differences in milk yield rather than milk composition. The significant impact of sward type on dry matter intake (DMI) was evaluated in Chapter 5, which demonstrates that animals grazing PRG-250N swards had the lowest intake (16.3 kg DM/cow), compared to PRGWC which was intermediate (17.5 kg DM/cow), while MSS (18.7 kg DM/cow) achieved the greatest intake. The economic analysis revealed that PRG-250N and PRGWC-125N achieved similar net farm profitability, even though PRGWC-125N had lower fertiliser costs but required more purchased grass silage. Therefore, MSS-125N emerged as the most profitable system, driven by higher milk output and lower N fertiliser use, even under varying milk and fertiliser prices. The experiments reported in this thesis demonstrate the potential for more diversity in pastures to maintain grass production and quality with a significant reduction in chemical N fertiliser, while increasing both the productivity and economic performance of dairy cows within spring calving pasture-based systems.