Table 6.
Estimated nitrogen (N) surplus, nitrate leaching loss and nitrous oxide (N2O) N loss (kg N/ha per year), and nitrous oxide emission (equivalent to CO2 tonne per year) for lower input ryegrass + plantain-based (LIRG + PL; perennial ryegrass, Italian ryegrass, white clover, and plantain diverse mix, and plantain–white clover mix swards, 3.5 cow/ha and herbage received 150 kg nitrogen/ha), lower input ryegrass-based (LIRG; perennial ryegrass–white clover mix sward, 3.5 cow/ha and herbage received 150 kg nitrogen/ha), and higher input ryegrass-based (HIRG; perennial ryegrass ryegrass–white clover mix sward, 5 cow/ha and herbage received 300 kg nitrogen/ha) irrigated diary systems over two milking years, 2017/18 and 2018/19. Values estimated using Overseer® (Version 6.3.4).
| Item | 2017/18 | 2018/19 | ||||
|---|---|---|---|---|---|---|
| LIRG + PL | LIRG | HIRG | LIRG + PL | LIRG | HIRG | |
| Nitrogen surplus | 230.0 | 235.0 | 376.0 | 242.0 | 239.0 | 368.0 |
| Nitrate N leaching | 17.0 | 26.0 | 39.0 | 18.0 | 28.0 | 40.0 |
| N2O N loss | 5.8 | 6.3 | 8.9 | 6.2 | 6.7 | 9.3 |
| N2O emission 1 | 60.0 | 65.0 | 92.0 | 64.0 | 69.0 | 96.0 |
1 Nitrous oxide global warming potential = 290 times that of CO2 for a 100-year timescale.