How do cocksfoot (orchardgrass), tall fescue, brome and perennial ryegrass grow under water stress with different levels of nitrogen? Monoculture pastures were established, at Ladbrooks and Ashley Dene, Canterbury, New Zealand, in the agronomic year of 2014/2015 and this thesis reports assessments for the years 2018/2019 (Year 5) and 2019/2020 (Year 6). The sites contrasting soil types and plant available water content (PAWC). The Wakanui soil at Ladbrooks (LB) had a PAWC, of ~194 mm compared with ~131 mm for the stony Lismore silt at Ashley Dene (AD). Total dry matter production at LB was 18100 kg ha-1 in Year 5 and Year 6 for the N+ pastures but 5360 and 6000 kg ha-1 for those unfertilised (N-). At AD the pastures yielded 9340 and 6000 kg ha-1 for N+ and N- in Year 5 but both yielded ~7395 kg ha-1 in Year 6. The temperature growth rates ranged from 0.3 kg DM °Cd-1 ha-1 during the water restricted summer dry periods, for N- plants, at Ashley Dene, up to 13.1 kg DM °Cd-1 ha-1 for N+ plants during spring 2019 at Ladbrooks when water was non-limiting. The highest N+ pasture yields at Ladbrooks were explained by the higher photosynthetically active radiation (PAR) interception and higher radiation use efficiency (RUE) coupled with greater water extraction, water use (WU) and water use efficiency (WUE) than at Ashley Dene. Plant physiology traits showed that under nitrogen or water deficit, the species had different response strategies. Cocksfoot and brome maintained their leaf area by keeping the cells turgid, with a higher relative water content and osmotic potential than perennial ryegrass and tall fescue. These species protected their photosynthetic apparatus and chlorophyll concentration by accumulating higher levels of proline, particularly when nitrogen was provided to enable its synthesis.
This research was part of the Hill Country Futures programme (BLNZT1701) funded by Beef + Lamb New Zealand, MBIE, Seed Force New Zealand and PGG Wrightson Seeds. Marcus was supervised by Professor Derrick Moot and Associate Professor Rainer Hofmann.