Yield and botanical composition of subterranean clover in response to ALS inhibiting herbicides and waterlogging : A thesis submitted in partial fulfillment of the requirement for the degree of Master of Agricultural Science at Lincoln University

Subterranean clover is an important component of dryland pastures as it has high growth in early spring and improves soil quality through nitrogen fixation. An issue when establishing subterranean clover swards is broadleaf weed control. This thesis investigated the herbicide tolerance of seven subterranean clover cultivars to two acetolactate synthase (ALS) inhibiting herbicides, imazethapyr and flumetsulam. A strip-split plot experiment was established at Lincoln University, Canterbury in April 2018. All cultivars established successfully with >110 seedlings/m2. Herbicides were applied when the subterranean clover was at the 4-5 trifoliate leaf stage. Both herbicides increased the total subterranean clover yield for the season, with ‘Napier’ and ‘Antas’ being the highest yielding at ~6500 kg DM/ha. This was nearly double the yield of the lowest yielding cultivar, ‘Trikkala’. This increase in subterranean clover yield was due to the reduction in competition due to both herbicides eliminating ~1000 kg DM/ha of broadleaf weeds. The cultivar*herbicide interaction at the first harvest demonstrates that the cultivars had different responses to the herbicide. Specifically, flumetsulam and imazethapyr increased the yield of ‘Antas’ and ‘Napier’. ‘Coolamon’ yield was increased by flumetsulam but not by imazethapyr while the herbicides had no effect on the remaining cultivars at the first harvest. The second part of this thesis investigated whether the yanninicum subspecies of subterranean clover is more suitable to be used in winter wet conditions in New Zealand. An experiment was established at Lincoln University in July 2018. Two cultivars ‘Monti’, ssp. yanninicum, and ‘Coolamon’, ssp. subterraneum, were exposed to four watering treatments for eight weeks. ‘Monti’ was found to be more tolerant of waterlogging, having a 46% reduction in shoot dry weight in the waterlogged treatment compared with its highest yielding treatment. In contrast, ‘Coolamon’ was less tolerant of waterlogging with an 83% reduction in shoot dry weight compared with its highest yielding treatment. ‘Coolamon’ shoot dry weight was also reduced when watered 3x a week compared with ‘Monti’ which increased growth under the same treatment. The morphological strategy that allowed ‘Monti’ to be more tolerant to waterlogging appeared to be the production of lateral roots near and at the soil surface, which would allow the roots to absorb more oxygen. Photosynthetic rates decreased under waterlogging but the reduction was higher for ‘Coolamon’ than ‘Monti’ due to increased stomatal closure. ‘Monti’ produced anthocyanins in a response to waterlogging, showing that the plants were stressed, which may have provided an unknown protective factor. Eight weeks after treatments finished neither cultivar had recovered fully from waterlogging due to the previous effects on their root systems. This comparison has suggested that the ssp. yanninicum ‘Monti’ was more tolerant of waterlogging which suggests investigation of the impacts on other cultivars is warranted.