Yield and phytotoxicity responses of subterranean clover (Trifolium subterraneum L.) sprayed with different herbicides for broadleaf weed control

Abstract

A key factor for integrating subterranean clover into pastoral systems is its success in the establishment year. With appropriate management, establishment ensures productivity and persistence in future years. This thesis investigated the relative herbicide tolerance of subterranean clover at the seedling stage for pasture establishment in New Zealand rain-fed environments. This involved quantification of the field response to herbicides, at different seedling development stages of subterranean clover cultivars. Four experiments, two arranged as split-plot, and two as split-strip-plot, were established in Canterbury, New Zealand in autumn 2016. Emergence of Experiments 1 and 2 occurred in late-March, while Experiments 3 and 4 were delayed by lack of rainfall. A cultivar*herbicide interaction was identified in all experiments, confirming that cultivars were different in their response to herbicides. The cultivar ‘Narrikup’ showed the greatest herbicide tolerance to imazethapyr across all experiments, with sown clover yields of 2600-3500 kg DM/ha. All yanninicum cultivars (‘Monti’, ‘Napier’, and ‘Trikkala’) were not suited to the environmental conditions of the season and failed to persist following herbicide applications. The subterranean clover subspecies brachycalycinum cultivar ‘Antas’ showed variable herbicide tolerance, with responses of developmental delay as well as yield depression. The white clover control was consistently the lowest yielding at <1000 kg DM/ha, with no response to herbicides. Subterranean clover cultivars had visible phytotoxicity responses to imazethapyr which were related to plant pubescence. The phytotoxicity scores were correlated to yields within each experiment. ‘Whatawhata’, ‘Woogenellup’, and ‘Narrikup’ cultivars showed the greatest tolerance and benefit from imazethapyr, with total clover yields of 3500-4500 kg DM/ha for the growing season, >2000 kg/ha more than their unsprayed unweeded controls. For cocksfoot/clover mixtures only ‘Narrikup’ had no reduction in total dry matter yields compared to unsprayed unweeded controls when imazethapyr was applied. Cocksfoot productivity was slowed by imazethapyr up to 24 weeks after application, with yields 750 kg/DM lower than the unsprayed unweeded controls. Pastures recovered to be no different in November, and cocksfoot can be expected to continue to provide summer grazing after the annual clovers set seed. The early reduction in cocksfoot productivity allowed >20% increases in the clover component of imazethapyr treated swards. Experiments 3 and 4 found that the ALS-inhibiting herbicides imazethapyr and flumetsulam, and photosynthesis-inhibitor bentazone were the least damaging herbicides to subterranean clover. Experiment 3, where plants were treated at the 1-2 trifoliate leaf stage had higher subterranean clover yields when compared to Experiment 4, where herbicide was applied at the 4-6 trifoliate leaf stage. The early seedling herbicide application had less impact on yields than prolonged competition from weeds. Combined sown+resident clover yields showed that sowing cultivar mixes can improve subterranean clover herbicide tolerance and increase total yields. Both application times for Imazethapyr, flumetsulam, and bentazone had mean total clover yields the same as or greater than the unsprayed unweeded controls. 2,4-DB had a negative impact on the development of all subterranean clover plants. For the remaining herbicides bentazone + MCPB, bromoxynil + diflufenican, and MCPB, ‘Antas’ and resident ‘Woogenellup’ were susceptible to developmental delays as a result of application, while ‘Denmark’ and ‘Narrikup’ were less adversely affected. Imazethapyr + saflufenacil, and glyphosate treatments killed all vegetation and left ground bare in all experimental applications, confirming they are unsuitable for use in subterranean clover-containing pastures. This research confirmed a cultivar*herbicide interaction to a range of herbicides, and identified imazethapyr, flumetsulam and bentazone as suitable for use during establishment of subterranean clover based pastures. Longer term effects, such as those on subsequent regeneration and further investigations into effects on development, as well as the apparent brachycalycinum susceptibility are advised.