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‘Whatawhata’ is a naturalised genotype of subterranean clover in New Zealand that has been identified as different to the parent of origin ‘Mt Barker’. A field experiment was conducted from the 10th of May 2021 to the 2nd of May 2022 to characterise and quantify key aspects of growth and development of the ecotype and compare it with the cultivars ‘Denmark’ and ‘Woogenellup’. Evaluation against ‘Denmark’ and ‘Woogenellup’ allowed recommendations to be made for the establishment and management of the cultivar for New Zealand grazing systems. ‘Whatawhata’ is a small leaved plant with a prostrate growth habit and exceedingly hairy petioles, trifoliate leaves, and runners. It is differentiated from ‘Denmark’ by the visible leaf mark and structural pubescence, while ‘Woogenellup’ has a large light green leaf with inconsistent hairiness on structural components. ‘Whatawhata’ can be classified as a ‘Late’ flowering cultivar with phenological development consistent with that of ‘Denmark’ at all stages. In comparison with ‘Woogenellup’, the ‘Whatawhata’ strain is slightly later to produce its first trifoliate leaf and reach flowering. ‘Whatawhata’ required 260±21.2 Cd to produce its first trifoliate with phyllochron thereafter 64.4±7.0 Cd/leaf. ‘Woogenellup’ thermal time requirement in the field (230±21.2 Cd) and controlled experiment (188±9.29 Cd) was consistent with published values. Thermal time requirement up to the fourth trifoliate leaf stage was between 417-489 Cd. Runner initiation occurred with a change in photoperiod. The first runner leaf appeared at the same time for all cultivars (683±39.5 Cd). Runner phyllochron was not different for ‘Whatawhata’ (83.3±9.45 Cd) and ‘Denmark’ (64.5±9.45 Cd) but was longer for ‘Woogenellup’ (96.1±9.45 Cd). ‘Whatawhata’ (952±10.9 Cd) and ‘Denmark’ (962±10.9 Cd) required the same amount of thermal time after the 21st of June to produce their first flower. This was longer than for ‘Woogenellup’ (883±10.9 Cd). Legume yield of ‘Whatawhata’ (1898±166 kg DM/ha) was consistent with that of ‘Denmark’ (1986±166 kg DM/ha) and ‘Woogenellup’ (2482±166 kg DM/ha). ‘Whatawhata’ herbicide tolerance is comparable to ‘Denmark’ and ‘Woogenellup’ with a similar response reported for pastures sprayed with flumetsulam and imazethapyr. However, the application of recommended herbicides for weed suppression did not increase total DM yield compared with unsprayed pasture. Phytotoxicity scores supported the lack of difference, with none of the pasture components reaching the commercial threshold (7) for high yield suppression. Cooler temperatures (

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.

The increase in herbicide-resistant weeds in Arkansas crop fields has led to the need for new herbicide modes of action for use in all crops. This need has led to the introduction of technologies that can be devastating to conventional rice crops. Field observation, noted that insecticide seed treatments used in rice could potentially reduce the effects of off-target movement of herbicides onto rice crops and possibly reduce the negative effects of some herbicides applied directly to rice. Research was conducted to determine if insecticide seed treatments could reduce the harmful effects of drift from imazethapyr and glyphosate onto conventional rice, and if so, which insecticide seed treatments provided adequate protection. In addition, research was conducted to determine if thiamethoxam, a popular insecticide seed treatment, could reduce the negative effects of some acetolactate synthase (ALS)-inhibiting, preemergence and postemergence herbicides for better or future use in rice. The use of insecticide seed treatments containing thiamethoxam and clothianidin resulted in less rice injury, more groundcover, and increased grain yields following simulated herbicide drift events compared to a fungicide-only seed treatment. Thiamethoxam seed treatment also reduced the amount of injury caused by ALS-inhibiting herbicides applied to imidazolinone-resistant varieties of rice. In addition, thiamethoxam reduced injury from the preemergence and postemergence herbicides; however, individual interactions were observed in terms of yield for the herbicides. Overall, this research confirms the hypothesis that insecticide seed treatments can provide some safening from low rates of harmful herbicides and reduce some of the negative effects of injurious herbicides commonly used in rice production.

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 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.

Off-target movement of glyphosate onto rice is a perennial concern when rice is grown in close proximity to glyphosate-tolernat crops. If differential tolerance to sub-lethal rates of glyphosate exists among rice (Oryza sativa) cultivars, these cultivars could be utilized in breeding programs or glyphosate-drift sensitive areas. Field and greenhouse experiments were conducted in 2006 and 2007 to examine differences among rice cultivars in response to sub-lethal rates of glyphosate, and to examine imidazolinone-tolerant rice response to imazethapyr and sub-lethal rates of glyphosate applied sequentially to determine the potential for either herbicide to predispose rice to greater injury. In the field experiment, glyphosate was applied at 0, 45, and 90 g ae/ha at the 3- to 4-leaf, panicle initiation, and boot growth stages of 10 rice cultivars. Cultivars inherently differed in plant height, flag leaf length, and yield. Glyphosate applied at the 3- to 4-leaf stage reduced plant height and yield of cultivars less than 5%. Height of all cultivars was affected more by glyphosate at 90 g/ha applied at panicle initiation, but the greatest yield reduction across all cultivars was from 90 g/ha glyphosate applied at boot stage. Relative flag leaf length of some cultivars was reduced by glyphosate more at one location than the other, and this reduction differed by growth stage. In field and greenhouse experiments, glyphosate was applied at 0, 45, and 90 g ae/ha at 14, 7, 3, 1, and 0 d prior to applications of imazethapyr at 0, 105, or 210 g ai/ha to the imidazolinone-tolerant cultivar `CL 161'. Imazethapyr at those rates was also applied 14, 7, 3, 1, and 0 d prior to receiving glyphosate at the above rates to determine any predisposition of CL 161 to either herbicide. Glyphosate reduced relative plant height, relative dry mass, and relative yield of CL 161. There was no interaction between imazethapyr and glyphosate on relative plant height, relative dry mass, relative leaf chlorophyll content, or relative yield; therefore, influence is independent. There is no evidence that sequential applications of imazethapyr or sub-lethal rates of glyphosate will predispose CL 161 to greater injury from either herbicide.

Italian ryegrass is the most problematic weed of small grains in North Carolina. This weed has traditionally been controlled in wheat with diclofop. However, after many years of continuous use, much of the Italian ryegrass in the state is now resistant to diclofop. A field experiment was conducted at three locations to determine control of Italian ryegrass and response of imidazolinone-tolerant wheat to imazamox and other herbicides. Imazamox ammonium salt applied POST in the fall to 3- to 4-leaf Italian ryegrass at 35 to 53 g ae/ha controlled Italian ryegrass 90 to 100% 10 weeks after treatment. Late-season control ranged from 83 to 98% at two locations and 24 to 55% at the third location. Late-season control from spring-applied imazamox on 1- to 3-tiller ryegrass ranged from 53 to 58% at one location and 7 to 16% at the other locations. Split application of imazamox at 27 g/ha in the fall and 27 g/ha in the spring was less effective than fall-applied imazamox at 53 g/ha at two of three locations. However, split application was more effective than spring application. Fall-applied imazamox was less effective than fall-applied diclofop on diclofop-susceptible Italian ryegrass but more effective on mixed populations of diclofop-resistant and -susceptible biotypes. Pendimethalin applied PRE at 1120 g ai/ha increased late-season control 10 to 33 percentage points when used in combination with fall-applied imazamox at 35 g/ha. Imazethapyr ammonium salt at 70 g ae/ha was less effective than imazamox, while imazethapyr at 47 g/ha plus imazapyr isopropylamine salt at 16 g ae/ha and imazamox were similarly effective. Imazapic ammonium salt at 70 g ae/ha and imazamox at 53 g/ha were similarly effective at two locations, while imazapic was more effective at the third location. Imidazolinone herbicides caused only minor, temporary chlorosis on the wheat. In greenhouse experiments, a diclofop-susceptible biotype of Italian ryegrass was more sensitive to imazamox than a diclofop-resistant biotype. Imazamox controls Italian ryegrass in imidazolinone-tolerant wheat but is only marginally effective on some broadleaf species. A fallow-area field experiment was conducted at three locations in North Carolina to determine control of Italian ryegrass by imazamox as affected by imazamox rate, weed size at application, and mixtures with the broadleaf herbicides 2,4-D, dicamba, and thifensulfuron plus tribenuron. Interactions of imazamox rate, weed size, and broadleaf herbicides were not observed. Italian ryegrass control 56 days after treatment increased as imazamox rate increased. Pooled over weed sizes at application and mixtures, imazamox at 27, 35, and 44 g/ha controlled Italian ryegrass 44, 53, and 67%, respectively, at two locations and 83, 86, and 91% at the third location. Control decreased as weed size at time of application increased. Pooled over imazamox rates and mixtures, Italian ryegrass was controlled 86, 54, and 24% at two locations when herbicides were applied to 2-leaf, 4-leaf, and 2-tiller weeds, respectively. At the third location, Italian ryegrass was controlled 95, 94, and 71% when treated at the 2-leaf, 4-leaf, and 2-tiller stages, respectively. Small increases in control were sometimes noted with mixtures of imazamox and 2,4-D dimethylamine salt at 530 g ae/ha or thifensulfuron plus tribenuron at 18 plus 9 g ai/ha, while minor decreases were sometimes noted with mixtures of imazamox and dicamba diglycolamine salt at 140 g ae/ha. In a greenhouse experiment, mixing 2,4-D with imazamox did not affect Italian ryegrass control. Dicamba reduced control, while thifensulfuron plus tribenuron increased control.