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The inclusion of narrow-leaved plantain (Plantago lanceolata) in the traditional pasture system of perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) is preferred by many New Zealand farmers nowadays to assist with nitrogen loss mitigation and to improve summer production. Control of weeds using selective herbicides becomes more difficult after adding extra species to a grass/clover sward. The work in this thesis investigated weed control options for ryegrass/clover pastures in New Zealand that contain plantain. This included determining the tolerance of plantain to different herbicides and identifying the herbicide options most effective for weeds. There has been some breeding of phenoxy herbicide tolerance into the Agritonic cultivar of plantain in New Zealand. In this thesis, the level of tolerance in Agritonic plantain was compared with Tonic plantain to MCPB, MCPA, MCPB/MCPA mix, 2,4-D and 2,4-DB in two glasshouse experiments. The tolerance generally appeared to be 1.3 to 3.4-fold, so not large but potentially useful. The thesis also investigated the tolerance of plantain, white clover and perennial ryegrass to a range of herbicides applied to a mixed sward of these species at an early stage of establishment in spring. The effectiveness of the herbicides was also assessed for controlling weeds that established within the swards using two field trials. Half of the recommended rate of MCPB was less damaging to both cultivars of plantain than the recommended rates of MCPB and MCPB/MCPA, but could not control most of the weeds present. The recommended rate of MCPB/MCPA had very little detrimental effect on young clover or Agritonic plantain and gave good weed control. Flumetsulam was fairly safe to use in the plantain-based pasture though it suppressed plantain initially, which recovered after 3-6 months in each trial. Bentazone was safe for the plantain, ryegrass and clover and suppressed most of the weeds, but only if there was no rainfall in the hours after herbicide application. Mowing after each harvest controlled redroot, black nightshade and fathen and also suppressed docks temporarily. The most effective weed control strategy that was also selective involved a combination of bentazone + half rates of MCPB/MCPA followed by mowing which gave useful control of weeds including docks for many months. The mechanism of tolerance of Agritonic plantain to 2,4-D was investigated using radiolabeled herbicide (¹⁴C-2,4-D) in two experiments. Absorption/translocation and metabolism of the herbicide was studied in both this cultivar and Tonic plantain for comparison. The tolerance to 2,4-D in Agritonic plantain appeared to involve reduced translocation of the herbicide, though reduced absorption may have also contributed. Two glasshouse experiments were conducted to test the tolerance of mature plantain plants (both Agritonic and Tonic) to application of some herbicides suitable for use in weed wipers (glyphosate, clopyralid, aminopyralid, dicamba, picloram and triclopyr) to the seed-heads, simulating potential contact during wiper application to pasture weeds. Aminopyralid and a low rate of glyphosate were found to be the least damaging treatments and should be safe to use for weed wiping within swards containing plantain. A high rate of glyphosate and a glyphosate/metsulfuron mix caused the most damage to plantain, and this damage was greater following simulated rainfall after application.

Sequestration of herbicide into vacuoles is considered to be the main mechanism of resistance to both glyphosate and paraquat worldwide. In New Zealand, the first case of glyphosate resistance was found in ryegrass (Lolium) species, and the restricted herbicide translocation was found to be the main mechanism of resistance in the studied populations, presumably through sequestration. Overseas researchers hypothesised that the mechanism responsible for glyphosate resistance could also cause resistance to paraquat. We examined this hypothesis by comparing a known glyphosate-resistant population of perennial ryegrass with a known susceptible population after spraying with different rates (25–800 g ai/ha) of paraquat. The glyphosate-resistant population responded similarly to the susceptible population at the different rates of paraquat application. This result suggests that the restricted glyphosate translocation mechanism does not necessarily lead to paraquat resistance. These results also suggest that paraquat could be useful for controlling ryegrass when glyphosate resistance has evolved and the application of paraquat is permitted.

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