Managing phenoxy resistance in pasture weeds
Slender winged thistle
to MCPA and 2,4-D in
(Revised July 2014)
In maize crops and orchards, several alternative herbicides from different chemical groups are available which should control any resistant weeds that may develop. A potentially more troublesome problem is the resistance to phenoxy herbicides that developed during the 1980s in populations of nodding thistle (Carduus nutans), slender winged thistle (Carduus pycnocephalus) and giant buttercup (Ranunculus acris) in pastures in several parts of the country (Harrington & Popay 1987; Bourdôt & Hurrell 1988, 1991; Harrington & Hewage 1997). The resistant biotypes of these weeds can no longer be controlled by MCPA, MCPB, 2,4-D or 2,4-DB (Harrington 1989; Bourdôt et al. 1994). This makes selective control of these weeds in clover-based pastures extremely difficult, as discussed in an earlier strategy for these weeds (Harrington 1996, 2005).
Resistance management and prevention strategy
Control of phenoxy-resistant giant buttercup was initially easier to achieve, as two selective products were available to kill phenoxy-resistant giant buttercup. These are flumetsulam (eg Preside) and thifensulfuron (eg Harmony) (Bourdôt et al. 1994). Also, an application of bentazone (Basagran) 1-5 days after applying MCPA has given good results (Butcher et al. 1993). However, after about 20 years of using flumetsulam each year, resistance to this herbicide also developed in the Golden Bay area, with cross-resistance to thifensulfuron (Lusk 2012). As there is some reduced fitness in MCPA-resistant plants (Bourdôt et al. 1996), it is now suggested for those farmers spraying annually for giant buttercup to alternate between MCPA in one year and flumetsulam in the following year.
Phenoxy-resistant thistles are still susceptible to chemicals such as clopyralid (eg Versatill, Void, Tango), dicamba (eg Banvel, Kamba), aminopyralid (T-Max) and products containing picloram (eg Tordon 2G, Victory Gold). However, these are all very damaging to clover. The present recommendation for controlling phenoxy-resistant thistles is to add 100 ml/ha of Versatill or other clopyralid product to normal rates of MCPA or 2,4-D, but significant levels of clover damage must be expected. Thus spot-spraying of rosettes or using a weed wiper when thistles start to flower are other possible options with these more effective chemicals.
Where resistance occurs, biological thistle control should be given every encouragement. Various insects introduced to control nodding thistles are well established in some regions.
Since control of resistant thistles in pasture is difficult, farmers should concentrate on preventing resistance from occurring in the first place. Resistance has developed only where farmers have applied 2,4-D or MCPA to their pastures annually for a number of years. So farmers who do this should seriously consider the wisdom of continuing with the practice.
General strategy to prevent and manage resistance weeds in pasture
More reliance should be placed on using pasture and grazing management techniques to prevent weeds from establishing each year (Harrington 1996a). If the pasture can be kept dense and competitive throughout the year, weeds should become less of a problem. Sowing pasture species that will survive summer droughts is particularly important in dry parts of the country. Grazing strategies that prevent overgrazing in summer and pugging in winter should be adopted. Selective grazing of pastures, which can encourage weed growth, should also be avoided through good use of rotational grazing.
Once a weed species is well established in pasture, low densities of weeds should probably be left unsprayed. If herbicides are used only in those years when weed densities are high, resistance is unlikely to build up and so the chemicals will be effective in bad years when they are most needed.
The assistance of colleagues in initially devising this strategy in 1994 is gratefully acknowledged, especially Graeme Bourdôt, Anis Rahman, Trevor Patterson, Roger Field, Mike Cornwell, Bob Moffatt and Murray Willocks.
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Bourdôt GW, Hurrell GA 1991. Evidence for the heritability of MCPA-resistance in giant buttercup (Ranunculus acris). Proceedings of the 44th New Zealand Weed and Pest Control Conference: 270-274.
Bourdôt GW, Hurrell GA, Saville DJ 1994. Herbicide cross-resistance in Ranunculus acris L. subsp. acris. New Zealand Journal of Agricultural Research 37: 255-262.
Bourdôt GW, Saville DJ, Hurrell GA 1996. Ecological fitness and the decline of resistance to the herbicide MCPA in a population of Ranunculus acris. Journal of Applied Ecology 33: 151-160.
Butcher MR, Strachan CM, Field RJ 1993. Giant buttercup (Ranunculus acris) control on Golden Bay dairy farms. Massey University Dairyfarming Annual 45: 93-103.
Harrington KC 1989. Distribution and cross-tolerance of MCPA-tolerant nodding thistle. Proceedings of the 42nd New Zealand Weed and Pest Control Conference: 39-42.
Harrington KC 1996. Herbicide management and thistle control – how to avoid resistance. Plant Protection Quarterly 11: 273-275.
Harrington KC 2005. Managing phenoxy resistance in pasture weeds. In: Martin NA, Beresford RM, Harrington KC, ed. Pesticide resistance: prevention and management strategies 2005. New Zealand Plant Protection Society, Hastings, New Zealand. Pp. 159-161.
Harrington KC, Hewage N 1997. Resistance of slender winged thistle to MCPA. Proceedings of the 50th New Zealand Plant Protection Conference: 538.
Harrington KC, Popay AI 1987. Differences in the susceptibility of nodding thistle populations to phenoxy herbicides. Proceedings of the Australian Weeds Conference 8: 126-128.
Lusk CS 2012. Variation in susceptibility of giant buttercup (Ranunculus acris L. subsp. acris) populations to herbicides. Masters thesis, Massey University, New Zealand.