Managing triazine resistance in other arable crops
(Revised July 2020)
Initially, resistance to triazine herbicides, such as atrazine, within New Zealand only developed where maize was grown for many years in succession. However, black nightshade (Solanum nigrum) then developed resistance to triazine herbicides in arable cropping farms where crops were rotated annually rather than the same one grown each year (Harrington et al. 2001).
Once this was investigated, it became apparent that the main rotation leading to this resistance building up was sweet corn crops rotated with peas. With both of these crops, triazine herbicides, such as cyanazine, terbuthylazine or atrazine, were the main form of weed control being used. Thus although crop rotation was occurring, herbicide rotation was not as these are all Group C1 herbicides. Black nightshade plants resistant to triazine herbicides were surviving the herbicides used in both crop types.
Resistance to triazines has also developed in black nightshade plants growing within asparagus crops. This situation is easier to explain as asparagus is grown continuously for many years in the same paddock, usually with triazine herbicides, such as terbuthylazine, being applied in succession for weed control. Harrington (2005) has previously discussed control of triazine resistant weeds in asparagus.
Resistance management and prevention strategy
Resistant black nightshade plants have been successfully controlled in asparagus by mixing the Group C2 herbicide diuron (Karmex, Pirate or Agpro Diuron 800) with the triazine herbicide, and this mixture of diuron and terbuthylazine is sold as Fenican. There are a number of other non-triazine residual herbicides that can be used in asparagus crops in rotation with terbuthylazine, such as bromacil (Uragan WG), methabenzthiazuron (Tribunil) and linuron (Afalon, Linuron 50 DF, Linex Flo, Liron, Linuron 400SC). Depending on the weed species in which resistance develops, there can sometimes be cross-resistance from triazine-resistant weeds to herbicides, such as bromacil (which is also a Group C1 herbicide), and partially to linuron (Fuerst at al. 1986). But when considering overall practices to avoid herbicide resistance occurring in any weed species, techniques for weed control other than just residual herbicides should also be used. For example, knockdown herbicides, such as clopyralid (eg Versatill Powerflo, Void, Archer, Multiple) and glyphosate, can be used at certain times during the year, as can cultivation. These can help kill weeds that are not being controlled by residual herbicides.
Other arable crops
One way to avoid resistance building up is to introduce other crop types into the rotation in which non-triazine herbicides can be used to control any resistant black nightshade plants that may be increasing in numbers. For example, a cereal crop such as wheat or barley could be grown and a hormone herbicide such as MCPA would kill any black nightshade. Sulfonylurea herbicides such as chlorsulfuron (eg Glean) or tribenuron (eg Granstar) would not be appropriate herbicides to use in cereals though, as they are ineffective on all black nightshade ecotypes. This can be overcome by mixing diflufenican or bromoxynil with the sulfonylurea herbicide to kill the nightshade. The crops grown over winter in rotation are not relevant as very little black nightshade will establish and successfully set seed over winter.
Another strategy would be to rely less heavily on triazine herbicides within pea and sweet corn crops. Suitable herbicides for controlling both triazine-resistant and susceptible black nightshade plants within peas include MCPB, bentazone, methabenzthiazuron, pendimethalin and clomazone, though they would all need to be applied while the nightshade was young.
Although dicamba is commonly used in maize crops for controlling triazine-resistant weeds, some cultivars of sweet corn are damaged by this herbicide. Likewise mesotrione cannot be used, and neither is flumetsulam registered for use in sweet corn, though nicosulfuron (eg Astound Ultra, Guardian) is suitable for this. Otherwise, the main herbicides suitable for use with triazines that would control black nightshade in sweet corn crops are topramezone (Arietta), the acetanilide herbicides at pre-emergence, which are acetochlor (eg Roustabout), alachlor (eg Corral), S-metolachlor (Metoken Gold) and dimethenamid-P (Frontier-P), or bromoxynil (eg Emblem Flo) at early post-emergence.
Although these strategies have concentrated mainly on black nightshade, they can be easily adapted for other species such as fathen if they should develop resistance in these situations.
The assistance of colleagues in reviewing early versions of this strategy is gratefully acknowledged, especially Trevor Patterson.
Fuerst EP, Arntzen CJ, Pfister K, Penner D 1986. Herbicide cross-resistance in triazine-resistant biotypes of four species. Weed Science 34: 344-353.
Harrington KC 2005. Managing triazine resistance in other arable crops. In: Martin NA, Beresford RM, Harrington KC, ed. Pesticide resistance: prevention and management strategies 2005. New Zealand Plant Protection Society, Hastings, New Zealand. Pp. 151-153.
Harrington KC, Ward AJ, Wells DM 2001. Herbicide resistance in black nightshade and Onehunga weed. New Zealand Plant Protection 54: 152-156.