Managing sulfonylurea resistance in cereal crops
Chickweed Stellaria media may develop resistance
(Revised July 2014)
Sulfonylurea herbicides such as chlorsulfuron are used extensively in cereal crops throughout the world, but unfortunately resistance to these herbicides can develop quite quickly within weed species (Rahman et al. 2001). In the early 1990s in Southland populations of chickweed (Stellaria media) were found that had developed resistance to chlorsulfuron (which is sold in products such as Glean, Basis, Centric and Tackle) after 10-12 years of using this herbicide annually on cereal crops (Bourdôt 1996). This resistance has now also developed in chickweed in other parts of New Zealand such as Canterbury. A strategy for managing this problem has previously been published for New Zealand conditions (Harrington 2005).
Resistance management and prevention strategy
Products that are not suitable alternatives to chlorsulfuron are those containing thifensulfuron (eg Harmony, Backup, Chord, Ranger, Synergy Thifen) as this was found to give no control of chlorsulfuron-resistant chickweed by Seefedlt et al. (2001). Although they found tribenuron (eg Granstar, Granit, Sero, Tillmate) to be effective, it would probably be best to avoid both this and iodosulfuron (Hussar) when designing a herbicide rotation, as these belong to the same chemical group B (ALS inhibitor / sulfonylurea) as chlorsulfuron. Often mutations that cause resistance to one chemical from a group will result in cross-resistance to others in the group, and different types of mutation are known to exist for sulfonylurea herbicide resistance.
Simply adding bromoxynil (Emblem Flo or Bromotril) to the chlorsulfuron each year (a common practice for controlling black nightshade) will not prevent herbicide resistance from occurring in chickweed, as it is not very susceptible to this second herbicide.
If populations of chickweed resistant to chlorsulfuron do develop within a cereal paddock, a herbicide from another chemical family that does control chickweed should be used, either in combination with chlorsulfuron or to replace chlorsulfuron. Chickweed has always been poorly controlled by some of the alternative herbicides available, such as MCPA or 2,4-D (phenoxy acetic group herbicides). One herbicide that would be suitable is mecoprop (a phenoxy propionic group herbicide), and this is available in several formulations (eg Trimec, Supamec, Hat-Trick, Legend, Image, Duplosan Super, Saxon). Other possibilities include bentazone (a Group C3 photosynthesis inhibitor sold in products such as Basagran, Broadstar, Troy, Pulsar), diflufenican (a Group F nicotinanilide herbicide sold in products such as Cougar, Jaguar, Firebird, Quantum and Tiger) and isoproturon (a Group C2 substituted urea sold in Protugan, Twister and IPU 500) (Seefeldt et al. 2001). Even if resistant chickweed is not yet present, it would be sensible to use some of these herbicide in alternate years instead of Group B herbicides every year.
The other possibility is to not grow cereal crops year after year but to rotate with other crops in which herbicides other than Group B herbicides can be used for weed control.
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.
Bourdôt GW 1996. The status of herbicide resistance in New Zealand weeds. In: Bourdôt, G.W. and Suckling, D.M. ed. Pesticide resistance: prevention and management. New Zealand Plant Protection Society, Rotorua, New Zealand. Pp. 81-89.
Harrington KC 2005. Managing sulfonylurea resistance in cereal 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. 165-166.
Rahman A, James TK, Seefeldt S 2001. The current situation with herbicide resistant weeds in New Zealand. Proceedings of the 18th Asian-Pacific Weed Science Society Conference: 500-508.
Seefeldt SS, Peters E, Armstrong ML, Rahman A 2001. Cross-resistance in chlorsulfuron-resistant chickweed (Stellaria media). New Zealand Plant Protection 54: 157-161.