In the Grapes of Wrath, John Steinbeck wrote, “Two are better than one, because they have a good reward for their labor. For if they fall, the one will lif’ up his gellow, but woe to him that is alone when he falleth, for he hath not another to help him up …” Little did Steinbeck know that this sound advice also applies to herbicides.
In 2012, Norsworthy et al. published 12 Best Management Practices (BMPs) for reducing the risks of herbicide resistance in a special issue of Weed Science. One of the BMPs was:
Use multiple, effective MOAs [Mechanisms of Action] against the most troublesome weeds and those prone to herbicide resistance.
Many growers have failed to heed this recommendation even as herbicide resistance has continued to increase. There are several reasons given for the lack of adopting this recommendation. Cost is perhaps the most common reason given. However, as herbicide-resistant weed biotypes continue to develop and spread throughout the PNW, the cost of failing to heed this recommendation may become greater than the cost of heeding it.
I recently read an article from the Australian Herbicide Resistance Initiative (AHRI) titled ‘The herbicide mixture is greater than the sum of herbicides in the mix’. The article discusses research conducted by Dr. Roberto Busi involving extensive herbicide resistance testing in rigid ryegrass (Lolium rigidum), which is closely related to Italian ryegrass (Lolium perenne subspecies multiflorum). Their work reinforced the message that herbicide mixtures are a powerful tool against herbicide-resistant weeds. They found that while 34% of their ryegrass populations were developing resistance to trifluralin (for example, Treflan HFP) and 11% were developing resistance to triallate (for example Avadex MicroActiv), when these two herbicides were combined in a mix, there was no resistance detected at all. A similar result was reported for all pre-emergent herbicide mixtures tested.
We conducted a field trial near Dixie, WA in 2020 to look at several different herbicide treatments with and without Avadex MicroActiv for the control of Italian ryegrass and downy brome (a.k.a. cheatgrass). Like many other places in the PNW, herbicide-resistant biotypes of both Italian ryegrass and downy brome were present in this field. In addition to Avadex MicroActiv, the other herbicide treatments included Zidua, Zidua + Sencor, Beyond, and PowerFlex HL. None of these treatments by themselves provided commercially acceptable control of Italian ryegrass or downy brome (Table); however, Avadex MicroActiv followed by Zidua or Zidua + Sencor provided good control of Italian ryegrass. Good control of downy brome was achieved when Avadex was followed by any of the herbicide treatments except PowerFlex HL applied in the spring.