Delving into mode of action diversity benefits

Farmers have a wide range of actives to choose from to control grass- weeds in wheat, particularly at the pre-emergence timing. New chemistry, such as aclonifen and cinmethylin, set the standard, but a wide range of older chemistry is still effective too.

 Smart use of all options available will help farmers get better weed control and prevent the development of resistance, according to John Cussans of NIAB TAG.

“There are actives from six HRAC Mode of Action (MoA) Groups that farmers can use at pre-em against black-grass and Italian ryegrass in cereals. A seventh, bixlozone from HRAC Group 13 is due in the near future, plus there is glyphosate, which is a unique MoA pre-drilling, and the post-em chemistry. We need to harness this diversity in herbicide options to maximise control and minimise the risk of herbicide resistance development.”

Mr Cussans says over the last decade, weed control has shifted to the pre-em timing, often with a high loading of flufenacet used to improve control. But, with a greater choice of actives now available, farmers should look to diversify MoA, rather than increase the loading of one or two actives.

“In many trials, the effect of diversifying MoA is confounded by the effect of simply adding more herbicide in total, since as you stack more herbicides, you are simultaneously increasing the diversity and the total load. This trial design makes sense, since using products at full rate, which is how farmers use them, is what we generally recommend for both efficacy and resistance management.

“But, such trials don’t really tell you if you get better control because you have diverse MoA, or the effect is from just applying more herbicide in total. We wanted to investigate directly the benefit of diversifying herbicide programmes (by adding more MoAs)in isolation; separating out this effect from the effect of higher total herbicide ‘load’.”

ll you if you get better control because you have diverse MoA, or the effect is from just applying more herbicide in total. We wanted to investigate directly the benefit of diversifying herbicide programmes (by adding more MoAs)in isolation; separating out this effect from the effect of higher total herbicide ‘load’.”

NIAB set up a matrix trial with five pre-em MoA in total, and the highest total loading was equivalent to five label rates of herbicide. Hence, at the lowest total dose, the trial compared the efficacy of one active applied at label rate, compared to five actives in a stack at 1/5 of the label rate for each active, aswell as the permutations with two, three and four actives as shown in the table below.

1990 first cases of herbicide resistant Italian ryegrass

“The trial set up is a bit different to the standard herbicide trial and includes application rates – high and low – that we wouldn’t recommend using on-farm,” says Mr Cussans.“But, it did show how the two factors of total dose and mode of action diversity affect control. “Most interestingly, we could see straight away that against Italian ryegrass, mode of action diversity is seen in improved efficacy, even at low dose rates (see chart below). Where as, against black-grass, we see less direct benefit from more modes of action and greater rate responses to increased total dose or ‘load’.

“These results shouldn’t change how herbicides are used. It is better in the long- term to mix modes of action for resistance management, as well as apply at label rates. But it does highlight what is driving the increase in control we see from stacks and sequences.