Variability of pre-em herbicide performance under dry conditions

A leading herbicide expert reports that soil moisture has a huge influence on the efficacy of pre-emergence herbicides and that there are significant differences in how pre-emergence herbicides perform under dry conditions.

 

John Cussans of NIABTAG says that wide spread weed resistance, particularly to both ACC-ase and ALS inhibitor herbicides, has put more emphasis onto pre-emergence herbicides which, as a consequence, have become the mainstay of any black-grass management programme. “Good weed control from pre-em herbicides is dependent on good seedbeds with adequate moisture. An analysis by NIABTAG of their herbicide efficacy data and associated September and October rainfall data confirms that dry conditions adversely affect the performance of most residual herbicides.”

 

John says that this analysis suggests that seasons where levels of rainfall lead to compromised pre-emergence herbicide performance are uncomfortably frequent.

 

He also reports that there are differences between herbicides when it comes to performance under dry conditions. “We have looked at several physical and chemical criteria including Vapour Pressure and Henry’s Constant (dimensionless) to help predict herbicide activity in dry soils. Unfortunately this information confirms field results that flufenacet, the bedrock of all pre-em programmes, is vulnerable to dry conditions, as is DFF and chlorotoluron. Prosulfocarb and flupyrsulfuron are a little less vulnerable and pendimethalin less so. The efficacy of tri-allate (Avadex 15G) from is the least affected by dry conditions.”

 

John reminds growers that even if tri-allate has been shown to be the least affected by dry conditions, it would be foolish to apply it if the soil was so dry that weed germination will not take place. “Nor should tri-allate (Avadex 15G) be used alone in any programme. Nothing is good enough to use alone against black-grass.”

 

Dr. Dominic Lamb of Gowan reports a huge increase in the interest in Avadex 15G Granules as the essential starter herbicide this season. “As starter herbicide, Avadex Granules fulfills a key role in the control of difficult-to-control grass-weeds such as black-grass, annual meadow-grass and wild-oats as well as some common broad-leaved weeds including cleavers, charlock, chickweed, poppy, field pansy, speedwell, forget-me-not, fumitory, mayweeds and red deadnettle in both winter wheat and winter barley. It is an excellent start to any programme and its ability to work in dry conditions is a really useful attribute.”

 

“Many farmers have invested in their own applicators or have identified a contractor to apply their Avadex this year, as they see the value of integrating this active ingredient into their weed control programme. This is particularly so as the level of black-grass control this year has been disappointing and we are starting off with a much bigger weed population to tackle.”

 

Dominic Lamb reminds growers that, in addition to winter crops – winter wheat, winter barley, durum wheat, triticale, winter rye, winter field beans - Avadex 15G is recommended in many spring crops too, including spring barley, peas, spring field beans, forage legumes, sugar beet, fodder beet, mangels and red beet.