Smart formulation for precise crop protection

Abstract

Post-emergence selective herbicides are an important part of the management of competitive weed species within crops, with delivery through the use of formulation being a key factor in their effective use. As a result of increasing environmental concern and stricter regulation on herbicide usage, there is a progressive move away from solvent based herbicide formulations to more environmentally friendly water-based systems, such as suspension concentrates (SC). Effective delivery of the active ingredient is often a major limitation to SC formulations, being highly dependent on chemical additives known as adjuvants. In this project we have examined the effect of varying the adjuvant chemistry on the delivery of SC formulated herbicides using wheat (Triticum aestivum) as the crop and blackgrass (Alopecurus myosuroides) and ryegrass (Lolium multiflorum) as the weeds targeted for control. The herbicides selected for the study were cyhalofop-butyl, diuron, metolachlor, flufenacet and propyzamide. Several biomarkers of primary herbicide delivery were assessed, including stress-inducible genes as determined using qPCR, and absorbed active ingredient and immediate detoxification products, monitored by liquid chromatography coupled to mass spectroscopy (LCMS). Changes in the expression of the stress-responsive genes catalase, GST23, GSTF1, gstu2, HSP90, RGA4, and ATP synthase over the timescales of interest (0 – 16 hours post treatment) proved unreliable. Instead, emphasis was placed on identifying primary detoxification products of herbicides effectively formed spontaneously on uptake in planta. Of the five herbicides tested, metolachlor and flufenacet proved the most tractable, as both undergo spontaneous conjugation with glutathione when taken up into plant cells, with the conjugates being readily detectable by LCMS. With further refinement of the analytical technology, this approach focussed on flufenacet, a herbicide of increasing importance in grass weed control, which following its glutathionylation is then processed into a sequential series of catabolites of the conjugate notably glutamyl-cysteine, cysteinyl-glycine and cysteine flufenacet conjugates. By monitoring the levels of these flufenacet metabolites over time, it proved possible to measure the uptake, bioavailability, and metabolism of the herbicide as a dynamic process. Wheat, blackgrass, and ryegrass were treated with flufenacet formulations containing 10% (w/w) of a “Tween” and “Tween L” series of adjuvants. Herbicide uptake and metabolism were greater in both blackgrass and ryegrass than in wheat with all adjuvants tested. Whereas varying adjuvant type had little effect on flufenacet uptake in wheat, with the weeds, the Tween series showed a step-wise change in uptake rate correlating with the level of ethoxylation, with Tween 22 (8 repeat units of polyethylene glycol) resulting in the greatest uptake and Tween 20 (20 repeat units of polyethylene glycol) resulting in the least. To explain these results, the droplet properties of the applied formulations were observed by looking at contact angle, surface tension, dry down distribution, and drying time, to determine influence on uptake. Additionally, the surface characteristics of leaves of each plant were explored looking at the composition of the waxy cuticle by gas chromatography coupled mass spectroscopy (GC-MS) and gas chromatography couple flame ionisation (GCFID). Wheat tended to have higher chain length compounds composing the waxy cuticle, giving rise to a more hydrophobic surface and a reduced contact angle of applied herbicides. Both blackgrass and ryegrass shared several similarities in waxy chemical composition, such as 1-Hexacosanol being the predominant chemical within each, giving rise to similar levels of contact angle regression on herbicide application. Using metabolite formation as a highly sensitive marker of the initial stages of herbicide uptake our studies demonstrate that varying Tween adjuvant chemistry can enhance delivery in grass weeds, while having no significant effects on the crop. This differential uptake can be explained by differences in the formulated herbicide’s behaviour on the surface of these grasses, which are in turn dependent on surface hydrophobicity. Further refinements in adjuvant chemistry show new potential in enhancing grass weed control in wheat using existing selective herbicides. With further study these results could be applied to additional actives and weed species to best optimise herbicide formulations for delivery and control.