Herbicides Explained: Selective, Non-Selective and Residual

Herbicides are grouped first by selectivity. Non-selective products such as glyphosate, glufosinate and paraquat kill almost any green plant they contact and are the tools for total vegetation control on roadsides, fence lines, fallow land and industrial yards. Selective products — the phenoxy group (2,4-D, MCPA, dicamba), sulfonylureas (metsulfuron) and ACCase inhibitors (clethodim) — remove target weeds while leaving the crop or turf unharmed.

The second axis is residual activity. Contact/systemic herbicides (glyphosate, 2,4-D) have no soil life and must hit green leaf. Residual herbicides — simazine, bromacil, diuron, imazapyr, tebuthiuron — bind to soil and keep killing germinating weeds for months, which is why roads departments, railways and municipalities specify them for bareground contracts, and why they must never be used where desirable plants root.

Mode of action matters because weeds adapt. Southern Africa has confirmed glyphosate-resistant rigid ryegrass and hairy fleabane. The defence is rotation and mixture: alternate HRAC groups (9 → 10 → 22), add residual partners, and never run the same chemistry season after season.

Practical field notes: glyphosate is crippled by hard borehole water — always condition with ammonium sulphate first. Ester formulations of 2,4-D and triclopyr drift onto vines, tomatoes and citrus; use amine salts near sensitive crops. And buy pack sizes to match the contract: municipal tenders overwhelmingly specify 5L for turf selective mixes and 20L/25L for glyphosate, simazine and imazapyr programmes.