Simazine, Metribuzin, Hexazinone and Tebuthiuron: Photosystem II Herbicide Guide

Simazine, metribuzin, hexazinone and tebuthiuron are soil-active herbicides that block photosynthesis. They range from selective crop herbicides to products for total vegetation control on non-crop land, so the first question for a buyer is the situation the product is registered for. Soil type, leaching risk and resistance then decide how each can be used.

Averstar's catalogue lists simazine 90% WG, metribuzin 70% WP, hexazinone at 25% and 75%, and a tebuthiuron 20% product. This guide explains how the actives differ and which documents to request. The approved destination-market label always decides the permitted crops or sites, weeds, dose, timing and restrictions for the product actually supplied.

Quick answer: Use metribuzin and simazine only in the crops their labels list, and hexazinone or tebuthiuron for the forestry, sugarcane, pasture or non-crop sites on their labels. All are HRAC Group 5, like atrazine, diuron and linuron, so they do not rotate with each other. Check soil type and groundwater restrictions, because several are prone to leaching.

How photosystem II herbicides work

These herbicides bind to the photosystem II complex in chloroplasts and stop the electron transport that drives photosynthesis. Treated weeds turn yellow, then brown, starting from leaf margins and tips. The HRAC classification lookup places simazine, metribuzin and hexazinone (formerly C1) and tebuthiuron (formerly C2) together in Group 5. Our atrazine and linuron and diuron guides cover other Group 5 herbicides.

Simazine and metribuzin in crops

University of Hertfordshire Pesticide Properties Database (PPDB) records describe them as follows:

• Simazine: a soil-acting herbicide that controls most germinating annual grasses and broadleaved weeds, absorbed through roots and foliage. Example uses include fruit such as apples, pears and berries, vines, hops, almonds, canola, chickpeas, beans and lupins.
• Metribuzin: a pre- and post-emergence herbicide with contact and residual activity against grasses and broadleaved weeds in soybeans, potatoes, wheat, barley, sugarcane, tomatoes, peas, lentils and asparagus. Crop varieties can differ in tolerance, so labels may restrict use on sensitive varieties or soils.

Hexazinone and tebuthiuron for tough sites

• Hexazinone: a broad-spectrum herbicide against many annual, biennial and perennial weeds, including woody plants, absorbed through roots and foliage. Example uses include sugarcane, forestry, pastures, rangeland, alfalfa, fruit and fallow land, each under its own label conditions.
• Tebuthiuron: a systemic herbicide with low selectivity, taken up by roots, used against herbaceous, woody, annual and perennial weeds on non-cropland such as rights-of-way, roadsides and industrial sites.

Because both affect most vegetation they reach, keep them away from desirable trees and plants whose roots may extend into the treated area, as the label describes. Our non-selective herbicides guide compares other options for total vegetation control.

Soil, leaching and registration

The PPDB rates hexazinone, tebuthiuron and metribuzin as having high leachability on its groundwater (GUS) index, and simazine as transitional. Labels therefore commonly restrict use on sandy or shallow soils and near wells or shallow groundwater; include these restrictions in distributor training and translated label artwork.

Resistance to photosystem II herbicides has been recorded in many weed species, so plan mixtures and rotations with other HRAC groups. The PPDB records all four actives as not approved in the European Union, while other markets still register certain uses, so confirm the current status in each destination market before preparing samples.

Compare the catalogue formulations

• Simazine 90% WG: water-dispersible granules at a high concentration, tested for dispersibility and suspensibility.
• Metribuzin 70% WP: a wettable powder that must stay dry and is tested for suspensibility and wettability.
• Hexazinone 25% / 75%: offered at two strengths; each is a separate registered product with its own dose, so the quotation, certificate and label must name the same one.
• Tebuthiuron 20%: a product for non-crop vegetation management; confirm the formulation type and concentration on the specification and label before quoting.

Penn State Extension's overview of pesticide formulations compares these types, and FAO/WHO pesticide specifications describe the properties buyers test.

A product selection checklist

• The registered situation: a named crop, forestry, pasture or non-crop land.
• Target weeds, including woody plants where hexazinone or tebuthiuron is considered.
• Soil texture, organic matter and depth to groundwater against label restrictions.
• Nearby trees, crops and water that the product must not reach.
• The herbicide groups already used, so Group 5 fits a resistance plan.
• Concentration and unit matched on quotation, certificate of analysis and label.
• Packaging, safety data sheet and translated label artwork.

Build the inquiry around the intended use

Send the crop or site, the target weeds, the soil type, the herbicides already used, the preferred product and concentration, the packaging and the destination market through our contact form. For a first order, a pesticide sample evaluation plan helps compare samples against the agreed specification before scaling up.