Weeds and Herbicides

How agricultural herbicides are grouped

Farm herbicides are sorted in a few overlapping ways, and you need all of them to read a label. Selective herbicides kill some plants and spare the crop; non-selective ones kill most green plants they touch. Pre-emergence products go on the soil and stop weeds as they germinate; post-emergence products are sprayed on weeds that are already up. Contact herbicides kill only the tissue they hit, while systemic ones move through the plant to the roots.

The grouping that matters most for long-term control is mode of action: the plant process the herbicide blocks. Weed scientists number these groups, and the numbers are printed on many labels.

Our articles on clethodim, sethoxydim and Clearcast and on metsulfuron, halosulfuron and rimsulfuron go deeper into two of these groups.

Choosing a weed killer for a farm

There’s no single best agricultural weed killer, because the right choice depends on what you’re killing and what you need to protect. Work through it in order:

1. Identify the weeds. Grasses and broadleaves need different products, and perennials with deep roots need a systemic herbicide.
2. Check crop tolerance and growth stage. Many herbicides are safe on a crop only within a window of growth stages.
3. Read the rotation restrictions. Some residual products limit what you can plant in that field the following season.
4. Plan for weather and water. Rain soon after spraying can wash off a foliar product, while pre-emergence products often need rain to activate.
5. Follow the label. In the US the label is a legal document, and rates, timings and protective equipment are not optional.

Our guides to using Roundup safely and to spotting herbicide injury and drift cover two common problems.

Spraying well

A good product badly applied gives poor control and drift onto neighbors. Calibrate the sprayer at the start of each season, choose nozzles that give the droplet size the label asks for, keep the boom low, and avoid spraying in strong wind or during a temperature inversion on calm, cool evenings and early mornings, when fine droplets can hang in the air and travel a long way. The UTV sprayer guide listed above covers tank size and pumps for smaller jobs.

Herbicide resistance

When one mode of action is used year after year, the few weeds that survive it set seed and take over. Glyphosate-resistant Palmer amaranth and waterhemp across the US Midwest and South, and multiple-resistant ryegrass in Australia, are the best-known examples. The International Herbicide-Resistant Weed Database tracks confirmed cases by species, country and group.

The standard advice is to rotate and mix modes of action that are each effective on the target weed, use residual pre-emergence products to cut the number of weeds a post-emergence spray must kill, spray small weeds at full label rate, and stop survivors from setting seed. Non-chemical tactics carry a lot of the load on farms that have resistance under control.

Herbicides near water

Only products specifically labeled for aquatic or near-water use should be used in or beside ponds, ditches, canals and streams. Several active ingredients have aquatic formulations, including some glyphosate products, imazapyr, imazamox, diquat, triclopyr and certain 2,4-D amine products, but the standard farm version of the same ingredient may not be approved near water, because of the surfactant or the formulation. In the US, applying pesticides to or over water can also require coverage under a Clean Water Act pesticide permit. Ordinary farm herbicides often carry their own water rules too: labels can set buffer distances from wells, streams and ponds, and some residual products warn against use on sandy soils over shallow groundwater. Our article on herbicides approved for use near water lists the main options.

Weed control without relying on one product

Integrated weed management combines herbicides with crop rotation, competitive crops and planting densities, cover crops, stale seedbeds, tillage or cultivation where it suits the soil, and keeping field margins and machinery clean. Some weeds point to a soil problem: corn marigold, for example, is a weed of light, acidic arable soils, and liming to correct the pH is part of controlling it. Check current local recommendations for the herbicides that work on it in your crop.

Our AI farm advisory includes a weed risk in each field report, alongside pest and disease risk, with an organic and a chemical control option. It is a prompt to walk the field; satellites can’t identify a weed species. The hub’s article on seven weed control methods compares the non-chemical options.