Scouting for weeds

Timely weed scouting makes it possible to know 1) what weed species are in a field, 2) where they are, and 3) how severe the infestation is – all of which are very important to know in order to design a management program that fits the specific needs of that field.

Scout early, scout often:

Since many weed species become more challenging to manage as they get larger, scouting should start early and be repeated at several key times through the season in order to catch weeds when they are young. Herbicide applications are often not designed to be effective against mature weeds, increasing management costs.

Herbicide-resistant Palmer amaranth and waterhemp must have herbicide application before they reach 4 inches tall in order for effective herbicides to kill the plants.

Weed species identification:

Accurate weed ID is important for effective management because herbicide recommendations vary according to species, as do some mechanical, cultural, and biological strategies. Some species can look similar to each other other species from afar, but may have drastically different management requirements. They should be examined closely to determine herbicide programs. Weed identification resources are available here, or by contacting your state’s extension weed science specialist.

Scouting procedure:

The goal is to get a representative idea of the weed populations throughout the whole field. For a 100-acre field, make 5-10 stops that are well spread out through the field. At each stop, walk 10 paces (or 30 feet) and record the following:

  • Weed species present
  • Life stage or height of weeds
  • Lifecycle (summer annual, winter annual, perennial)
  • Severity of the infestation based on number of plants (Low, medium, high)

Pre-plant scouting:

Fields should be scouted prior to planting for the following reasons:

  • to ensure effective burndown treatments before plants become too large to control effectively with chemicals
  • to evaluate the potential need for alternative pre-plant weed management strategies such as tillage
  • To ensure that crop seeds are being planted into weed-free fields

Post scouting:

This should be done soon after planting to determine the efficacy of pre-treatments and to decide if further treatments are necessary. Again, particular attention should be paid to the height of the plants; weeds should not be allowed to grow past several inches tall, in order to maintain effectiveness of post treatments and minimize competition with the crop.

Scouting for resistance:

Scout for weed species that are known to have herbicide resistance, especially if they have been sighted in your area or county. Click here for a current list of herbicide resistant weeds in your state and check out our interactive United States map of resistant pigweeds (coming soon). Failure of an herbicide application to control a weed does not necessarily mean it’s herbicide-resistant. While genetic resistance must be confirmed through laboratory testing, these signs can be evidence of potential resistance:

  • Spreading area of a single weed species
  • Failure of an herbicide application to control a weed species that it is usually effective on at the applied dose, especially if there are other surrounding weeds are successful controlled
  • Within the same weed species in the same field, there are surviving plants and controlled plants

If you suspect resistant weeds, contact your local extension agent. Collect a sample and provide them with information about the field conditions and the size of the weed population. If there are only a few plants, remove them from the field before they have a chance to spread seed.

Precision tools for weed scouting:

Numerous crop scouting apps have been developed for smartphones and tablets, and some of them are particularly useful recording weed info and doing weed ID in the field. Here are some examples:

List of crop scouting apps with good weed scouting capabilities – University of Missouri

ID Weeds app, by University of Missouri

ScoutPro

FarmLogs

eCropScout

Connected Farm

AGRIplot

Resources:

Brief Summary of Scouting principles – United Soybean Board

North Carolina State University weed scouting recommendations

South Dakota State University – Site-specific weed scouting guidelines

Weed Identification Key – University of Missouri

University of Missouri Weed ID and Herbicide Injury Guide – $18.00

Penn State University Herbicide Injury Guide

Common Weeds of No-Till – Purdue University

Weed ID Guide for Southeastern US grasses – Virginia Tech

Weed photos for use in weed ID – WSSA

Comprehensive list of weed ID sites