Scouting for Weeds

Field scouting is the most important part of an integrated weed management program.

Only through routine monitoring will you be aware of developing weed problems. The key to a good weed management program on any field is the correct identification of the weed species present. This can only be accomplished with a good field scouting program.

It's impossible to know which weed control strategies to try until you know which weeds you are dealing with.

Useful Tools For Weed Scouting

Scouting is easiest when you carry some basic tools and equipment into the field with you. These include a clipboard with scouting forms, field maps, a hand trowel, a sharp knife, a hand lens, plastic and paper bags for collecting samples of unknown weeds, labels for bags, a 0.25 square metre sampler, and pencils and markers.

Optional extras that can make the work more efficient include a pocket calculator for calculating weed densities and a GPS unit. GPS units are useful for marking the location of weed patches, which will make it easier to find those patches and monitor the effectiveness of your weed control program as time goes on.

It is also a good idea to have some basic weed reference guides (ex: Weed Seedling ID Guide) and your provincial Guide to Crop Protection with you or in your vehicle.

For weeds in field crops, follow these steps:

  1. Walk a zig-zag pattern through the field, stopping at least 5 times at widely-separated points along the way. Your pattern should take you across the entire field and allow you to finish where you started (ex: the approach where you left your truck). Note that some weeds may be concentrated in low, wet, or saline spots in the field or along field margins. If such weeds are a problem, adjust your sampling accordingly.
  2. While walking through the field, identify all weed species present and record the growth stage of each weed, especially if herbicide controls are being considered. Note the condition of the crop (if it is a good stand or not) and record the growth stage of the crop. A good crop stand that is more advanced than the weeds is more competitive and suffers less yield loss.
  3. If the weeds appear to be concentrated in specific areas of the field, draw a map of the field and mark their locations. Making a field map of these weed patches will make future weed control planning easier. Hand held GPS units are useful for weed mapping. Mapping will also aid in determining the extent of the weed problem over the whole farm and calculating control costs and potential rotational crops.
  4. At each field stop, determine the number of weeds present. For best results, take 4 or 5 samples at each of your 5 major field stops. Randomly take your samples by tossing a 0.25 square metre count hoop or a brightly coloured ball into the field. Tie some bright ribbon to your count hoop or paint it a bright colour to make sure you don't lose it in the field. This will give you a total of 20 to 25 counts per field. Consider taking more samples if the field is large (greater than 100 acres) or non-uniform (with many low spots and hills). With this data, you will have an excellent picture of all the weed problems in a field.
  5. After all counts have been taken, total the count for each weed and divide by the total number of counts. This will give the average number of weeds per 0.25 square metres. Multiply this number by 4 to get an average number per square metre. Most threshold information for weeds is based on a weed density per square metre.
For more information on integrated weed management, contact Manitoba Agriculture/MASC Service Centre