University of Manitoba: Natural Systems Agriculture: Swedish Model
University of Manitoba Faculty of Agricultural and Food Sciences Department of Plant Science

How a Swedish Model Can
Help You Use Fewer Chemicals


Print a copy of the Swedish Model
Spring Crop | Fall Crop, Spring Treatment

The Swedish Model is a decision support model for using reduced rates of herbicides in crops. Crops must be thoroughly scouted at appropriate times. The model works with all crops, all weeds and all herbicide combinations. The user of the model must be experienced enough to recognize and judge the vigor of the crop, be able to identify the weeds, be familiar with a herbicides efficacy in controlling specific weeds.

An example situation will be used throughout the steps below to illustrate the process.

Crop: barley (3-4 leaf stage, 350 plants/m2), weeds: wild oats (3-4 leaf, 10 plants/m2), herbicide: Puma Super.

Step 1

There are two models, one for a spring annual crop and one for a winter annual crop. Choose the correct model for your situation. Example: spring crop

Step 2

Pick one of the three sections on the circle: NORMAL START, WEAK START or VIGOROUS START of the crop. Estimate the ability of the crop to compete with the weeds. This would involve judging the difference between the vigor of a good barley population to the relative inability of flax to compete against weeds. Within a crop, the user must judge the crop density and health of the crop to pick the correct third of the circle. Example: Vigorous start.

Step 3

Within the correct third of the circle, pick the appropriate weed density: LOW, MEDIUM or HIGH. There are no specific densities listed, but past experience with other weed populations should allow you to place yourself in the correct 1/9th of the circle. Example: Medium.

Step 4

Still in the correct 1/9th of the circle start at the centre of the circle for your next decision. How competitive are the weeds: WEAK or STRONG? You will need to be familiar with the biology of each weed to make this decision. You will likely have a number of weed species in the field. Concentrate on the weed(s) that are the most competitive and have the highest density. Example: Strong.

Step 5

Still in the correct 1/9th of the circle move toward the outside of the circle either up the WEAK or STRONG section. The next decision is Ease of Control: EASY or DIFFICULT. By now you should have a herbicide(s) in mind for controlling the weeds in the crop. You must now judge the herbicides ability to control the weeds that are present. Will it be EASY or DIFFICULT for the herbicide to control the weeds? The Manitoba Guide to Crop Protection has tables at the beginning of the book ranking the herbicides ability to control the weeds. Control ranges from E=excellent, G=good, F=fair and P=poor. Example: Easy.

Step 6

Continue moving toward the outside of the circle, up from either EASY or DIFFICULT to the next decision: Are the weeds BIG or SMALL? Look at the herbicide label to find the range of weed staging that is recommended. For example: if a weed is labelled to be controlled from the 1-4 leaf stage, the weed would be BIG if it was in the 3-4 leaf stage. Example: Puma label for wild oat control: 1-6 leaf stage, 3 tillers. Therefore Small.

Step 7

Continue moving toward the outside of the circle, up from either BIG or SMALL to the next decision: the weather. This decision circle is blank. You must decide if the weather is suitable for making a reduced herbicide recommendation. Remember that the best conditions for a herbicide to work are: good moisture conditions (crop and weeds not under stress), warm but not hot or cold temperatures, relatively high humidity conditions, in other words good growing conditions. If the crop or weeds are under stress from any factor it is best not to reduce the herbicide rate for satisfactory weed control. Example: Good weather conditions.

Step 8

Continue to move toward the outside of the circle to the last ring in the circle. This is the recommended fraction of a full rate of herbicide that would give satisfactory weed control under the circumstances you find yourself in. Example: ¼ of the full rate (0.31L/ac, 20 Ac per jug) of Puma or 0.078L/ac, 80 Ac per jug.


Use this model at your own risk. No warranty expressed or implied. The manufacturer of a herbicide will not guarantee weed control efficacy at reduced herbicide rates.

For further information contact: Gary Martens, Plant Science Department, University of Manitoba at 204-474-8227,

Copyright and Liability

This page created August 2004.