University of Manitoba: Natural Systems Agriculture: Predicting Weed Emergence Using Temperature
University of Manitoba Faculty of Agricultural and Food Sciences Department of Plant Science

Predicting Weed Emergence
Using Temperature


Prediction of weed emergence periodicity can help farmers implement more informed weed control strategies. Timing of weed control measures is important for optimum weed control. Weed emergence is determined primarily by two factors: soil temperature and soil moisture availability. These two factors are influenced by weather and soil management such as tillage.

Study Objectives

  • The aim of this study was to characterize the emergence periodicity of several annual weed species in canola fields in Manitoba, and
  • To determine the influence of tillage system (conventional vs. conservation)</font> on these emergence periods.

Experiment Description

Canola fields were randomly selected in southern Manitoba for this study. Tillage systems used on these fields were classified as conventional (at least two passes with a field cultivator) or conservation (no-tillage, narrow openers at seeding). Weeds were counted in marked quadrats every two to four days from April 1 to bolting of canola. Once counted weeds were tagged so they would not be counted again. The earliest emerging weeds were not protected from pre-seeding tillage or herbicide application. Weeds were protected from in-crop herbicide by covering quadrats.

Soil temperature was measured continuously throughout this period. The soil temperature data was used to calculate cumulative growing degree days (GDD), a measure of heat accumulation. Weed emergence was then correlated to accumulated GDD.

How are Growing Degree Days (GDD) calculated?

GDDdaily = ([Tempmax + Tempmin]/2) - Tempbase

accumulated GDD = GDDday1 + GDDday2 + GDDday3 ....

where Tempmax is maximum daily temperature,
Tempmin is minimum daily temperature,
and Tempbase is the temperature at which no germination is expected to occur (0 degrees C in this study)


Emergence periodicity of wild oat and green foxtail as influenced by tillage in 2000.
Solid black line and diamonds = conservation tillage; dashed line and open diamonds = conventional tillage.

Follow the blue line in the figure at right. In 2000, 400 GDD occurred at approximately May 18. This means that:

  • under no-till conditions, about 38% of wild oat emerged by May 18, but only 18% under conventional tillage
  • only 5% of green foxtail emerged in both systems at this time

Most of the weed species studied emerged earlier under no-till conditions. The exceptions were wild mustard and red root pigweed which emerged at the same time under each tillage system. Soil moisture and temperature to a depth of one inch was not different between the two tillage systems in this experiment, therefore, it was determined that earlier onset of weed emergence under no-till was due to the fact that weeds were emerging from a shallower depth. The number of weeds that ultimately emerged was the same in both systems.

Canola emerged earlier than weeds in this experiment especially in the no-till fields. Canola growing under no-till emerged 65 GDD before weeds, whereas canola growing under conventional tillage emerged 45 GDD before the weeds. This suggests that canola under no-tll has a better competitive advantage with weeds than canola growing under conventional tillage. If the crop is ahead of the weeds, there is less competition from the weeds and less yield penalty.

How can you use this information to predict your weed emergence?

We've developed a weed emergence calculator using GDD data from Winnipeg. This calculator requires Microsoft Excel. To use follow directions below:

  1. Click on icon at right.
  2. When File Download box appears, click on Open.
  3. When Microsoft Excel box appears, click on Enable Macros.
  4. If Query Refresh box appears, click on Enable automatic refresh.
  5. After file has finished downloading, press ctrl + d on keyboard to update GDD accumulation.
  6. Click OK when asked if you want to replace the content of destination cells.
  7. Note yellow box has now been updated to include current GDD accumulation.
  8. Columns under the headings in the green box have current weed emergence.
  9. Close window when done. You have the option to Save or you can close without saving.

Note: You may need to adjust the macro security settings in Excel in order for the calculator to function properly (under "Tools", select "Options", then select the Security tab, then click on Macro Security and select "Medium"; you will then need to re-open the calculator file).

Because the above calculator uses GDD data from Winnipeg, the results may not be the same for your area. You can calculate your own GDD and insert the accumulated GDD in the model yourself. Weather data is available online from various sources.

Once you have this information, use the formula above to calculate your accumulated GDD. This number can then be plugged into the Weed Emergence Calculator. Note that historically GDD calculated from air temperature are similar to those being calculated from soil temperature.


  • Using the Weed Emergence Calculator can help improve timing of control operation whether it be herbicide application or non-chemical strategies such as in-crop harrowing.

Further Reading: W.J. Bullied, A.M. Marginet, and R.C. Van Acker. 2003. Conventional- and conservation-tillage systems influence emergence periodicity of annual weed species in canola. Weed Science 51:886-897.

Copyright and Liability

This page created August 2004.