Weed herbicides make up a major portion of the annual cropping expenses on most prairie farms. Wise choices and savings in herbicide use can make a significant difference in net return. Knowing whether certain weed populations are high enough to warrant spraying or not is crucial in making these choices.
Research studying the effect of weed densities on crop yield has focused on the grassy weeds especially wild oats, green foxtail and volunteer cereals. One of the reasons is that herbicides that kill these grassy weeds are more expensive than broadleaf herbicides. Also they tend to be more specific to 1 to 3 weeds. For example considering threshold levels of Target would mean that you would have to consider up to 20 different species but the only species to consider for Avenge is wild oats. Manitoba Agriculture has compiled some recommendations and charts in a fact sheet and in their provincial weed control guide.
Economic threshold determinations have to evaluate the increased value of the crop that can be expected with the use of the herbicide less the cost of herbicide and application. There are a number of other considerations also. Any weed species that reduce grade because they are hard to clean out have a higher economic cost than just crop competition. Also control of species which might not be economic this year but would prevent problems in future years needs to be considered. For example a species difficult to control in next years crop needs to be controlled this year in preparation for the following crop. Along with this the control of new hard to kill weeds before they have a chance to spread is important. A few examples are scentless chamomile and cleavers. Likewise hard to kill perennial weeds that can become competitive quickly should be controlled in consideration for future years.
This leaves us again focusing on wild oats, green foxtail and possibly volunteer cereals as the most economic species to evaluate thresholds levels on before herbicide applications. To come up with accurate evaluations the effect of various population densities in reducing crop yield must be determined. This is what research has focused on. The following charts summarize some of these results. These values have been determined for crops which have good stand establishment and are growing well. If a crop is doing poorly yield losses may be greater than these figures.
| Table 1. Effect of Green foxtail density on yield loss in wheat, barley and canola.* | ||||
|
Green Foxtail Density (plants/sq.m.)
|
||||
|
50
|
100
|
200
|
300
|
|
| Crop |
Percent Yield Loss
|
|||
| Wheat |
2
|
6
|
9
|
14
|
| Barley |
1
|
3
|
6
|
8
|
| Canola |
3
|
5
|
10
|
15
|
* For green foxtail counts if the majority of the green foxtail has not emerged within 1 week of the crop or daytime temperatures at spraying are not equal to or higher than 20°C then yield loss will be insignificant.
Source: O'Donovan, Alberta Environmental Center (Vegreville)
| Table 2. Effect of weed density for three weeds on yield loss in canola. | ||||
|
Weed Density (plants/sq.m.)
|
||||
|
1
|
4
|
10
|
14
|
|
| Weed |
Percent Yield Loss
|
|||
| Wild Oat |
3
|
6
|
10
|
12
|
| Vol. Wheat |
1
|
6
|
11
|
14
|
| Vol. Barley |
3
|
8
|
14
|
17
|
Sources: Dew & Keys, Agriculture Canada (Lacombe)
O'Donovan, Alberta Environmental Center (Vegreville)
| Table 3. Effect of weed density for three weeds on yield loss in flax. | ||||
|
Weed Density (plants/sq.m.)
|
||||
|
1
|
3
|
7
|
10
|
|
| Weed |
Percent Yield Loss
|
|||
| Wild Oat |
6
|
10
|
16
|
19
|
| Vol. Wheat |
6
|
15
|
27
|
33
|
| Vol. Barley |
6
|
16
|
31
|
39
|
Sources:Dew & Keys, Agriculture Canada, (Lacombe)
Friesen et al. University of Manitoba (Winnipeg,)
| Table 4. Effect of wild oat density and relative leaf stage on yield loss in wheat. | ||||
|
Wild Oat Density (plants/sq.m.)
|
||||
| Wild Oat Leaf Stage |
2
|
6
|
10
|
16
|
| Compared to Crop |
Percent Yield Loss
|
|||
| 1 leaf ahead of crop |
2
|
6
|
10
|
15
|
| same as crop |
1
|
4
|
6
|
9
|
| 1 leaf behind crop |
1
|
2
|
3
|
5
|
Source: O'Donovan, Alberta Environmental Center
(Vegreville)
| Table 5. Effect of wild oat density and relative leaf stage on yield loss in barley. | ||||
|
Wild Oat Density (plants/sq.m.)
|
||||
| Wild Oat Leaf Stage |
1
|
5
|
10
|
15
|
| Compared to Crop |
Percent Yield Loss
|
|||
| 1 leaf ahead of crop |
1
|
5
|
10
|
14
|
| same as crop |
0
|
2
|
4
|
6
|
| 1 leaf behind crop |
0
|
1
|
2
|
2
|
Source: O'Donovan, Alberta Environmental Center (Vegreville)