Conservation tillage places increased emphasis on spray application for weed control. Although the relative abundance of weeds may decrease under conservation tillage, cultivation is no longer a management option when weeds do become a problem. As a result, many zero-tillers spray a field at least 3 times a year - before seeding, in-crop, and post-harvest. Pre-harvest weed control and fungicide and insecticide applications will further increase the spraying frequency.
Spraying is both time consuming and expensive, and must occur within a narrow time frame. It is therefore important to take this task seriously and do as good a job as possible. A good yield may depend on it.
A zero-till environment differs from a conventionally tilled field in several ways, notably through the presence of standing stubble (Figure 1). Stubble can intercept spray, reducing the dosage reaching the weed. The stubble can also change the environment in which weeds grow, by reducing the impact of wind, sun, and drought conditions. As a result, the weed is established in a sheltered environment, which may change the way it responds to a spray. Changes in wind speed in the canopy may also affect spray drift, an ongoing concern for applicators and their neighbours. This paper will summarize recent research on the impact of this crop residue on spray targeting, weed growth, and off-target drift, and provide general recommendations on how to best achieve good weed control.
A dense wheat stubble canopy can intercept a significant amount of spray. A single wheat stubble culm can capture between 2 and 3 µL of spray, a similar amount to a foxtail plant (Table 1). While this does not seem like much, a population of 500 culms/m2 could capture between 10 and 15 L/ha of spray, representing a significant fraction of the dosage applied to the field. Weeds growing near this stubble will experience a significant reduction in spray dosage, up to 50 % depending on their location. How can this loss of dosage best be minimized?
Table 1: Spray volume distribution among weeds and stubble culms, sprayed with two spray qualities.
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Proportion of total output |
|
(L/ha) |
(µL/plant) |
(#/m2) |
(L/ha) |
(%) |
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Smooth Pigweed |
Fine |
90 |
14.6 |
10 |
1.46 | 1.6 |
|
500 |
73 | 81 | ||||
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Medium |
180 |
44.9 |
10 |
4.49 | 2.5 | |
|
500 |
225 |
125 | ||||
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Giant Foxtail |
Fine |
90 |
5.94 |
10 |
0.59 | 0.66 |
|
500 |
30 | 33 | ||||
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Medium |
180 |
9.68 |
10 |
0.97 | 0.54 | |
|
500 |
49 | 27 | ||||
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Wheat Straw |
Fine |
90 |
2.10 |
10 |
0.21 | 0.23 |
|
500 |
10.5 | 12 | ||||
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Medium |
180 |
3.36 |
10 |
0.34 | 0.19 | |
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500 |
16.8 | 9.3 | ||||
Since standing stubble is vertically oriented, a spray droplet falling straight down from the nozzle is more likely to miss the stubble than a droplet that has a more horizontal trajectory. Applicators can use this to their advantage by adjusting spray quality, nozzle orientation, and travel speed.
Larger droplets are more likely to travel in a vertical compared to a horizontal direction since they are primarily under the influence of gravity. Small droplets, on the other hand, move in response to air currents, and often travel horizontally. As a result, a coarser spray is less likely to be intercepted by standing stubble (Figure 2).
Orienting the spray forward has long been recommended to increase coverage on grassy weeds. Since grassy weeds are vertically oriented, droplets coming from the side are more likely to hit the weed than those coming down from the top. Unfortunately, forward orientation in a stubble canopy will predispose the spray to interception by stubble (Figure 2). Even though this approach is still favoured for coverage of grassy weeds, the downside is that overall less spray will be available to hit the grasses in a stubble environment.
Faster travel speeds have two main effects on herbicide sprays. In the first place, faster speeds require higher flow rate nozzles, which produce a coarser spray. This should favour the penetration of stubble. On the other hand, faster speeds cause the spray's movement to become more horizontally oriented, even if the nozzles point straight down. Because of this effect, faster travel speeds usually mean less canopy penetration and less spray available to hit weeds (Figure 3).
Wind velocities near the ground are significantly lower in a stubble canopy than on bare soil. Even in a sparse stubble stand, velocities averaged 16% lower in stubble compared to a bare environment (Figure 4). Velocity reductions were most dramatic at low wind speeds and closer to ground level.
Wind velocity can have major effects on early plant growth and development. Pigweed plants grown in a bare environment had lower growth rates and smaller leaf areas than those grown in a stubble environment, even though they were at the same growth stage after the test period (Table 2).
Table 2: Plant growth parameters for smooth pigweed grown in two field environments, 32 days after planting.
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Standard |
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| Parameter |
Units |
Canopy |
Mean |
Error |
| Plant height |
(cm) |
Stubble |
8.83 |
0.54 |
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Bare |
4.50 |
0.22 |
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| Biomass |
(g) |
Stubble |
3.50 |
0.41 |
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Bare |
1.59 |
0.16 |
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| Leaf number |
(No./plant) |
Stubble |
8.17 |
0.31 |
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Bare |
8.00 |
0.45 |
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| Projected leaf area |
(cm2) |
Stubble |
14.8 |
1.1 |
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Bare |
7.8 |
0.4 |
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As a result of the differing environments, plants grown in the stubble proved to be more susceptible spray targets. For example, smooth pigweed plants grown in a stubble environment were much more susceptible to a herbicide application than those grown in a bare environment (Figure 5). This effect could be due to greater spray capture by the larger projected leaf area, increased presence of soil pathogens, or more permeable cuticles of the plants grown in a stubble environment.
The reduction of spray drift will always be an important goal for applicators. There are relatively few days available for spraying, and applicators must make the most of the time they have available. The rules of drift management, i.e., choice of proper weather conditions, nozzle type, carrier volume, boom height, and travel speed, are discussed in some detail in a recent SAF Farm Fact (Spray Drift - Causes and Solutions). But for those practicing conservation tillage, a stubble canopy provides a unique beneficial factor that is not mentioned in traditional publications on spray drift.
A stubble canopy reduces spray drift significantly in several ways. First, by reducing the wind speed near the ground, spray is less subject to displacement. At a 50 cm height and a wind speed of 20 km/h in a bare environment, wind speed was reduced by an average of 15% (Figure 4, Figure 6). This translates directly into a drift reduction.
Drift collection: A second way by which stubble reduces drift is through capture of the drift cloud that moves downwind. The cylindrical shape of wheat stubble makes for effective spray drift collectors, and wind tunnel tests have shown dramatic drift reductions arising from the presence of stubble (Figure 7).
The final way in which stubble contributes to reduced spray drift is through increased dispersion of the spray cloud. The stubble provides what are called "roughness elements" at ground level. This rough surface causes mechanical turbulence when a wind passes over it. Mechanical turbulence contributes to the upward dispersion of a spray cloud, which reduces the spray dosage contained in the drift cloud at ground level. The reduced dosage is less harmful to non-target plants and animals.
Maximum penetration of stubble is achieved with:
- coarser sprays
- slower travel speeds
- vertical nozzle orientation
A stubble environment provides shelter for early plant growth, resulting in weeds that may be more susceptible to herbicides
Stubble reduces spray drift by:
- reduced wind speeds near the ground
- increased dispersion of the spray cloud
- improved capture of spray drift by standing stubble.