The weed control measures used in soybean production need to be overhauled. The development of resistance in weeds has made current technology and management increasingly expensive and comes accompanied with heavy yield losses. Precision management solutions can reduce chemical use in several ways.
Soybean is important for the global economy as it is used as a source for oil and protein (for people and livestock) and as biodiesel.
Soybean is the crop with the largest monoculture in the world and with 90% of it grown in Argentina, Brazil, the United States, China, and India.
Weeds are the number one problem in soybean cultivation in all these countries and are estimated to result in a 37% reduction in yields on average.
The current measures are clearly not working for soybean farmers.
The main reason for this is the development of resistance in weeds. Worldwide, regardless of the weeds species and the soybean cultivars used, farmers use only a few broad-spectrum herbicides and similar agricultural technology in the monoculture.
In the USA more than 93% of soybean crops use GR (glyphosate-resistant) technology, and in Brazil and Argentina, it is 80% and 99%, respectively. Roundup or glyphosate is used as the herbicide to control weeds growing in these GMO crop fields.
Since glyphosate is a broad-spectrum herbicide also used for many other crops and weeds, it is extensively and frequently used over large areas. This has led to the development of chemical resistance in the weeds. So, farmers spend an increasing amount of herbicides and are yet not able to control the negative impact of weeds on their soybean yield.
Of the top ten weeds found in soybean fields in the USA, half have already developed resistance to herbicides, and they are the
- Common waterhemp
- Giant ragweed
- Lamb quarters
- Horseweed/ marestail
- Common sunflower
The other five weeds are Velvet leaf, Giant foxtail, Woolly cupgrass, Burcucumber, and Annual morning glory.
Research to find new herbicides, which can control the resistant weeds
One of the solutions is the wider use of precision farming methods based on site-specific control.
Drew Lyon, an extension weed specialist from the Washington State University, says, “The new site-specific weed management technology is currently popular in high-value crops.”
Figure 1: Weed treatment can be restricted to patches, grids in patches, or individual plants, Christensen et al. 2009. (Source: Site‐specific weed control technologies)
Instead of treating the whole soybean farm, it is possible to treat only patches/subfields, grids in patches, or target individual weed plants to control them, as shown in Figure 1. The amount of herbicide or treatment is also proportional to the weed density using variable rate application.
To use precision farming methods, we need
- Weed management models
- Precision weed control implementation
There are numerous methods and approaches
Figure 2: Using cameras of different light wavelengths for weed identification, Martin Weiss et al. 2008. (Source: Precision farming for weed management: techniques)
The camera differentiates weeds from the soil and the soybean, based on color, shape, and size of weeds and crops, as shown in Figure 2.
- In patch and grid level treatments, an image of the field is taken by remote sensing by drones that are flown over the fields by farmers or by drone operators. Satellite imagery uses the same technology.
- In spot treatment where individual weed plants are treated, sensors attached to the boom feed the images directly to the computer to help in decision making.
During scouting by drones, many photos are taken, that are “stitched” using stitching software to get a composite map of the field, which is then analyzed.
In spot application, the pictures of the ground are analyzed in real-time.
Figure 3: The images a–e, show maps derived from remote sensing on (a) soil classification, (b) weed distribution map, (c) weed control recommendations, (d) application map, and (e) yield map (as a result of soil quality and weed control), Weiss et al. 2008. (Source: Precision farming for weed management: techniques)
A combination of computers,
This technology can also count weeds in a given area, for example per square meter to find out the intensity of infestation. Then using threshold levels based on the crop and weed type, the intensity is calculated creating treatment models. For example, Figure 3 b shows a model with four zones,
- Grids with no weeds
- Low-density grids with 10-25 weeds/m2
- Medium-density grids with 25-60 weeds/m2
- High-density grids where there are more than 60 weeds/m2
Specific weed maps can be created for each weed or a group (grasses or broad-leaved species) as in Figure 3c.
As it is possible to detect different species of weeds from drone images as well as sensors on application booms, species-specific herbicides can be used, reducing reliance on any single chemical.
Farmers can use insights on weed identity, density, and location, to control
In either case, farmers can opt to use conventional technology, and still cut costs by reducing herbicide use, since areas with no weeds get no treatment at all. This can reduce areas to be sprayed by around 60% or even more, by targeting only the weed patches.
However, there is advanced variable rate application technology already on the market.
The smaller the treated area, the greater is the reduction in herbicide use.
“Several weed sensing systems and precision implements have been developed over the last two decades,” says Dan Crummett. “The lack of knowledge about the economic and environmental potential for increasing the resolution of weed control,” remains an important barrier in increasing the resolution of weed control, he explains.
Weed maps depicting the occurrence of one or more types of weeds are used to develop prescriptive maps, using economical thresholds. The treatment is adjusted based on intensity, so not only is a treatment restricted where necessary, but even the amount of herbicides can be varied, resulting in huge savings in chemical use.
Depending on the scale of application there are many technologies available on the market.1. Drones for weed patches: Weeds patches can be treated by automated sprayers on drones. Drones are not allowed for spraying in the European Union, as there are concerns due to drift produced during spraying. Drones are allowed in the USA, but pilots have to adhere to strict Federal Aviation Administration operational rules.
2. Patch sprayers: Tractors with GPS can read uploaded prescription maps to apply herbicides at a variable rate for a single weed type based on weed density, as shown in Figure 3d.
Special patch sprayers have booms with direct injection sprayers or a series of nozzles for variable rate application. In patch sprayers, sections of the boom can be controlled separately. Patch sprayers can deliver 3-6 levels of spray treatments based on previously prepared prescription maps. Here the maps are complex, where weeds are classified into three or more types. The sprayer also has three or more separate tanks that can deliver different herbicides, based on the location of each kind of weed, with a control system regulating the rate of application.
3. Grid application: Using machines with individually controllable nozzles and injection sprayers allow a higher resolution of application within the patches, and application is controlled by adjusting the rate of sprays. Modern nozzles have a control system that uses a pulse-width-modulated rate, which can control the spray frequency and droplet size. Each nozzle system has many types fitted, so farmers can choose on the go. A good control system can alone reduce your herbicide use by 2-5%.
4. Spot application: Machinery against individual weeds usually scout the ground using sensors mounted on the boom and send the images to a computer in the vehicle. A software analyzes images to identify weeds, calculate the rate, and apply the correct herbicide only over the weeds. These usually use injection sprayers. Some of the new technology can be used day and night! At the end of the trip, farmers know the type and count of weeds.
Injection sprayers have the highest application efficiency and reduce herbicide use by nearly 95% compared to conventional broadcast spraying of whole fields.
Automated Steered Mechanical Methods
Mechanical methods can also be used to manage weeds in soybeans.
Automated steering technologies for inter-row hoeing in combination with a camera on sensors is one possibility.
In comparison to conventional machine hoeing, in soybean farms, the automated method controls more
One of the main disadvantages of site-specific application technology is the high capital cost of sophisticated machinery. However, these sprayers can be used also for applying fertilizers, pesticides, and fungicides, to distribute the costs. Moreover, 60-95% reduction of herbicide costs coupled with an average 37% increase in soybean yield, should help to recover the investment in a few years.
Part of a Package
Precision technology has the potential to
prevent the development of resistance against any new herbicides, and limit the
damage by older herbicide use. Yet,
Using cover crops is another way to prevent weeds from establishing, by acting as mulch in the soybean growing season. Wide-row intercropping may be another way to help suppress weeds. Based on the information of weeds, their location, and intensity, farmers can plan ahead for weed management in the next soybean cycle, as weeds grow mainly from seed banks.