Weed Control in Organic Soybean Farms

Experts all agree that there is no silver bullet to control weeds in the organic cultivation of soybeans. Farmers need to use a combination of measures to keep weeds at bay in the short and long term.

Soybean vs Weeds

Weed control is the main problem in organic agriculture including in soybeans.

It is more difficult to control weeds in soybeans because of their compact foliage, which takes longer than grain crops to produce an interlocking canopy that shades inter-row area.

On the other hand, weeds have various natural traits that help them spread and establish. These include high seed numbers, seed dormancy, wide seed-dispersal, and discontinuous germination.

Weeds can also have allelopathic effects due to the chemicals they secrete, which affects germination, nodulation, and seed quality. For example, common lambsquarters (Chenopodium album), sunflower (Helianthus annuus), and yellow nutsedge (Cyperus esculentus) can reduce the germination of soybeans by 40-90%.

Weeds not only compete with soybeans for space, light, and nutrients, but they can also transmit diseases and pests, and cause problems during harvesting.

Since chemical treatment is not allowed in organic farming, Integrated Weed Management (IWM) that relies on avoidance, prevention, and extermination of weeds with cultural, mechanical, and biological means is ideal. The aim is to bring the number of weeds down to a level they no longer affect soybeans, see Figure 1.

Figure 1: Multiple strategies to control weeds through Integrated Weed Management (IWM). (Image credits Joel Gruver)

Cultural Weed Control

Cultural operations help in preventing weeds, and in improving crop ability to compete against weeds.

• The cultural methods that increase the competitiveness of soybeans are increasing crop cover, variety selection, planting time, and improving soil conditions.

• Prevent weeds from establishing in the fields through good sanitation, crop rotation, and cover crops.

These are some of the first practices an organic farmer should consider in weed control. This also reduces the reliance on heavy machinery and herbicide treatments, improving the overall ROI of a farm. 

Clean Start

Weed prevention can start by ensuring that the field is free of weed seeds. Weeds come from seed banks present in the soil or are transported as contaminants of crop seeds into a farm. So, control these two sources.

• Reduce the weed seed bank: Mow weeds in the edges of the field or after harvest to prevent weeds from going to seed. Hand weeding to control small patches of new weeds can stop their future spread.
• Clean seeds: Check the crop seeds and remove any contaminants to prevent introducing new species or augmenting existing weeds’ populations. Seeds should also be free of fungal or viral diseases that can affect germination or growth rate. Sparse crop cover or weak growth will leave empty spaces in fields that can be easily colonized by weeds.
• Intensify weed control: Most of the damage to the yield happens during initial crop development, resulting in an 80% drop in yield. This is called the critical period of weed control and is the time when farmers should intensify their weed management.
• Clean machinery: Weeds can be carried on farm machines and equipment used for tilling and cultivation from one field to another. Cleaning the equipment will not only reduce weeds but also diseases. Starting operations on farms with lower weed infestations is another possibility. 

Selection of Variety

Soybean cultivars vary in their ability to compete with weeds, due to vigor and growth rate. Cultivars that emerge quickly, and have a fast-early development, will achieve canopy closure sooner and minimize weed establishment by cutting light penetration. This can decrease reliance on mechanical control.

Planting Time

Varying sowing time to eliminate weeds is often advocated. Weeds have different times when they germinate, so farmers are probably only altering the weed sets that soybeans have to compete with when they alter the planting date. Hence, growers need to plan their planting time carefully considering which weed species are a problem in their fields.

Generally delayed planting can encourage faster germination of soybean crops in warmer conditions and provide an opportunity to tackle early germinating weeds by shallow tilling.

Increase Crop Cover

The soybean crop population can be increased to limit light available for weeds. There are several ways of achieving this.

• Closer and more planting: Decrease the distance between the rows to 20 inches, so that soybean can quickly cover the intermittent ground with their foliage. Depending on the cultivation, the number of seeds can also be increased. “When blind cultivation such as rotary hoeing will be used, seeding rates should be increased by 10 to 20 percent,” recommend NC State Extension scientists.
• Check planter success: Planting efficiency of machines can impact crop cover. Adjust the planter for proper depth placement of seeds and optimum seed rate. However, machines are not always 100% efficient, so farmers should pause during planting to check the placement of seeds in the fields. Maintenance of the equipment can also be helpful in this regard.

Crop Rotation

Scientists have shown that having more species of weeds is better for soybean yield than having one or few very dominant weeds. When dominant species are controlled, a greater number of weeds can establish, and reduce the impact on soybean yield.

Crop rotation prevents any weed species from gaining dominance. When crops with varying life-cycles and management are used it prevents the same weed from building pressure.

“Multiple year crop rotations change the weed mix and yearly weed pressures. A rotation can break disease and pest cycles providing healthier, more vigorous competitive crops,” says Dan Rossman from the Michigan State University Extension.

Moreover, crop rotation by itself increases soybean yield. In the long-term, rotating with crops like wheat or sorghum which produce high residue also improves soil fertility by increasing soil carbon and nitrogen levels. 

Use Cover Crops

Incorporating green cover crops in the rotation before soybeans are one of the surest ways to bring weeds under control. Cover crops limit weeds by reducing space, light, and nutrients for weeds. After the cover crops are cut, the mulch continues weed control while soybean is growing. Cover crops can also be allelopathic and produce chemicals that control certain weed types.

Ryegrass, vetch, turnips, and oats are used in the tropics. In the sub-tropics, it is pigeon pea, crotalarias, velvet beans, jack-beans. In the USA, rye, cowpea, triticale, or clover are recommended. Cover crops can be allowed to grow longer in winter and for a shorter duration in summer. Care should be taken to cut or suppress them before seed-set, to prevent them from becoming weeds. 

Improve Soil Conditions

Improving soil fertility will boost the growth of soybean plants and give them a competitive edge.

However, the quality of manure will matter. Manure that is not properly composted can introduce weed seeds.

The pH of the soil can be increased to make it more alkaline if the field has weeds that like acidic soils. 

Mechanical Operations

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Figure 2: Image credits: (Weed Management for Organic Soybeans)

The purpose of mechanical operations in organic weed control is to remove any weed that has emerged by uprooting, injuring, or burying them.

During cultivation, the short stature of soybeans should be kept in mind as they cannot tolerate too much soil throwing and burying. The timing of tillage will therefore determine the machinery used. Speed, machine adjustment, and operator skills are all factors that will affect success in controlling weeds with mechanical operations. 

Scouting

Regular and early scouting is a must to check for weed problem areas, and to identify the species. 

Tillage

The type of tillage can affect weed control. Methods that turn up topsoil to suppress early emerging weeds and expose late-germinating types are the best, like moldboard plowing or rotary hoes. Disk and chisel plowing are not as efficient in removing weeds that can compete with the emerging crops; hence they can be combined with blind cultivation and flame weeding. 

Pre-emergence

Before the soybean has germinated, shallow blind cultivation can control early weeds that are just sprouting by loosening the topsoil and drying the weed seeds. Flex-tine, spike-tooth, and spring-tooth harrows, or rotary hoe can be used for blind cultivation. They don’t disturb deep correctly placed soybean seeds, see Figure 2.

Deep-rooted perennial weeds control will need other mechanical means. 

Post-emergence

Up to two to five blind cultivations can be undertaken once every five days after emergence, with the flex-tine harrow or rotary hoe.

Later, between-row cultivation is used when both weeds and crops are larger but before the crop canopy closes. As it can disturb the crop it should be used sparingly. S-tine and wide sweep cultivators can be used to remove larger weeds, and rolling cultivators that heap soil to or away from crop rows for smaller weeds.

To prevent damage to soybean crops, organic farmers are using satellite GPS navigation to mark rows while sowing. This map is used for all post-emergence cultivation to avoid damaging soybean. 

Flame weeding

This technique is useful against broadleaved weeds and is suitable for tilled and non-tilled fields. As fire can also affect soybean crops, it is useful only when the crop is young and the field is wet, see Figure 3.

Figure 3: Flame weeding is “terminating weeds without waking sleeping seeds,” says Joel Gruver

No-Tillage

An absence of tillage combined with mulching prevents the germination of weeds and the survival of their seedlings. This changes the composition of weeds and decreases their density.

For example, up to 90% of cyperus grass tubers are eliminated by no-tillage. Sorghum residue mulch reduces populations of Brachiaria plantaginea by 96%, Sida rhombifolia by 91%, and Bidens pilosa by 59%.

In addition to weed control, no-tillage also improves physical and chemical soil properties. Also, the soil microbe community becomes more diverse and numerous and helps in nutrient cycling. 

Biological Treatments

In addition to cultural and mechanical means, biological options, using natural enemies such as pathogens and predators of weeds can be used to control weeds. 

Bioherbicides

Many weed-specific bioherbicides being developed around the world to replace chemicals in conventional farming are also suitable for organic cultivation.

Fungi, bacteria, or microbial products that infect weeds are used as bioherbicides. The advantage of using microbes is that they are living organisms, which will survive and multiply in the field in succeeding crop cycles and continue weed-control.

Bioherbicides can be used at any stage of the crop as seed treatment, and soil, foliar, and post-harvest applications. They can be liquid, granular, or dust in form. These bioherbicides are expected to get cheaper as demand drives supply. 

Pests and Predators

Known predators or pests of weeds like parasitic wasps, painted lady butterflies, etc. that target only specific weeds can also be introduced in the fields as biological weed control. 

Chemical Control

The IWM uses a combination of methods to avoid chemical use. As a last resort, natural plant extracts and some chemicals that are allowed in organic cultivation can be used.

Since these are expensive, their use can be optimized by using precision farming techniques. Drone images can be analyzed to locate the exact spots in the fields that have weeds, and estimate their severity. A prescription map uploaded to smart farm machinery for directed spot applications can further reduce the amounts of herbicides used.

Herbicides that are certified for weed control in organic farming are essential oils such as clove oils, plant extracts like corn gluten meal products, and distilled vinegar. 

Know Your Weeds

The weeds species -grasses and broadleaved- have unique requirements. Often when a farmer manages to control one weed, they could be creating conditions optimum for another weed. So, it is necessary to be able to identify weeds and their intensity, and know more about their growing conditions to be able to maintain soybean crop productivity. By using IWM, over the years, investment in weed control in organic cultivation can be reduced to improve ROI.