Weed management options within the organic production system

Farmers have struggled with the presence of weeds in their fields since the beginning of agriculture. Weeds can be considered a significant problem as they tend to decrease crop yields by increasing competition for water, sunlight and nutrients while serving as host plants for pests and diseases. Since the invention of herbicides, farmers have used these chemicals to eradicate weeds from their fields. The use of herbicides not only increased crop yields, but also reduced the labor needed to eliminate weeds. Today, some farmers have a renewed interest in organic weed management methods since the widespread use of agrochemicals has led to alleged environmental and health issues. It has also been found that in some cases the use of herbicides can cause certain weed species to dominate fields because the weeds develop resistance to the herbicides. In addition, some herbicides are capable of destroying weeds that are harmless to crops, leading to a potential reduction in biodiversity among farmers. It is important to understand that in an organic seed control system weeds will never be eliminated but only managed.

Weed control is an essential aspect of successful crop production. Lack of weed control can result in total yield loss due to weed competition and weeds acting as a reservoir of pathogens through disease and insect damage. Weed control should be considered an ongoing effort and not just a seasonal effort. It is more cost effective to prevent an infestation than to eliminate a weed species once the production area is infested. Weed control should start in the previous crop, by monitoring, controlling and managing weeds. Successful weed management uses a multifaceted approach (crop and herbicide rotation, cover crop, mulch, cultivation) rather than relying solely on herbicides to control weeds. Knowing which weeds will be present and understanding their growth habits will allow the grower to achieve better weed control through the judicious application of the many weed control methods available.

Crop rotation

Crop rotation involves alternating different crops in a systematic sequence on the same land. This is an important strategy for building a strong long-term weed control program. Weeds tend to thrive on crops with similar growth requirements to theirs, and cultural practices designed to aid cultivation can also benefit weed growth and development. Monoculture, which involves growing the same crop in the same field year after year, results in an accumulation of weed species adapted to the growing conditions of the crop. When various crops are used in a rotation, weed germination and growth cycles are disrupted by variations in cultural practices associated with each crop (tillage, sowing dates, crop competition, etc.).

Cover crops

Rapid development and dense crop cover will suppress weeds. Including cover crops such as rye, red clover, buckwheat, and oilseed radish or overwintering crops such as winter wheat or forages in the cropping system can suppress weed growth. Highly competitive crops can be grown as short-lived “smother” crops in the rotation. In addition, cover crop residue on the soil surface will suppress weeds by shading and cooling the soil. When choosing a cover crop, always consider how the cover crop will affect the next crop. Additionally, the decomposition of cover crop residues can release allelochemicals that inhibit weed seed germination and development.

Intercropping

Intercropping involves growing a smother crop between the rows of the main crop. Intercropping is capable of suppressing weeds. However, the use of intercropping as a seed control strategy should be approached with caution. Intercropping can significantly reduce yields of the main crop if there is competition for water or nutrients.

Mulching

Mulching or covering the surface of the soil can prevent weed seed germination by blocking light transmission, which prevents seed germination. Allelopathic chemicals in the mulch can also physically suppress seedling emergence.

Planting patterns

Cultivated population, spatial arrangement and choice of cultivar (variety) can affect weed growth. For example, studies have shown that narrow rows and higher seeding rates reduce the biomass of late-emerging weeds by reducing the amount of light available to weeds below the canopy. Likewise, fast-growing cultivars can have a competitive advantage over weeds.

Varietal selection

Careful selection of crop varieties is essential to limit weed and pathogen problems and to meet market needs. Any crop variety that can quickly shade the ground between rows and can grow faster than weeds will have an advantage.

Tillage system

Tillage systems alter the dynamics of the soil seed bank and the depth to which weed seeds are buried. Studies have shown that nearly 75% of the seed bank is concentrated in the top 5 centimeters of soil in no-till fields. However, in the moldboard plow system, the seed bank is more evenly distributed over the depth. Other conservation tillage systems are intermediate to these two systems. The emergence of weed seedlings is often more uniform when buried shallow and can result in better weed control. Weed seeds closer to the ground are more susceptible to being eaten or damaged by insects, animals, other predators and pathogenic organisms.

Soil solarization

During the summer and fall, organic farmers sterilize their soil through solarization. In this process, a clear plastic wrap is placed over an area after it has been plowed and tightly sealed around the edges. Solarization works when the accumulated heat under the plastic wrap becomes intense enough to kill weed seeds.

Pre-emergence organic herbicides:

corn gluten meal

Corn Gluten Meal (CGM) is a biological pre-emergent herbicide and is the by-product of the corn wet milling process. The protein fraction of CGM is approximately 60% protein and 10% nitrogen. CGM produced the greatest inhibitory effect and reduced root formation in several weed species, including creeping bentgrass (Agrostis palustris) and crabgrass (Crabgrass spp.).

Mustard seed flour

Mustard seed meal (MSM) is the by-product of the seed oil pressing process. Research has shown that MSM added to soil inhibits weed emergence and growth. As with CGM, MSM is a natural non-selective herbicide that will not discriminate between weeds and crop plants, therefore care must be taken to control the target species (weeds) and ensure sufficient crop safety. .

Post-emergence organic herbicides:

Ammonium nonanoate

Racer® (40% ammonium nonanoate) is a pelargonic acid soap formulation with a changing registration history. It is a non-selective contact herbicide for the control of small (2.5-5 cm tall) annual broadleaf and grass weeds. Repeat applications may be required to control most grasses or larger (5 cm) broadleaf weeds. It has been cleared for non-agricultural use in organic crop production and, with the addition of new formulations, may be cleared for use in organic crop production.

Vinegar (5, 10, 15 and 20% acetic acid)

There are a number of biologically approved products that contain vinegar (eg, Weed Pharm®, 20% acetic acid) that contain vinegar (eg, 5%, 10%, and 20% acetic acid). Vinegar (acetic acid) is a non-selective contact herbicide. Generally, vinegar is less effective at controlling grasses than broadleaf weeds and more effective on annuals than perennials.

Clove oil

Clove oil is the active ingredient in a number of biologically approved post-emergence non-selective herbicides (e.g. Matratec®, Matran® EC and Matran®, 50% clove oil ). Clove oil is a post-emergence, non-selective, contact herbicide for the control of actively growing annual and perennial grasses and broadleaf weeds. As a non-translocating contact herbicide, its effectiveness increases with application rate and weed size. As with other contact herbicides, when weeds are of similar size, broadleaf weeds are easier to control than grasses. The effectiveness of clove oil-based weed control can be as good or better than acetic acid-based herbicides, and can be applied at lower application volumes and still be effective. Adding certain organically approved adjuvants (eg, garlic and yucca extracts) is proven to increase weed control with clove oil. Repeat applications may be necessary because larger annual grasses may regrow.

(The authors are scientists from KVK Kathua of SKUAST Jammu)