Farmers have struggled with the presence of weeds in their fields since the beginning of agriculture. Weeds can be considered a significant problem because 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. Using herbicides not only increased crop yields but also reduced the labor required to remove weeds. Today, some farmers have a renewed interest in organic methods of managing weeds since the widespread use of agro-chemicals has resulted in purported environment and health problems. It has also been found that in some cases herbicides use can cause some weed species to dominate fields because the weeds develop resistance to herbicides. In addition, some herbicides are capable of destroying weeds that are harmless to crops, resulting in a potential decrease in biodiversity on farmers. It is important to understand that under an organic system of seed control, weeds will never be eliminated but only managed.

Controlling weeds is an essential aspect of successful crop production. The lack of weed control can result in the total yield loss due to weed competition and with weeds acting as a reservoir for pathogens through disease and insect damage. Weed control should be considered a continuous endeavor not just a seasonal effort. It is more cost effective to prevent an infestation than eliminating a weed species once the production area is infested. Weed control should start in the previous crop, by monitoring, controlling, and managing the weeds. Successful weed management uses a multifaceted approach (rotating crops and herbicides, cover crops, mulches, cultivation) rather than relying solely on herbicides to control the weeds. Knowing which weeds will be present and understanding their growth habits will enable the producer to achieve greater weed control by the wise application of the many weed control methods available.

Crop rotation

Crop rotation involves alternating different crops in a systematic sequence on the same land. It is an important strategy for developing a sound long term weed control program. Weeds tend to thrive with crops of similar growth requirements as their own and cultural practices designed to contribute to the crop may also benefit the growth and development of weeds. Monoculture, that is growing the same crop in the same field year after year, results in a build-up of weed species that are adapted to the growing conditions of the crop. When diverse crops are used in a rotation, weed germination and growth cycles are disrupted by variations in cultural practices associated with each crop (tillage, planting dates, crop competition, etc).

Cover crops

Rapid development and dense ground covering by the crop will suppress weeds. The inclusion of cover crops such as rye, red, clover, buckwheat and oilseed radish or over wintering crops like winter wheat or forages in the cropping system can suppress weed growth. Highly competitive crops may be grown as short duration 'smother' crops within the rotation. Additionally, cover crop residues on the soil surface will suppress weeds by shading and cooling the soil. When choosing a cover crop, consideration should always be given to how the cover crop will affect the succeeding crop. In addition, decomposing cover crop residues may release allelo chemicals that inhibit the germination and development of weed seeds.

Intercropping

Intercropping involves growing a smother crop between rows of the main crop. Intercrops are able to suppress weeds. However, the use of intercropping as a strategy for seed control should be approached carefully. The intercrops can greatly reduce the yields of the main crop if competition for water or nutrients occurs.

Mulching

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

Planting patterns

Crop population, spatial arrangement, and the choice of cultivar (variety) can affect weed growth. Fr example, studies have shown that narrow row widths and a higher seeding density will reduce the biomass of later-emerging weeds by reducing the amount of light available for weeds located below the crop canopy. Similarly, fast growing cultivars can have a competitive edge over the weeds.

Variety selection

Careful selection of crop varieties is essential to limit weeds and pathogen problems and to satisfy market needs. Any crop variety that is able to quickly shade the soil between the rows and is able to grow more rapidly than the weeds will have an advantage.

Tillage system

Tillage systems alter the soil seed bank dynamics and depth of burial of weed seeds. Studies have found that almost 75% of the seed bank was concentrated in the upper 5 cm of soil in no-till fields. In the moldboard plough system however, the seed bank is more uniformly distributed over depth. Other conservation tillage systems are intermediate to these two systems. Weed seedling emergence is often more uniform shallow buried weed seeds and may result in better weed control. Weed seeds closer to the soil are more likely to be eaten or damaged by insects, animals, other predators and disease causing organisms. 

Soil solarization

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

Pre-emergence organic herbicides:

Corn gluten meal

Corn gluten meal (CGM) is an pre emergence organic herbicide and is the by-product of the wet-milling process of corn. 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 (Digitaria spp.).

Mustard seed meal

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

Post-emergence organic herbicides:

Ammonium nonanoate

Racer® (40% ammonium Nonanoate) is a soap formulation of pelargonic acid with a changing registration history. It is a non-selective contact herbicide for controlling small (2.5 to 5 cm tall) annual broadleaf and grass weeds. Repeated applications may be needed to control most grasses or larger (5 cm) broadleaf weeds. It has been cleared for non-crop use in organic crop production and with 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 organically approved products that contain vinegar (e.g., Weed Pharm®, 20% Acetic Acid) that contain vinegar (e.g., 5%, 10%, and 20% acetic acid). Vinegar (acetic acid) is a non-selective contact herbicide. Typically, vinegar is less effective in controlling grasses than broadleaf weeds and more effective on annual species than perennials.

Clove oil

Clove oil is the active ingredient in a number of organically approved post-emergent 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 emerged annual and perennial grass and broadleaf weeds. As a contact, non-translocated herbicide its effectiveness increases with application rate and decreasing weed size. As with the other contact herbicides, when weeds are of similar size, the broadleaf weeds are easier to control than the grasses.  Clove oil weed control efficacy can be as good, or better than acetic acid herbicides, and can be applied at lower application volumes and remain effective. There is evidence that adding certain organically approved adjuvants (e.g., garlic and yucca extracts) will increase weed control with clove oil. Repeated applications may be necessary because larger annual grass weeds may grow back.

(The Authors are Scientists at KVK Kathua of SKUAST Jammu)