Organic Weed Management – General Tips

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. At the same time, it is important to understand that under an organic system of seed control, weeds will never be eliminated but only managed. 

Components of Organic weed management: 

I  Cultural Weed Management:  

• Buy quality crop seed with low/no weed seeds present.

• Do not allow weeds to grow until seed formation.

• Thoroughly compost (>130°F for ≥15 days) all manure and plant residues to ensure destruction of weed seed.

• Stale seedbed technique- Prepare soil for planting and bring weed seeds to the surface and allow weeds to germinate. Once germinated, kill weeds with light tillage/minimal soil disruption. This may be repeated based on need followed by which planting of the main crop can be done.

• Improve soil tilth, aeration and fertility to optimize crop growth.

• Increase crop density through narrow row spacing and increased seeding rate.

• Use transplants rather than seeds, when possible.

• Plant at optimal soil temperatures to prevent slow germination of crop.

• Choose competitive crop cultivars.

• Manage fertility according to crop needs; avoid excess application.

• Crop rotation: It involves alternating different crops in a systematic sequence on the same land. It is an important strategy for developing a one 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: These crops smother weeds by out-competing for light, water and nutrients. Some also release allelopathic chemicals that suppress weed germination. They may reduce weed emergence by 75-90%  E.g: Sudan grass, buck wheat, annual rye grass, sesbania etc. 

• 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. There are different types of mulches.

1. Living mulch: Living mulch is usually a plant species that grows densely and low to the ground such as clover. Living mulches can be planted before or after a crop is established. For example, a living mulch of Portulaca oleracea broadcast before transplanting broccoli suppressed weeds without affecting crop yield.  

2. Organic mulches: Some materials like straw, bark, and composted material can provide effective weed control. Producing the material on the farm is recommended since the cost of purchased mulches can be prohibitive, depending on the amount needed to suppress weed emergence. Organic mulches have the advantage of being biodegradable. For example, cut rye grass, spread between planted rows of tomatoes was more economic than cultivation. Fresh bark of conifers and oak as well as rapeseed straw gave good control of weeds when they were laid as mulches under the trees in apple orchards.  

• Planting patterns: Crop population, spatial arrangement, and the choice of cultivar (variety) can affect weed growth. For 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.  

• Sanitation: It is possible to prevent many new weeds from being introduced onto the farm and to prevent existing weeds from producing large quantities of seed. The use of clean seed and thoroughly composting manure before application can greatly reduce the introduction of weed seeds and difficult weed species. Other sanitation factors to consider would include thorough cleaning of any machinery which might have been used in weedy fields and the establishment of hedge rows to limit windblown seeds. 

• Nitrogen fertility: Nitrogen fertilizer can affect the competition between crops and weeds and in the subsequent crops. Nitrogen fertilization may result in increased weed growth instead of increased crop yield. Selective placement of nitrogen in a band can favour the crop over the weed. Ex.: Nitrate is known to promote seed germination and seed production in some weed species. Avoid pre-plant broadcasting of soluble nutrients that may be more readily utilized by fast-growing weeds than slow-growing crops and may even stimulate weed germination. 

• Water management: Effective water management is a key to control weeds. There are a number of ways that careful irrigation management can help to reduce weed pressure on crops: 

1. Pre-germination of weeds: In pre-germination irrigation or rainfall germinates weed seeds just before the crop is planted. The newly germinated weeds can be killed by light cultivation or flaming. 

2. Planting to moisture: Another technique similar to pre-germination is planting to moisture. After weeds are killed by cultivation, the top 2 to 3 inches of soil are allowed to dry and form dust mulch. At planting, the dust mulch is pushed away and large sized seeds can be planted into the zone of soil moisture. These seeds can germinate, grow, and provide partial shading of the soil surface without supplemental irrigations that would otherwise provide for an early flush of weeds.

3. Buried drip irrigation: Drip tape buried below the surface of the planting bed can provide moisture to the crop and minimize the amount of moisture that is available to weeds closer to the surface. If properly managed, this technique can provide significant weed control during dry period.

II Mechanical Weed Management: 

Mechanical removal of weeds is both time consuming and labor-intensive but is the most effective method for managing weeds. The choice of implementation, timing, and frequency will depend on the type and number of weeds. Observing the composition of the seed bank can help a farmer make practical weed management decisions. Burial to 1 cm depth and cutting at the soil surface are the most effective ways to control weed seedlings mechanically. Mechanical weeders include cultivating tools such as hoes, harrows, tynes and brush weeders, cutting tools like mowers and stimmers, and dual-purpose implements like thistle-bars. The optimum timing for mechanical weed control is influenced by the competitive ability of the crop and the growth stage of the weeds.  

III Thermal Weed Management: 

1. Flamers: Flamers are useful for weed control. Thermal weed control involves the use of flaming equipment to crate direct contact between the flame and the plant. Flaming can be used either before crop emergence to give crops a competitive advantage or after the crop has emerged. However, flaming at this point in the crop production cycle may damage the crop

2. Soil solarisation: During summer, organic farmers sterilize their soil through solarization.

3. Infrared weeders: Infrared weeders are a further development of flame weeding in which the burners heat ceramic or metal surfaces to generate the infrared radiation directed at the target weeds. In general, flame weeders are considered  to be more effective because they provide higher temperatures, but burner height and plant stage are important too. Infrared weeders cover a more closely defined area than those of the standards flame weeder

4. Freezing: Would be advantageous only where there is an obvious fire risk from flaming. Liquid nitrogen and solid carbon dioxide (dry ice) can be used for freezing weeds.

IV Biological Weed Management: 

1. Allelopathy: Allelopathy is the direct or indirect chemical effect of one plant on the germination, growth or development of neighboring plants. 

2. Beneficial organisms:

• Use of bio control agents for weed control:

• Use of fish for weed control: Eg:Grass carp or white amur, silver carp and common carp fishes for Lemma, Hydrilla, Potamogeton weeds. 

• Use of competitive plants for weed control: Cassia sericea forParthenium hysterophorus and Brachiaria mutica  for Typha sp. 

• Commercial mycoherbicides:

Conclusion: 

Herbicides have been widely used for weed management around the globe because of its great advantages. However, it is a well known fact that these toxic herbicides are harmful to our environment and animal kingdom including human beings. It has also been found that in some cases herbicides usage can cause some weed species to dominate fields as they 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. In this perspective, non-chemical methods of weed management are of great importance. An effective weed management implies the use of multiple approaches to manage weeds that will yield greater impact than relying on a few practices. There is a need to develop a weed management strategy that is designed for the needs of your farm which not only has a positive effect on crop growth but also works in harmony with the nature. 

Authors

Dr. Sunil Kumar K and Dr.Madhurima Vinod

Research Associate (Agronomy), 

Research Associate (Agril. Entomology),

ZBNF Project, ZAHRS Brahmavar, 

Udupi Karnataka - 576213