Weed Management without Herbicides

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During the conversion to organic or specialty crop farming, weed management becomes the biggest production issue. Non-chemical weed control can have many different approaches and success can only be achieved when several approaches are integrated into the weed management program (IWM).

Preventative Measures

Minimize the Weed Population from the Start by:

Variety choice - choose competitive varieties with rapid initial growth
Seedlings rather than seeds - use transplants instead of direct seeding
Land choice - avoid land overrun with weeds or difficult to control perennial weeds such as thistles and quackgrass.
Seedbed - prepare the seedbed far in advance of planting, to allow a high percentage of weeds to emerge prior to planting, making control easier.
Promote crop growth - the faster the crop grows, the more competitive it will be. Band or place fertilizers ensuring the crop has the competitive advantage over weeds.
Fertilizer - use manure that is free from weeds - make sure the manure has been fully composted, or do not use manure.
Soil Coverage - if your crop is not fast growing with a lot of soil coverage, apply a mulch, sow a cover crop or a complementary crop between the rows.
Crop Rotation - create a varied rotation with highly competitive crops, followed by non-competitive crops, include temporary grasslands and green manure (fertilizer) crops in your rotation.

Direct Weed Control

The timing of all measures will directly determine the amount of manual weeding needed and the subsequent profitability of the crop.

Field scouting

Field scouting is a key component of an IWM system. It involves the systematic collection of weed and crop data from the field (weed distribution, growth stage, population, crop stage, etc.). The information is used, in the short term, to make immediate weed management decisions to reduce or avoid economic crop loss. In the long term, field scouting is important in evaluating the success or failure of weed management programs and for making sound decisions in the future.

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

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. Fast growing crops or crops exhibiting allelopathic properties can also suppress weeds. 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, always consider how the cover crop will affect the succeeding crop.

Intercropping

Intercropping involves growing a smother crop between rows of the main crop. Intercrops suppress weeds. However, approach the use of intercropping as a strategy for weed control carefully. Intercrops can greatly reduce the yields of the main crop if competition for water or nutrients occurs.

Nitrogen fertility

Nitrogen fertilizer can affect the competition between crops and weeds, and in the subsequent crops. For example, nitrate promotes seed germination and seed production in some weed species. Nitrogen fertilization may increase weed growth instead of increasing crop yield. Selectively placing nitrogen in a band can favour the crop over the weed. Using legume residues as opposed to chemical nitrogen fertilizer to supplement nitrogen needs of the crop can enhance weed suppression. Legume residues release nitrogen slowly with less stimulation of unwanted weed growth.

Planting patterns

Crop population, spatial arrangement and cultivar (variety) choices can affect weed growth. For example, narrow row widths and a higher seeding density reduces 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.

Tillage system

Tillage systems alter the soil seedbank dynamics and depth of burial of weed seeds. Studies show that almost 75% of the weed seedbank is concentrated in the upper 5 cm of soil in no-till fields. In the moldboard plow system however, the seedbank is more uniformly distributed over depth. Other conservation tillage systems are in between these two systems.

Weed seedling emergence is often more uniform from shallowly buried weed seeds and may result in better weed control. Weed seeds closer to the soil surface are more likely to be eaten or damaged by insects, animals, other predators and disease-causing organisms.

Pre-emergent or "Blind" Harrowing

Blind harrowing consists of waiting until after the crop has been planted, the seed is sprouted and the shoot is not yet emerged. At this crucial time, the field is harrowed to kill the small weed seedlings that have already sprouted. The crop will emerge shortly afterwards, having gained a head start on the next flush of weeds. When properly carried out, this method can be most effective in controlling annual weeds in large seeded crops. In cereals, this blind harrowing will often provide adequate control of annual broadleaf weeds. In row crops, it can be used to keep weeds down until the crop plants are large enough to withstand other mechanical control methods. Any type of light harrow can be used. Blind harrowing must not be done on crops previously underseeded with grass or legume seeds.

Weeder Harrow

A weeder harrow has spring tines that are gentle enough not to harm the cultivated crop, while uprooting or covering the smaller annual weeds. Therefore, the relative size of the cultivated plants and the weeds is important. Weeder harrowing will be most successful when used before the crop emerges and can also be used post emergent in many crops. Some crop population loss will result. Speed and pressure settings are important to minimize plant loss. They can be used most effectively on field vegetables, corn, soybeans and cereals.

Rotary Hoe

The rotary hoe has "fingers" that lift and mix the soil, uprooting small weeds. It is important to work at 10-20 km/hr for satisfactory results. Best results are obtained during late morning or afternoon hours when the hot sun can dry out the uprooted weeds. Also, crop plants tend to be more pliable at this time and injury is reduced. Rotary hoes tend to cause less crop damage than harrows. They are also effective in breaking up a soil crust and mixing surface-applied herbicides into the soil, which will improve the weed control. On light soils or under loose soil conditions, care must be taken to keep the rotary hoe working at a shallow depth.

Inter-Row Cultivation

Shallow inter-row cultivation or scuffling of row crops uproots small weeds and cuts off larger weeds. Various types of equipment can be used but, when shovels are used, allow for up to 50% overlap for thorough weed control. Shields should be used to protect small crops.

To provide adequate weed control in long-season crops, scuffling will probably have to be done more than once. The first cultivation is the most crucial since weeds that escape this pass will usually grow to maturity. Here again the relative size of the crop to the weeds is important and probably the limiting factor in achieving 100% success. Use blind harrowing, weeder harrows and/or rotary hoes first to complement the inter-row cultivation to achieve success in non-chemical (organic) weed management.

Mowing

Mowing or cutting weeds may control weeds in orchards, roadsides, lawns, etc. Harvesting hay or cereal crops also helps to control weeds. Mowing cereal stubble in August can reduce weed growth and weed seed production, especially if the cereal had been underseeded to another crop such as red clover to compete with the new weed growth. The best time to mow perennial weeds is usually at the bud stage when root reserves are low and before seed set.

For further information please visit: http://www.omafra.gov.on.ca/english/crops/organic/organic.html