Agriculture

Weed Management In Organic Crop Systems

Weeds can be defined simply as plants growing where they are not wanted. Managing weeds on an organic farm presents a major challenge.

The use of synthetic herbicides, defoliants and desiccants is prohibited. As a result, weed control in organic systems focuses on management techniques designed to prevent weeds, as well as the production of a crop vigorous enough to out-compete weeds.

The presence of weeds in a crop does not automatically result in damage or yield loss. For example, grassy weeds such as wild oats or green foxtail in a grain crop being cut as green feed will not reduce its feed value.

Similarly, a few weeds in a pea field may reduce wind damage and help raise the pods higher off the ground, allowing for easier harvesting, while late-emerging weeds may not result in a yield loss in that year. In addition, late weed flushes could provide suitable post-harvest grazing if the field is fenced.

Each situation has to be assessed within the context of the entire farm operation. Sustainable weed management must be properly planned using all available information.

(Refer to the Manitoba Agriculture, Food and Rural Initiatives (MAFRI) publication, Guide to Field Crop Protection, for tables on yield losses attributed to varying levels of certain weed infestations in conventional cropping systems).

Managing Weeds Effectively

A combination of methods is usually the most effective means of dealing with weeds and should include:

  • weed avoidance techniques
  • managing the growth environment - giving the crop a head start
  • direct treatment

To effectively manage weeds, the producer must also understand the biology and growth habit of both weeds and crops. Weed type and concentration are often responses to the condition of the soil and previous crop history.

Weed Avoidance Techniques

Healthy Soil

Maintaining favourable soil conditions is the “first line of defense” against weeds. A biologically-active soil with good drainage supports vigorous crop growth, allowing a higher level of crop competition with weeds.

Improving soil conditions may require several growing seasons, but it is important in reducing weed problems. For example, certain weeds tolerate soil compaction and drainage problems better than many crops. Therefore, a compacted soil with poor drainage may result in competitive weeds and poor crops. Similarly, high levels of available nitrogen favour weed growth, whereas crops thrive on the slow release of nutrients from mature compost or soil organic matter.

An imbalance of nutrients usually favours weed growth, especially where crop competition is reduced. It often occurs when large manure applications have been made on a regular basis in the same field. Soil testing is an important tool in monitoring nutrient levels to allow corrections to be made to any imbalances.

Crop Rotations

The rotation of crops is a central component of all sustainable farming systems. It offers the most effective means of minimizing weed, insect and disease problems, while maintaining and enhancing soil structure and fertility.

Diversity is the key to a successful crop rotation, which should include practices such as:

  • Rotating early-seeded, late-seeded and fall-seeded crops. This is effective in managing such weeds as wild oats, green foxtail, winter annuals (such as stinkweed and flixweed) and some perennial weeds.
  • Rotating grassy, broadleaf and legume crops. Each of these plant groups has specific rooting habits, competitive abilities, nutrient and moisture requirements. The soil incorporation of green manure crops (such as sweet clover or peas) is a useful practice that suppresses weeds, disrupts their life cycles, and provides the additional benefits of fixing nitrogen and improving soil properties.
  • Managing the frequency that a crop is grown within a rotation. This affects the population of certain weed species.
  • Rotating highly-competitive crops (such as alfalfa or barley) with less-competitive crops (such as flax or lentils).
  • Using perennial phases in crop rotations, combined with mowing or intensive grazing, to control perennials and to interrupt the life cycles of annual weeds in cultivated crops.
  • Using cover crops such as fall rye or sweet clover to suppress weeds and prevent erosion on fallow land.

General Sanitation

Weed seeds can be moved around and carried on to the farm from neighbouring areas by water, wind, animals or equipment. Removing weeds along fence lines, shelterbelts, road allowances and other non-crop areas also prevents them from spreading to cultivated areas.

While only a few weeds on field margins can pose a real threat of spreading into adjacent fields, the complete elimination of field margin weeds may be damaging to beneficial insects that require weeds as host species. Other animals that use the area may also be affected.

The best timing of weed control operations such as mowing or tillage may be a compromise between reducing the seed-set of weeds and damaging some wildlife. Delaying operations until late July allows ground-nesting birds to raise their broods. If non-crop areas are especially weedy, they may be seeded to competitive native grasses. Movement of weeds into fields can also be reduced by sowing the field margins to native species. Chaff collector

Figure 1
Chaff collector

Forage Sanitation

Ideally, hay should be produced on-farm, since buying hay can introduce new weed problems. The hay may contain weed seeds from new types of weeds, and the seeds may survive the digestive tract of the animals. Although proper manure composting kills most weed seeds, some may survive and be applied via compost to crop and forage land while still viable.

Clean Machinery

Remove all weedy material (seeds and vegetative parts) before moving any equipment from field to field. This is especially important when doing custom work.
A tarp over grain, soil, or feed being transported will prevent contamination along roads or in yards.

Seedbed Preparation

Any spring tillage should be limited to a depth of four inches (10 cm) to stimulate early weed seed germination. When weeds have developed secondary roots, cultivate no deeper than 2-4 inches (5-10 cm). Planting the crop immediately after into a firm, well-prepared seedbed will encourage rapid crop germination. Tillage should be limited, however, on land subject to erosion or if soil moisture is low.

Clean Seed

Sowing only clean seed virtually eliminates the introduction of weeds that are foreign to the farm. Be sure to obtain purity and germination information from your seed supplier to determine the actual weed content of the seed. Purity results will indicate weed species and numbers found in a representative sample of the seed lot. Pedigreed seed is required to conform to Seeds Act and Regulation standards and must have minimal weed seed content.

Chaff Collection

A chaff saver behind the combine can be used to collect weed seeds (see Figure 1). Chaff collectors are effective in collecting crop seeds that blow over (creating a potential volunteer problem) and in removing large numbers of seeds of later-maturing weeds.

Chaff collection in separate field plots at Scott, SK showed the potential benefits of the practice: up to 16,000 shepherd's purse seeds, 5,000 canola seeds, and 2,000 lamb’s quarters seeds per square metre were collected in the chaff. Chaff collection prevents the movement of weed seeds within a field and helps to reduce the expansion of weed patches.

Collected chaff also provides useful livestock feed, although the weed seeds should be cooked, ground, or pelleted before being used as feed.

Managing the Growth Environment - Giving the Crop a Head Start

Any crop management techniques that contribute to a vigorous, competitive crop are considered tools of weed management. Good crop husbandry practices contribute considerably to weed control at very little extra cost. Producers must also be mindful of the fact that many practices that work well in conventional systems are not conducive to organic systems. For instance, certain species, varieties and equipment may work well in one system but not in the other.

Crop Selection

Some crops are better able to tolerate competition and suppress weeds than others. Factors that increase competitive ability include: rapid germination, early emergence, seedling vigor, rapid leaf expansion, large stomate number, rapid canopy development, increased plant height, early root growth and an extensive root system.

The most competitive crops are (in order): fall rye, winter wheat, barley, oats, spring wheat, canola, flax, lentils, and other pulse crops. Because there are many varieties of each crop, the use of cultivars that achieve early canopy closure, develop rapidly, are taller and have more tillers should be considered.

Perennial and biennial crops are very competitive with weeds as well. Crested wheatgrass, bromegrass, alfalfa and sweet clover, for example, offer competition that lasts for more than one season and eliminates the need for tillage that can stimulate the germination of annual weed seeds. Mowing of forages also reduces weed seed production.

Allelopathy

Certain plants secrete compounds (phytotoxins) that inhibit the growth of other plants. This phenomenon is called allelopathy, and the effect may be produced directly by living plants or indirectly through the process of plant decomposition.

Both crops and weeds can have allelopathic effects. Allelopathic crops include rye, oats, sunflowers, barley, wheat, mustard, buckwheat, clovers (red, white, sweet), tall fescue, creeping red fescue, hairy vetch and perennial ryegrass. In rotation, these crops help to suppress weeds in subsequent crops, but because they can also suppress subsequent crop growth, this practice must be carefully monitored. These crops can also be used as cover crops and green manures. The allelopathic effect is complex and requires the correct combination of crop and weather conditions.

Intercropping

The practice of intercropping, where two crops are grown at the same time, has been found to be beneficial in suppressing weeds, increasing crop competition and providing allelopathic effects. Two crops use light, water and nutrients more effectively than one crop, resulting in fewer resources available for the weeds.

Intercropping is most successful when the two crops have complementary growth patterns and resource needs. For example, an intercrop of peas and oats controls weeds in several ways: the oats provide early competition with weeds while the peas are becoming established; the peas then climb on the oats, blocking out light to the soil; the rooting patterns of the two crops also differ; the oats compete more with grassy weeds for nutrients and the peas compete with the broadleaf weeds; the oats also take up excess nitrogen that would otherwise stimulate weed growth.

Other successful intercrops include: oats and pulses (such as lentils or beans), flax and wheat, flax and medic, wheat and lentils, flax and lentils, barley and peas.

Seeding rates need to be adjusted so that the two crops compete with the weeds, but not with each other. Seeding each crop at two-thirds of its normal rate has produced good results.

Seeding Date

Practices that encourage an early, vigorous start to the crop or that remove early-germinating weed seedlings give the crop a head start and allow it to compete more successfully with weeds. The critical period in which crops are most sensitive to competition varies. For example, in the case of wheat, this time is about 2-4 weeks after emergence. Early seeding is successful with cereal crops, peas and lentils, but not for other crops such as beans or buckwheat.

Seeding Rate

Increasing seeding rates 20-30% above normal can increase the competitive ability of a crop. Higher rates will also result in earlier maturity (2-3 days), shorter plant height, reduced tillering and possibly higher yields (if moisture conditions are adequate), but at the same time may put the crop at a higher risk for lodging.

Increased seeding rates should also be used where either post-seeding or post-emergence tillage is planned. This will help compensate for any damage caused by the tillage.

Row Spacing

Narrow row spacing provides the best competition with weeds in most cereal, pulse and oilseed crops.

Less competitive crops such as flax and lentils benefit the most from narrow rows, as there is less open space for weeds to grow.

Seeding Depth

To achieve the optimum competitive ability for the crop, seeding depth should be no deeper than required for quick and even emergence. Variables include seed size, soil type and moisture conditions. Avoid deep tillage prior to seeding, but consider packing if the soil is loose; this firms the soil and brings moisture closer to the surface.

Cultivating field peas

Figure 2
Cultivating field peas

Direct Treatment

At times, direct weed control is necessary, even with judicious agronomic practices and a thoughtful, tolerant attitude. In developing effective weed management strategies, growers need to be aware of the pros and cons of all the tools available to them.

Mechanical Weed Control

Tillage can be very effective in reducing weed populations, and because it favours some species over others, it can be used to alter weed communities. Tillage does have potential drawbacks, however, as it:

  • Tends to dry out the soil. n May cause soil degradation and loss through erosion and leaching.
  • Increases nitrogen loss through volatilization.
  • Reduces the population of some beneficial insects.
  • Stimulates weed seed germination.
  • Brings up dormant weed seeds and may spread about the rhizomes of perennial weeds.

Hand Weeding

This option is practical on a large scale only if the weed numbers are small.

Out-of-Crop Tillage

This practice is used typically after the harvest of one crop and before the next one is seeded. It may also include summerfallow tillage.

  • Post-harvest tillage is used to destroy winter annual, biennial and some perennial weeds. Shallow tillage avoids burying weed seeds.
  • Pre-seeding tillage, if used along with delayed seeding, may reduce crop yields by delaying harvest or reducing moisture in tilled soils.
  • Summerfallow tillage should be as shallow as possible to avoid bringing new weed seeds to the surface. It may be used to control perennials such as Canada thistle, perennial sow thistle and quackgrass. Tillage is most effective when the soil surface is dry and the air temperature is high. Fallow is typically considered a “last resort” method. Where soil erosion is a high risk, tillage should be integrated with other methods to protect the soil.

In-Crop Tillage

Since a certain degree of crop damage will occur with in-crop tillage, seeding rates should be increased by 20-30% so as to allow for potential plant losses (see Figure 2).

  • Post-seeding tillage has shown to be effective before the crop emerges using harrows or rod-weeders on a hot day when the soil surface is dry.
  • Post-emergence tillage is effective on shallow-rooted weeds using a harrow. This practice is generally safe in the 2-4 leaf stage of cereal crops (but not recommended in oats), and in peas and lentils shorter than four inches (10 cm).

Other Control Methods

  • mulching
  • mowing
  • burning
  • flaming
  • allelopathy
  • biological controls (includes the use of livestock, the introduction of insect predators, the introduction of fungal and bacterial compounds, and increasing the beneficial biota by maintaining their habitat)

Conclusion

Successful weed management in organic crop production systems requires an understanding of plant characteristics in relation to specific field conditions. This serves to provide producers with a reference point from which they can plan crop rotations that are best suited for their individual situations.