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 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
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
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.
This option is practical on a large scale only if the weed numbers are small.
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.
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
- 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)
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.
Alfalfa Seed Production
The recommendations for alfalfa seed production techniques are presented as a general guideline of the steps involved. Adaptation to local conditions is a prime consideration.
The most important factor in field selection is the weed spectrum present. Good recommendations for perennial weed control in established alfalfa seed fields are not presently available. Consequently, it is important to eliminate perennial weed problems before seeding down to alfalfa. Sweet clover is considered a weed in these circumstances. It is advisable to avoid fields with a history of sweet clover to control the problem of carry-over seed in the soil.
The importance of starting alfalfa seed production on a clean field, requires field management practices to start two years earlier. Certified production requires using land that has not grown alfalfa two years prior to seed down. Begin by assessing the weed spectrum and initiating an appropriate weed control program. Do not plant rapeseed the two years prior to alfalfa establishment to avoid the problem of volunteer rapeseed. Cereals are the preferred crops to grow at this time as they allow the broadest range of weed control. If perennial grasses, such as quack grass, are present consider a fall application of Roundup or an equivalent.
Although alfalfa is adapted to a wide range of soil conditions, best results are achieved when using deep, medium textured, well drained soils. Avoid poorly drained or saline soils. For certified production ensure neighbouring fields pose no hazard. Keep other alfalfa at least 165 feet (50 metres) from the seed field. A field sheltered from the wind benefits the bees, increasing the chances for pollination in the field. Keep in mind the difficulties of maintaining isolation posed by removing stray alfalfa plants from bush areas. Another requirement for good establishment is a well worked seedbed. Summerfallow the year prior to establishment is advisable.
The land should be well worked to produce a firm fine seedbed. Some emphasis must be placed on making the bed firm enough to allow for a relatively shallow seed placement. We recommend placing the alfalfa seed no deeper than 1/2 inch (2 cm ). A firm seedbed also allows moisture to move upward in the soil profile and provides more upward in the soil profile and provides more moisture for the germinating seed.As a rule of thumb, the heel of your shoe should not sink more than 1/4 inch (1 cm) into a good seedbed. To produce a bed this firm generally requires some packing. The only caution here is on the clay soils. If you pack them too much you may run into crusting problems after a rain. However, in most other soils it is virtually impossible to overpack. Some growers have had good success seeding into undisturbed stubble fields. This practice conserves the maximum amount of soil moisture and provides a firm seedbed but limits the weed control options that are available. Growers wanting to seed into stubble should select only the cleanest fields.
Although most growers still use a companion or nurse crop when establishing new fields, an increasing number now plant without one. On sandy soils a companion crop may be necessary to control wind and water erosion. The advantages to seeding without a nurse crop are:
- less competition for light, water and nutrients
- greater seedling vigor
- more flexibility in your weed control program
- increased winter hardiness
- increased seed production in the first harvest year
The negative aspect of not using a nurse crop is the increased opportunity for the weeds to flourish. The loss in income from not using a nurse crop is normally offset by the increased seed production in the first year.
Flax and canola are the preferred companion crops. Seed them in a separate operation, prior to and at right angles to the alfalfa seeding. This method minimizes the competition between the alfalfa and the nurse crop. Another method is to exclude the nurse crop from every fourth or fifth run of your seeder, drilling the alfalfa using only these runs. Depth control in this case usually ends up as a compromise with the alfalfa being seeded slightly too deep. When a companion crop is used, its seeding rate should be cut by at least one-third to cut down on the competitive effect on the alfalfa. Reducing the seeding rate by one-third or one-half rarely reduces the final yield of the nurse crop by more than 10 or 20 percent.
Seeding Rates, Dates, and Row Spacing
It has been shown at numerous locations throughout North America that the highest seed yields are obtained from fields containing from 14,000 to 30,000 plants per acre (35,000 to 75,000 plants per hectare) planted in 24 inch to 48 inch (60 to 120 cm) rows. The narrower rows and higher plant populations are more suited to the moister areas of the province. Seed yields from stands grown much denser than this have suffered. The following table, from work conducted at the University of Manitoba, illustrates the effect of stand density on seed yield.
Table 1: Plant Population and Seed Yields in 24 inch (60 cm) Rows at Glenlea, Manitoba – 1971