Mechanical Weed Control

In today's age of Roundup Ready and other multiple herbicide strategies, most farmers have long ago moved away from mechanical weed control after planting. However, there are a growing number of organic farmers that depend on mechanical weed control as a key strategy in their weed management program. It can also be useful for growers who for a variety of reasons (weed resistance, buffer zones, problem weeds, specialty crops, etc) want to reduce their dependency on herbicides and to integrate other approaches into their crop management plans.

Organic farmers use three main types of equipment for mechanical weed control - weeder harrow, rotary hoe, and inter-row cultivation. This equipment complements other weed control strategies including:

• using clean seed
• tillage
• stale seedbed
• appropriate planting dates, populations, and row widths
• diverse crop rotations
• cover crops
• controlling weeds in areas adjacent to the field
• hand weeding when needed for problem areas and problem weeds
• weed scouting and identification of new weeds.

Weeder Harrows

Weeder harrows are a relatively new implement to Ontario. Two generations ago, it was common to lightly harrow the crop between planting and emergence using a diamond or spike-tooth harrow. This trip over the field would disrupt weed seedlings, mostly before they emerged, to give the crop a head start on the weeds. Newer models come from European technology and use flexible tines that are +/- ¼ inch (6 mm) in diameter, 16-20 inches (40-50 cm) long on narrow spacings (less than 1 inch, 2.5 cm). Maintenance on these implements is relatively low.

Weeder harrows are used before emergence and sometimes after the crop is established to kill young weeds that are unemerged or less than one inch tall. Research has shown up to 50% weed reduction with each pass. Larger weeds and plants emerging from deeper than one inch (including crops) are able to tolerate this type of weed control. Weed kill is highest when the weather is sunny and warm. Penetration is less on clay soils than on sandy soils. The weeder harrow is used on both row-crops and on wheat and barley crops that have not been underseeded with forages.

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Rotary Hoes

Rotary hoes are used mostly in row crops such as corn or beans. They have long been used in Ontario to break up crusted soils to aid in emergence. Rotary hoes used either before or after emergence can effectively kill young weeds. In some cases "spoons" are welded onto the hoes to make them more aggressive for weed control. Narrow or worn spoons (<11 mm) are relatively poor for weed control. One advantage of the rotary hoe is that they can cover ground quite quickly. They may not work as well in soils with numerous stones.

Inter-row Cultivation

Inter-row cultivation is another tool that has been used for years in row crops. There are many options and attachments for the cultivator to improve the tool and to kill weeds. One of the drawbacks has been the ability of the operator to keep the cultivator on the row to reduce the effects of "iron toxicity" (cultivating out the row by accident). There are a number of new advances in row guidance systems that allow operators to drive faster and more accurately.

Crop Injury

These implements should not be used when the crop is at the emergence stage to reduce the risk of crop injury. Repeated use of these implements may cause some reduction in plant populations. Harrowing oats after emergence will cause more injury than to barley or wheat.

Costs, Adjustments, & Timing

The cost of using these implements is relatively low compared to other tillage/weed control tools. A soybean crop receiving 2 passes with the harrow or rotary hoe and twice cultivation may cost $25-35 per acre (total) for weed control.

With all three implements there are many adjustments to enhance their operation. Successful use of these implements relies upon using the correct adjustments. Weeds should be controlled early. The most effective weed control is when weeds are very small. Cultivation after weeds are 10 cm (4 inches) tall is usually too late to get good control and yield is already being affected. Success is also dependent upon proper timing relative to weed stages and weather. Farmers are using these implements in various widths (15-30', or wider) to improve efficiency.

Mechanical weed control will take more time in June than conventional herbicides, but it is a viable option for growers.

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