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Mechanical Weed Control in Organic Systems

Av Singh, Ph.D.

Survey after survey has organic farmers emphasizing a greater need for information on weed control. In particular, transitional farmers and farmers recently converted to an organic system are often overwhelmed by weed pressure. The organic farmer has many options to reduce weed problems such as crop rotation, intercropping, allelopathy, mulches, clean seed, cultivar selection, and can also play around with seeding rate, seeding date, and row spacing to minimize weed invasion. Despite all these tools, many will still have to rely on some form of mechanical weed control. In crops such as soybean, corn, and small grains (wheat, barley, oats) many organic farmers are turning towards two types of mechanical weeding equipment, namely the rotary hoe and the finger-weeder (flex-tine harrow).

Rotary Hoe
The rotary hoe has several adjacent spindled wheels with small spoon like structures at the end that serve to break and throw the soil from a 1-2 inch depth. The young weeds (primarily at the "white thread" or cotlyedonary stage) are effectively destroyed either by burial or by being pulled out and thereby facilitating the drying out of weed plants. The rotary hoe is most effective if used both at a crop pre-emergence stage (i.e., blind tillage) and again at a later stage. The timing of the second pass of the rotary hoe differs depending on crop. For example, the rotary hoe works well in corn from the emergence of the first leaf ("spike") to the 4-leaf stage, while in cereals the rotary hoe should not be used after emergence until at least the two-leaf stage.

Finger weeder in corn at the four-leaf stage
Rotary hoe at crop pre-emergence
Photo courtesy Yvon Douville

A common mistake in using the rotary hoe is going too slowly. The rotary hoe is most effective at speeds between 10-20km/h. One of the main advantages of using the rotary hoe is that a producer can cultivate large areas of land in a relatively short time. If weed pressure is high, producers will often decide on a second pass. It is advisable that the second pass be done a few days later and in the opposite direction of travel to the first pass. The best results of a rotary hoe are obtained in slightly dry and crusted soils, and are more variable on loosely bound or sandier soils.

Finger Weeder
The finger weeder has several long-springed tines in a comb-like formation that vibrate in the soil to disturb the soil surface. The resultant effect is that weeds are uprooted, buried, or mutilated by the finger weeder. The finger weeder is slightly better on more mature weeds (weeds having appearance of first leaves) than the rotary hoe. Crop stage is more critical for the finger weeder than the rotary hoe because of its stronger action. Similar to the rotary hoe, the finger weeder is an effective means of blind tillage, but ideally only one pass in the crop should be performed. In corn and cereals, the 3-5 leaf stage would produce best results.

Rotary hoe at pre-emergence
Finger weeder in corn at the four-leaf stage
Photo courtesy Yvon Douville

The passage speed at crop post emergence varies between 6-12 km/h. It is recommended to adjust the equipment before every pass according to the crop stage and the weed pressure. Tine tension can be altered to reduce crop impact, and similarly speed can be reduced to minimize damage to the crop.
Both the rotary hoe and the finger weeder perform best on fields with little or no surface residue. These weeding implements have no effect on well-established perennial weeds or deep-rooted annual weeds like wild oats. The tines of the finger-weeder will plug easily by corn stalks, quack grass rhizomes, or clumps of turf, and will drag this residue along damaging the crop.

Underseeding, a common practice among organic farmers can somewhat be limited by mechanical weeding. Organic farmers using rotary hoes or finger weeders alter their management practices to optimize weeding strategy and crop establishment. Often mechanical weeding is performed prior to forage seed germination, or alternatively a forage seed box is mounted on the weeding implement or forage seed is broadcast immediately prior the weeding operation.

General recommendations or precautions when using either a rotary hoe or finger weeder include: 1) If pre-emergence cultivation is planned then seeding depth should be at least 4 cm (1 ½ inch) deep; 2) Seeding rate should be increased by 5-10% to account for crop damage; and 3) Adjust equipment over a short distance before starting the rest of the field. There should be less than 5% of the crop plants being damaged (i.e., completely buried or uprooted). With these thoughts in mind, one last piece of advice that is freely shared from practitioners of mechanical weeding is, "DON'T LOOK BACK!" After adjusting the weeding equipment, it is often recommended that you not worry that your crops look a little "rough". Crops are resilient and will come back and harvest yields will not be affected.

It does seem rather odd to be discussing mechanical weed control at the end of our growing season, but I wanted to stir some interest in the use of these weeding implements. The rotary hoe (6m/20ft.) usually sells for under $7000 and the finger weeder (6m/20ft.- flex-tine weeder) is often sold for under $6000. The equipment has benefits for conventional crop producers, as well as vegetable growers, so the opportunity to share the equipment with neighbours is a possibility (check with your certifying body to find out the protocol with sharing equipment with transitional or conventional producers).

Lastly, many farmer cooperators with the OACC rely on mechanical weeding. One such transitional farmer is Andrew Kernohan at Parrsboro, NS. A testimonial of his experience with mechanical weeding, in particular the use of a finger weeder in cereals has been posted elsewhere on our website. Click here to read it.


For more information please call the Organic Agriculture Centre of Canada at 902-893-7256 or email oacc@dal.ca


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