| | Herbicide weed control | Mechanical weed control
Return to the Guide to Commercial Potato Production on the Canadian Prairies.
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Weeds compete directly with the potato plant for light, water and nutrients. Dense weed infestations restrict growth resulting in smaller tubers, lower dry matter content and poor quality. In addition to reducing yield and quality, weeds:
- Interfere with the harvesting operation.
- Interfere with in-field seed inspections
- Interfere with roguing of seed fields
- Restrict air flow through the canopy, which increases the potential for disease development
- Provide alternate hosts for diseases and the insects capable of spreading disease.
Competition between potatoes and weeds should be minimized from planting to the time of canopy closure; 6 to 7 weeks later. Weeds that emerge after row closure usually will not compete with the potato crop so long as the canopy is uniform and dense. Crop uniformity and density is determined by the stature of the variety and the uniformity of plant spacing. Plant misses as a result of poor quality seed, planting into cold soil or a malfunctioning planter will reduce crop density and competitiveness with weeds. It is important that the planter be operating efficiently not only to produce a high yield, but also to produce a uniform competitive plant stand. See Planting for Better Stand, Yield and Quality.
A potato grower’s weed control strategy should be an integrated approach consisting of cultural, mechanical and chemical methods to control weeds.
Cultural methods include field selection, management of weeds in crop rotation and preventing the entry of new weeds into a field. Growers should be familiar with weed problems from previous crops because this information is essential for field selection. It is important to select a field free from weed problems, which cannot be controlled by tillage and herbicides used in the production of potatoes. Potato growers should attempt to control perennial weeds such as Canada thistle and quack grass, in other crops in the rotation because there are few herbicides registered for use on potatoes that are effective against these weeds. It is also important to prevent the entry of new weeds into the field via equipment, livestock and manure. Ideally, escape weeds should not be allowed to set seed.
A combination of the following chemical and mechanical weed control methods is used to control weeds in the potato crop:
- Herbicides (pre-plant incorporated, pre-emergence, pre-emergence burn-off and post-emergence). Consult with your Provincial Guide to Crop Protection for information about pesticides registered for application on potatoes.
- Mechanical (harrowing, cultivation and hilling)
Herbicide Weed Control
Pre-plant incorporated herbicides are effective in controlling a broad spectrum of annual weeds early in the season. Incorporation of pre-plant herbicides dries and compacts the soil and buries crop residue, leaving it susceptible to wind erosion. Soil-applied herbicide should only be used in areas that receive adequate rainfall or where irrigation is available to activate the herbicide. Cultivation and hilling will disturb the herbicide treated soil allowing weeds to germinate, so apply pre-emergence herbicides after hilling.
Potatoes emerge approximately 15 to 30 days after planting. In that time, a significant number of weeds can germinate. An application of a non-selective herbicide just prior to emergence of potatoes will control annual weeds and set back perennial weeds. Burn-off herbicides are not affected by tillage operations. If hilling is performed just prior to emergence of potatoes than a pre-emergent herbicide may not be required.
Post-emergent herbicides are most effective when applied to weeds just after emergence and through the cotyledon stage of growth. Herbigation can be used to apply herbicides in Western Canada.
Herbigation is the process of applying an herbicide to the soil or plant surface by injecting the chemical into the irrigation water. Some herbicides are more effective if applied with irrigation water. This is especially true for pre-emergence herbicides that require moisture to be activated. Not all herbicides that are registered for use on potatoes can be applied through herbigation. Consult the chemical manufacturer or your local agricultural representative before herbigating.
It is important that a pre-emergence herbicide be in the top 3 inches (7.5 cm) of soil, when applied by herbigation. The amount of irrigation water required to incorporate the herbicide will depend upon the water holding capacity of the soil, soil moisture status at the time of herbigation and the desired incorporation depth. To ensure uniform chemical distribution, use only centre pivot or linear irrigation systems and apply when wind velocities are less than 10 mph (16 kph). Herbigation should not be carried out without having the proper anti-pollution systems in place to prevent pesticides from entering the water source. Refer to the following publications for information about anti-pollution safety devices on an herbigation system: Oregon State University’s Chemigation in the Pacific Northwest PNW 360 or the University of Nebraska’s Anti-Pollution Protection when Applying Chemicals with Irrigation Systems EC 89-730-B.
Always consult the current edition of your Provincial Guide to Crop Protection for detailed information concerning herbicides registered for potatoes, varietal restrictions, application rates, pre-harvest intervals and weeds controlled. Growers should use this reference as a guide and always read the herbicide label for specific instructions.
Mechanical Weed Control
Tillage is an effective tool for controlling annual weeds, however, if performed under the wrong conditions it can have a deleterious effect on the efficiency of harvesting operations, yield and quality. Tillage performed at any stage of growth will dry and compact the soil. Tillage performed when the plants are greater than 6 inches (15 cm) in height will prune roots and damage foliage. These effects can result in more than a 5% loss in yield. Tillage performed under wet conditions, will produce soil clods that will reduce harvesting efficiency and increase the incidence of blackspot bruising. For these reasons, potato growers should rely on herbicides for weed control and perform as few tillage operations as possible. Of the available tillage operations, only hilling is required in the production of potatoes. It is becoming more common for potato growers to perform only a single cultivation-hilling operation just prior to or shortly after emergence.
Mechanical tillage operations harrowing, cultivation and hilling operations can be performed before and after emergence; separately or combined. See above table.
Harrowing is performed with spring tooth harrows and if carried out when the weeds are just emerging, it can be an effective method of controlling weeds in the planted row. Harrowing can be performed separately or in combination with cultivation. Harrows should not be used after the plants are 2 inches (5.0 cm) in height, due to excessive damage to the roots, stolons and foliage.
Inter-row cultivation is performed with an S-tine shank cultivator or a rotary hoe and is most effective in controlling annual weeds between the rows. The cultivator can be adjusted to throw soil around the base of the plants, which will bury weed seedlings or it can be equipped with a spring tooth harrow to control weeds in the planted row. The S-tine cultivator is also used to loosen soil, which aids in the hilling operation. Inter-row cultivation should be carried out before the plants are 8 inches (20 cm) in height to avoid excessive root pruning.
Hilling is the only tillage operation necessary in the production of potatoes. The objective of hilling is to cover the daughter tubers with sufficient soil to prevent greening, minimize infection with late blight, minimize frost damage, improve drainage in the area of tuber formation and to facilitate harvest. Rotary hoes, discs, mouldboards, or power hillers equipped with a metal mould are commonly used to shape loose soil into a hill. The hilling implement should be adjusted to produce a wide, flattened hill ideal for protecting the tubers from sunlight, late blight spores and frost. To effectively control weeds, hilling must take place before the weeds get past the two true leaf stage. Hilling can be performed pre or post emergence. Regardless of the hilling implement used, emerged plants should not be covered with soil. This sets the plants back and delays growth. It is recommended that power hilling be carried out prior to emergence to avoid covering the plants. Post-emergent hilling, other than power hilling, should be completed before the plants are 8 inches (20 cm) in height to avoid damage to the roots and foliage.
Written by B. Geisel |
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