Selectivity ‑ An important concept in mechanical weed control

E. Johnson - Scott Research Farm


Some form of mechanical weed control is generally used by most organic producers. Post-emergence weed control with an implement such as a harrow or a rotary hoe may control weeds, but does not always improve crop yields. If weed interference is reduced, then why aren’t crop yields always improved?


Selectivity is defined as the ratio between weed control and crop injury. In general, mechanical weed control is most selective when the crops differ from weeds in growth habit, emergence time and maturity time.

Weeds with a short specific germination period are more easily controlled mechanically than weeds that germinate over a long time period. Large seeded crops can be seeded deep which allows highly selective mechanical weed control between seeding time and crop emergence. Most weed seeds emerge from the top two cm of soil with the maximum emergence depth at six cm. This technique can provide effective weed control and give the crop a competitive advantage. Pavlychenko observed that pre-emergence tillage provided the emerged crop with about a ten-day weed free period.

Wild oat can emerge from greater depths than most other weed species but pre-emergence tillage can still be effective. Wild oat has a mesocotyl that extends the growing point meristem away from the seed to near the soil surface. In cereal crops, the mesocotyl remains close to the seed, protecting the growing point from mechanical injury.

Fifty-two percent of organic growers surveyed in Saskatchewan use post-emergence harrowing as a weed control practice. Postemergence tillage generally has low selectivity as the negative crop covering effect can offset the positive weed killing effect. Studies in Europe show that the primary action with post-emergence harrowing is burying plants with soil and less than 25% of the weeds are uprooted by the treatment. The amount of soil burial required to control many weed species varies between two and three cm. This may be difficult to achieve in the field as it was found that less than 10% of harrowed plants were buried deeper than 1.5 cm. Partially buried plants generally survive, although dry soil conditions may increase the mortality of partially buried plants.

Sensitivity to soil burial is related to seed size. Small-seeded plants exhibit greater mortality when buried, particularly in early growth stages. The small seeded plants rely heavily on photosynthesis to supply their energy needs, while large seeded plants can draw on seed reserves in early growth stages.

Weed burial effectiveness depends on soil texture, weed growth stage, plant architecture, harrowing speed and depth, harrow setting, soil moisture and number of passes. Fine-textured soils have a higher mechanical resistance than coarse-textured soils, which impedes recovery of buried plants. The plant’s ability to resist bending is important in tolerating postemergence harrowing as bent plants are more easily buried. Therefore, plants in advanced growth stages tolerate postemergence harrowing better than seedlings. Weeds that form a rosette with their meristem close to the soil surface are more sensitive to burial than erect weeds with their meristems above ground level.

Soil coverage increases in proportion to harrowing depth. Weed control due to harrowing will be spatially variable in a field, as working depth and soil cover will fluctuate in response to soil conditions. Weed burial by a tine implement may vary due to ridges formed by the back row of tines. Kurstjens and Perdok reported that up to 80% of weeds were covered by ridges between the tines; however, the trench left by the tine covered less than 30% of weeds. This may be why multiple passes tend to result in more efficacious weed control as subsequent harrow passes may fill the trenches with soil. The amount of soil disturbance a harrow causes is controlled by the angle of the harrow tine in relation to the soil surface. A forward pointing tine was found to cause 42% more horizontal transport of soil than a backward pointing tine. The distance soil aggregates move is proportional to the square of the tool velocity. Therefore, increased harrow speeds result in more uniform soil covering of weeds but do not necessarily increase burial depth. Faster harrow speeds do not improve selectivity as they increase soil burial of both weeds and crop.

Soil moisture’s effect on post-emergence mechanical tillage efficacy is not clear. In controlled environment studies, moisture content did not affect survival of four weed species if they were completely buried. Dry conditions were very effective in improving mortality if plants were pulled from the soil and the roots were re-buried. Good soil moisture conditions might have resulted in the differential ability of spring wheat and wild oat to recover from harrowing. Above normal precipitation and good soil moisture conditions at and following harrowing were reported in the one year where harrowing improved spring wheat yield. The effect of soil moisture on harrowing efficacy may also be dependent on the weed species.

Multiple harrow passes may be required to achieve some level of weed control. A model developed by Rasmussen suggested that up to three harrow passes provided the optimum level of weed control and yield response in field pea. Four to five consecutive harrow passes in winter wheat were required to achieve up to 95% weed control. Three passes reduced weed biomass by about 75% in spring barley. Deep harrowing, high soil disturbance, and multiple passes may cause more weed burial; however, selectivity is not necessarily improved as there is generally a proportional increase in crop burial. Under low weed populations, increased crop covering due to consecutive harrow passes resulted in a decline in spring barley yields of 0.12 to 0.15% for every unit percentage of crop cover.

To minimize crop injury, it is usually recommended that harrowing be conducted in the same direction as crop rows. However, Wilson found that harrowing direction had no impact on weed control or crop injury in winter wheat. Harrowing may be more selective at the crop’s later growth stages, provided the weeds are in early developmental stages. Four passes with a flex-tine harrow resulted in up to 90% control of small broadleaf weeds with no damage to winter wheat when it was harrowed at a height of 20 to 25 cm. The established crop was able to push the tines sideways where inter-row weeds were controlled. Wilson also reported that winter wheat tolerated harrowing in the spring, but autumn harrowing caused severe crop injury.

Overall, yield responses to postemergence harrowing have been modest due to low crop-weed selectivity. Kirkland reported a 19% spring wheat yield increase in one year of a three-year study. In Denmark, post-emergence harrowing resulted in spring wheat yields ranging from 91 to 118% of the untreated check. Yield response in winter wheat has ranged from 0 to 10%. A model for harrowing in field pea suggested yield responses in the range of 0 to 5% corresponding to 0 to 70% weed control. However, Al-Khatib reported that post-emergence cultivation with a finger weeder resulted in an 18% yield increase in field pea. Weed control levels in harrowed spring barley averaged about 75%; however, yield responses did not exceed 10%. Models developed by Rasmussen suggest that crop yield response will be greater under high weed pressure, as the positive weedkilling effect will outweigh the negative crop damage.


Organic growers who use mechanical weed control techniques should time their operations to maximize selectivity. Preemergence tillage in deep seeded crops can result in highly selective weed control. Post-emergence harrowing has low selectivity, therefore organic growers need to scout their fields to determine if the practice is justified. If postemergence harrowing is warranted, every attempt should be made to minimize crop injury.


Funding provided by the Canada-Saskatchewan Agri-Food Innovation Fund

Originally published in Research Report 2002, Canada-Saskatchewan Agri-Food Innovation Fund