Post-Emergence Harrowing – Cameron de Wolf
Post-emergence harrowing is utilizing a form of tertiary tillage (tillage conducted after seeding, but before harvest). Harrowing provides a shallow, thorough stirring of the soil that kills sprouting and emerging weeds. It also knocks soil from weed roots. This method provides the crop with an advantage over the weeds that are present. Large seeded crops are more likely to survive a harrowing event than small seeded crops (ex. field peas vs. flax). Research has been completed in many different crop species (wheat, barley, field pea, canola, flax, etc.). Field peas have been found to be the most tolerant to post-emergence harrowing. Harrowing may not kill all weeds, but can damage the majority of them, offering a competitive advantage to the crop. This technique offers a low soil disturbance weed management method to discourage weed growth within a growing crop.
As a general rule, the smaller the weed, the better control that can be achieved (esp. at the thread stage of weeds). Since burial of crops and weeds is greater at faster speeds, operating speeds of 5 – 10 km/h is most effective in regards to crop burial and weed management. Post-emergence harrowing also works best if the soil is dry and operation is shallow (less than two inches).
Selectivity in regards to both crop and weed is a very important consideration. Overall aggressiveness of the operation must also be monitored, as well as overlapping. Thresholds for these factors (in regards to crop damage) should also be developed. Heavier seeding rates and deep seeding can decrease negative effects of in-crop harrowing. It is also important to disturb the crop plants as little as possible, which is different than pre-seeding harrowing (which favors soil disturbance). This can usually be achieved by shallow operation done parallel to crop rows.
Many different implements can be used, all with varied costs (rod weeder, cable weeder, flexible harrow, drag harrow, rotary harrow etc.). It also depends on the width of machine purchased, and if the implement is new or used. Many of these units have been deemed “obsolete” by conventional agriculture and can be purchased inexpensively (approx. $5000). However, there is also a growing niche market for these harrowing implements. Fuel cost must also be included, as several harrowing operations may be warranted in some cases. Within this technique, some crop damage is unavoidable and may increase a crop’s vulnerability to disease. Crop injury can be reduced by using low-disturbance methods, such as a flex-tine harrow.
Post-emergence harrowing can be a valuable weed management technique. Again, the producer needs to be aware of many factors, such as crop species, staging, plant life histories, and implement characteristics. It appears to be the most valuable in field pea crops, but must be completed when weeds are small to be most effective. Crops can be damaged if this method is used incorrectly.
Frick, B. Nd. PLSC 234 Course Materials. https://bblearn.usask.ca/webapps/blackboard/content/listContent.jsp?course_id=_56145_1&content_id=_864797_1&mode=reset (accessed March 1, 2014).
Johnson, E. Nd. Post-emergence harrowing for weed control. http://www.organicagcentre.ca/ResearchDatabase/res_emerg_weedctrl.asp (accessed March 2, 2014).
Natural Systems Agriculture – University of Manitoba. Nd. In-crop tillage for weed control. http://www.umanitoba.ca/outreach/naturalagriculture/articles/tillage.html (accessed March 2, 2014).
Sustainable Agriculture Network. 2002. Steel in the Field: A farmer’s guide to weed management tools. G. Bowman ed. Sustainable Agriculture Publications, University of Vermont, USA.