Despite ideal crop rotations, quality seed, best agronomic practices and tolerant attitudes, organic producers may find more weeds in their fields than they are comfortable with. Post-emergence harrowing is one option for in-crop weed management.
Like all weed management tools, the key to in-crop harrowing is to give the advantage to the crop and not to the weed. This is referred to as selectivity.
Post-emergence harrowing is not a highly selective tool. There is always some compromise between killing too many crop plants and failing to kill or damage many of the weeds.
Eric Johnson of the Agriculture Canada Scott Research Farm in Scott, Sask., has conducted extensive research to improve harrowing’s selectivity. Most of the following recommendations are based on his work.
Some crops tolerate harrowing better than others. In general, large-seeded crops are more likely to survive harrowing than small seeded crops. Field peas and chickpeas, for example, are better choices for harrowing than flax and mustard.
Similarly, some weeds are more vulnerable to harrowing than others. Green foxtail and lamb’s-quarters are more easily removed than wild oats.
For weeds, earlier control is more effective. Younger seedlings, especially those at the thread stage, where they have germinated but not yet emerged, are most vulnerable. As they develop, they become more able to withstand harrowing damage.
About a quarter of weeds removed by harrowing are damaged by the process. The rest are buried and
are unable to re-emerge or do so slowly. Crops generally bend less than smaller weeds, and are
thus less likely to be buried.
Burial of both weeds and crops is greater at increased speed. Some producers find that speeds between five and 10 km-h gives the best compromise between burying weeds and burying crops. Burial tends to be greater between harrow tines than at the tines. Multiple passes give more uniform burial and thus greater weed kill.
Generally, harrowing in the direction of crop rows is recommended, though studies have not verified this practice.
Some crop damage is inevitable. Heavier seeding rates allow for a good crop stand even with some damage. Deep seeding also improves a crop’s chances when harrowing. Ideally, harrowing is less than five centimetres deep, while the crop seeds are securely below that depth. Harrowing damage may increase a crop’s susceptibility to diseases. The decision to harrow should consider this additional disease potential, especially in lentils and chickpeas.
Harrowing works best if the soil is dry, though harrowing may further dry the soil and increase the chance that it will blow. Cool, wet weather after harrowing will help the crop recover, although rain may also help the weeds
Crop residues can plug the harrows and reduce selectivity by reducing weed kill and increasing crop kill. Adjusting the tines to reduce the level of disturbance can reduce plugging.
Finger weeders, or flex-tine harrows, further reduce crop damage.
Ideally, harrowing causes only a small amount of damage to the crop, and a much greater amount of damage to the weeds. Producers control some of the factors involved – timing, speed, angle and number of passes.
Still, for a producer with less experience in the technique, the look of the crop after harrowing can be an intimidating prospect. Some producers suggest that newcomers to the technology avoid looking back, and then go fishing for a couple days while the crop recovers.
For less risk-tolerant producers, a small-scale trial might be a good starting point.
Of course, tillage is never a tool to be used indiscriminately. Excessive tillage increases the risk of soil erosion, reduces soil organic matter and beneficial soil organisms, and increases salinization and nitrogen loss. It may even move parts of perennial weeds throughout the field and move weed seeds into better places to germinate.
Producers who decide to harrow balance the risk to the crop and the soil against the benefits of reduced weed competition.
For more information, see: Inter-row cultivation Ð effective weed control in field pea, Post-emergent harrowing for weed control and Post-emergence field pea harrowing Ð rotary or tine? in Spoke Program Research Reports 1997-2002, published by Agriculture Canada and Saskatchewan Agriculture.
All are available at www.organicagcentre.ca.
Frick is the Prairie co-ordinator for the Organic Agriculture Centre of Canada located at the University of Saskatchewan. She can be reached at 306-966-4975, at firstname.lastname@example.org, or www.organicagcentre.ca. The opinions expressed in this column are not necessarily those of The Western Producer.