It Can Be A Harrowing Experience

Brenda Frick, Ph.D., P.Ag.

Despite ideal crop rotations, quality seed, best agronomic practices, and tolerant attitudes, organic producers may find more weeds than they are comfortable with in their crop fields. One option for in-crop weed management is post-emergence harrowing.

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, at the Scott Research Farm, of Agriculture and Agri-Food Canada, has conducted extensive research to improve the selectivity of harrowing. Most of the following recommendations are based on his work.

Many factors can increase the selectivity. Firstly, some crops can tolerate harrowing better than others. In general, large seeded crops, are more likely to survive harrowing than small seeded crops. Field pea and chickpea, for instance, 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.

Plant stage can also be important. Recommended stages for harrowing are shown in the following table.

For weeds, earlier control is more effective. Younger seedlings, especially those at the thread stage (germinated, but not yet emerged) are most vulnerable. As they develop, they become more able to withstand both the damage and the burial of harrowing.

About a quarter of weeds removed by harrowing are actually damaged by the process. The rest are buried, and are unable or slow to re-emerge. Crops generally bend less than smaller weeds, and are thus are less likely to be buried. Burial of both weeds and crops is greater at increased speed. Some producers find that speeds between 5 and 10 km per hour gives the best compromise between burying weeds and burying crops. Burial tends to be greater between harrow tines than at harrow 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 2 in deep, while crop seeds are securely below that depth. Damage from harrowing may increase a crop's susceptibility to diseases. The decision to harrow should consider this additional disease potential, especially in lentil and chickpea.

Weather can also influence how effective harrowing is. Harrowing works best if 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. Rain may help the weeds recover as well.

Several types of harrow can be used for post emergence weed control. Crop residues can plug the harrows and reduce selectivity - both by reducing weed kill and increasing crop kill. Some adjustment of the level of disturbance can reduce plugging. A setting of 45? for rotary or tine harrows (opposite to the direction of travel) is recommended. Finger weeders, or flex-tine harrows, further reduce crop damage.

Ideally, harrowing causes a small amount of damage to the crop, and a much greater amount of damage to the weeds. The producers control some of the factors involved - timing, speed, angle, 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 new-comers 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 by both wind and water. It reduces soil organic matter, and beneficial soil organisms. It increases salinization and nitrogen loss. Tillage may even move parts of perennial weeds throughout the field, and weed seeds into better places to germinate.

Producers who decide to harrow for weed reduction balance the risk to the crop and to the soil against the benefits of reduced weed competition.

For more information, see "Inter-row cultivation - effective weed control in field pea?", "Post-emergent harrowing for weed control" and "Post-emergence field pea harrowing - rotary or tine?" on our website, or in Spoke Program Research Reports 1997-2002, published though the Agri-Food Innovation Fund, by Agriculture and Agri-Food Canada and Saskatchewan Agriculture, Food and Rural Revitalization.

Brenda Frick, Ph.D., P.Ag., is the Prairie Coordinator for the Organic Agriculture Centre of Canada at the College of Agriculture, University of Saskatchewan. She welcomes your comments at 306-966-4975 or via email brenda.frick@usask.ca

All photos are of Wayne Hovdebo harrowing wheat on June 5, 2003 at Birch Hills, Saskatchewan

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This article first appeared in The Western Producer, and is published here on the OACC website with permission.