Science Behind Mechanical Weed Control

REGINA - Dec 12/07 - SNS -- There is more science behind the art of mechanical weed control than many organic growers may appreciate believes Agriculture Development Fund (ADF) researcher Dr. Steve Shirtliffe sums up some new research into herbicide alternatives.

"Weed control in organic crops is difficult. Mechanical techniques offer some options for farmers. When you use mechanical methods, they tend to be not nearly as selective as an herbicide would be. A lot of these mechanical methods cause crop damage as well as weed damage, so you have to balance it out to make sure that you are not making the matter worse," said Shirtliffe.

In 2004, Shirtliffe and co-researcher Eric Johnson with Agriculture and Agri-Food Canada set out to explore the tolerance of oat, wheat and barley to mechanical weed control methods. Three years later, the research, funded in part by ADF, provided some interesting results.

Shirtliffe, an Associate Professor with the Plant Sciences Department at the University of Saskatchewan, said the research will benefit the growing organic sector. The research looked at several mechanical techniques, including in-crop harrowing, mowing and rolling. Shirtliffe said the biggest surprise had to do with oats.

"At the onset of our research, the thought was - and it was reflected in some production manuals - that post-emergent oat should not be in-crop harrowed. The information at the time suggested that wheat and barley were tolerant to this, but what we found out is that oat is indeed tolerant to it," explained Shirtliffe.

It's unclear why in-crop harrowing was previously not recommended for oat.

"We couldn't find any solid evidence, but speculate that because the position of the growing point of oat is closer to the surface, perhaps it was believed there would be damage. Our research showed oat, in fact, was often more tolerant than even wheat, which most people hold up as being a crop that is quite tolerant of in-crop harrowing," said Shirtliffe.

The findings provide organic oat producers with another option for weed control which previously was not recommended.

Another surprise came from the research into rolling flax as a weed control method. Shirtliffe said the results there were pretty clear. "We found out that it is probably not a good idea," but, he said, there was some anecdotal evidence that it might be effective.

"The idea was that you roll your flax with a roller that you would use for pulse crop production, and the thinking is that some weeds, like wild mustard, would be broken down by it and not come back, whereas flax with fibre in its stem would come back up and wouldn't be affected. Well, that never happened. It is something that we are not recommending at all. We looked at it for three years in a row at one location and it did not have any potential," said Shirtliffe.

Mowing to control weeds was an equal disappointment.

"We used wheat, oat and barley in the test, mowing them at different stages. The thinking was that the crop would come back quicker than the weeds - giving it a competitive advantage. In the end, we just didn't see any positive yield response or weed control benefit that would indicate that it is a practice that we would ever recommend," said Shirtliffe.

However, rotary hoeing did yield some positive results.

"My partner Eric Johnson looked at rotary hoeing. It looks like it might have some promise for organic growers - using a minimum-till rotary hoe. It is an implement we are not very familiar with in Western Canada, but it is used in the corn and soy bean belt. Multiple passes with a rotary hoe when the weeds are small is effective at killing some weeds, and there is quite good crop tolerance as well," said Shirtliffe.

While this research will benefit organic producers the most, Shirtliffe points out mechanical weed control techniques can also help non-organic producers reduce herbicide use.

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