Herbicides Weed Management

Reality check: herbicide resistant weeds

Kristen Callow, OMAF and MRA Weed Management Program Lead – Horticulture

From ONvegetables in The Grower, June 2013

So, which camp are you in?  Have you conceded and admitted to yourself that you have a resistance problem on your farm?  Or are you still in denial?  No one ever wants to admit that they have a problem that can potentially spread to others.  However, it is always best to admit it now, because it will catch up with you later and usually at a big expense.  For example: Jack who grows 8000 acres of soybeans in the southern U.S.  He didn’t admit that he had a problem until it was much too late.  Jack had glyphosate resistant palmer amaranth infesting his 8000 acres.  The problem was so bad that Jack went bankrupt because he could no longer grow soybeans economically.

Reality Check: Once you discover a herbicide resistant weed problem, it has likely been there for more than 3 years.

Reality Check: If you didn’t know you had a problem until 3 years later, you have likely spread the problem around your farm without even knowing it.

When it comes to the health of your farm, you should think about it as seriously as your own health.  Resistant weeds, once present are there for good and you will continue to manage them for your lifetime.  They are like cancer cells multiplying rapidly once they are present.

Reality Check: On average there are 100,000,000 weed seeds per acre and only 1,000,000 emerge each year.  Therefore, resistant weed seeds can remain in the soil for many years (Giant ragweed produces up to 5,000 seeds per plant with an average dormancy of 2 years; Canada fleabane produces up to 200,000 seeds per plant with minimal dormancy (86% of seeds can germinate immediately); Redroot pigweed produces up to 100,000 seeds per plant and has a seed dormancy of up to 40 years).

It is true that new management strategies are being developed to help you manage resistant weeds in various cropping systems.  Most of these strategies use multiple methods (Integrated Weed Management) to control the resistant weeds, such as; crop rotation, shallow cultivation (to prevent bringing weed seeds to the soil surface from deeper soil depths, tank-mixing herbicides with different modes of action that have activity on the resistant weed, using preemergence herbicides followed by postemergence herbicides, etcetera).  These are all strategies that were used in the past, prior to the introduction of herbicide tolerant crops.  It is now time to bring these management tools back.  Remember when weed control was an art?

Reality Check: Integrated weed management is the future of weed control.  There are no new herbicide modes of action coming any time soon.  Crop protection companies are working on new herbicide tolerant corn and soybean cropping systems.  There are no new technologies for horticulture producers.

It is not all doom and gloom … just reality… If you have resistance get on it now, spend the extra money to prevent a situation like what happened to Jack.  If you are not sure if you have a problem (yet) get your weeds tested (it is free: http://www.plant.uoguelph.ca/resistant-weeds/) and for heaven’s sake, do not let those ‘maybe resistant weeds’ go to seed.

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