Facing up to the new realities behind weed resistance

Tuesday, December 8, 2015

Effective weed control requires a willingness to invest in the appropriate strategies and equipment. That includes more than one nozzle type to accommodate all the different sprays you will be using

by DALE COWAN

We have a long history of weed resistance development in Ontario – from 2,4-D on wild carrot in the early 1960s to the current scourge of glyphosate and group 2 resistant Canada fleabane. We have all been aware of the multiple mode of action recommendations, the requirement to include residual chemistry in our pre-plant applications and the general need to mix up the chemistry we use to control weeds.

What we may not fully realize is how resistance to weed control strategies develops. Certainly the over-reliance on a single herbicide, such as glyphosate, has contributed to the current situation with glyphosate-tolerant fleabane, giant ragweed and now common ragweed.

But it is more than that. In addition to over-reliance, we have been using less than label rates and have tried to wait until all the weeds are up, so that we can spray once to get our money's worth. Improper application, too small carrier volumes resulting in poor coverage for post-emergence, improper nozzle selection, worn-out nozzles, improper choice of products, poor water quality and travelling too fast – all these activities can actually lead to increased selection pressure on any given weed population. In any field population of weeds, there is always a resistant biotype at very low levels.

With continued overuse of a herbicide that "works," and the nature of the weed, we slowly select for this resistant biotype until it becomes a significant portion of the weed population.

It takes several more seasons to realize we are dealing with the resistant biotype. We usually increase the rate or frequency of the current product until it's too late and the field weed seed bank is dominated by the resistant biotype.  

Controlling resistant biotypes is a numbers game. Most weed control scientists agree that probably one in every billion seeds is a resistant biotype. The following table illustrates, based on weed seed bank counts, how many acres you need to locate one resistant biotype. If you allowed 100 weeds to grow on one acre, it would require 10 million acres to find one resistant biotype. However if you had 3.2 million weeds per acre, you would only have to look at 300 acres to find the first resistant biotype.

You may ask how a field could ever get to 3.2 million seeds per acre. It's very easily done with Canada fleabane, a prolific seed producer with upwards of 250,000 seeds per plant. It would only take 13 plants per acre allowed to go to seed. This is one of the reasons that resistant Canada Fleabane can spread so fast.  

The real issue with this kind of weed population is the selection pressure that is placed on the next alternative chemistry. Indeed, we have group 2 resistant fleabane as well as glyphosate resistance.

What we all want are simple, easy and cheap weed control strategies, but what we actually need is complexity and diversity, requiring a willingness to invest in the appropriate strategies and equipment.   

We are awaiting registration of dicamba-tolerant soybeans and new dicamba formulations. This new technology will give us more pre-plant and in-season options for controlling some of our most resistant broadleaf weeds. The new reality will be the up front stewardship, as indicated on the labels. We will move from label recommendations to very prescriptive labels. These new labels will specify spraying conditions, rates, volume of carrier, additives, optimum environmental conditions and nozzle type.

The nozzle type will be one of the most important aspects for coverage and controlling offsite movement. With new products will come the need for better sprayer operation. Your spraying operation may require investments in more than one nozzle type to accommodate all the different sprays you may be using. Remember, the sprayer is required to haul the nozzles around the field. Make sure you have the correct configuration for the task at hand. BF

Dale Cowan is a Certified Crop Adviser in Ontario and the Senior Agronomist and Sales Manager for Agris and Wanstead Co-operatives, located in southwestern Ontario.