It is obvious as I talk to growers and consultants that the level of attention on weed resistance has increased dramatically. Now we will find out if it has increased enough for folks to look longer term and get out of a comfort zone.
Steve Powles from the University of Western Australia has a great saying: “If your weed control program is working — change it!” That statement does not jive with human nature. With most of us, if it worked good last year… we already know what we are going to do this year.
Change does not mean changing to something that does not work, but rather look for ways to increase crop diversity, herbicide diversity and technology diversity.
I recently heard another of Ken Smith’s presentations and he talked a lot about soil seed bank or the number of weed seeds in the soil. I believe he is definitely on the right track. I came up through the hey-day of IPM — integrated pest management. The basic philosophy in this system is to treat pests only after they are at economic threshold levels and that the best economic return on the pesticide investment is to control them to the point they are again below this level.
There is a lot of great research on economic thresholds for weeds and even some nifty computer programs that will tell you how many of each weed you can stand before you need to apply a herbicide. I was involved in developing one of these. However, weed resistance has changed Ken’s opinion on this and I agree with him completely.
In a lot of ways weed resistance is a numbers game. With Palmer pigweed, for example, the more seed that are present in the seed bank in a given field, the more weeds that can emerge and thus the greater the chances that one of these can be resistant. If it is resistant, then you begin to select for it with a herbicide. Since it is such a prolific seed producer, you can go from one plant to a solid infestation in about three years if you continue to use the same herbicide.
The same principle is true with a creeping type resistance. The more plants you have out there, the greater the chances are that some of them are becoming increasingly resistant to the same herbicide.
The way to keep this from happening is to reduce the soil seed bank by not allowing any weed seed production in the cropping system. This is a tall order but one that has become much more important.
Reducing the soil seed bank is important within crops and across crops. I have written several articles on barnyardgrass resistance in rice. Without some major changes in Clearfield use patterns and herbicide use patterns in Clearfield rice, I believe, the ALS inhibiting herbicides are destined to blow up on barnyardgrass.
However, a huge contributing factor in rice is the amount of barnyardgrass that is being allowed to go to seed in soybeans. When barnyardgrass is knocked down below threshold levels in the soybean crop but allowed to make seed, you are looking right down the gun barrel at all that seed production in the next year’s rice crop.
In many cases with early soybeans, the barnyardgrass flourishes after the leaves fall off. Therefore, while we are trying to hold our rice weed control programs together with duct tape and bailing wire, so to speak, we are shooting our foot off in the soybean crop. If one of those barnyardgrass plants happens to be resistant to Command or Newpath, for example, and it is not controlled in the next rice crop, the bailing wire and duct tape begins to unravel.
I followed Ken on the program the other day and challenged the consultant group to consider a “whole farm approach” to weed management. I will issue the same challenge to growers to consider using your consultants in more of a whole farm approach.
Successful resistance management is going to require looking beyond one crop and one year. It is going to require getting a resistance management program in place that maximizes crop, herbicide and technology diversity on a planned approach for the future.
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