rice seedlings

Residual herbicides return to weed control programs

The reluctance to invest $50 or so for two residual herbicide applications up front often costs $100 or more later.

I attempt to address the topic of weed control economics in general terms, rather than individual herbicide cost comparisons.

First, you have to control the weeds — it doesn’t take many weeds to cost more than the entire herbicide program. We used to say control them to levels below economic thresholds and at one point we even had a neat computer program to help you do that. Herbicide resistance changed all of that, and in hindsight, thresholds for weeds were a bad idea.

In the days when Integrated Pest Management (IPM) was the buzz, we tried to follow the lead of the entomologists in establishing economic thresholds. For them it is a good idea, but for weed scientists, not so. They are dealing with a mobile or transient pest from year to year. In the case of weeds, you grow most of your own from the soil seed bank.

As we gain resistant weeds and lose the effectiveness of herbicides, we cannot afford any escapes because that escape could be resistant. Therefore, the weed control goal should be zero tolerance.

The challenge is to get as close to 100 percent weed control as possible with the least amount of inputs. Get rid of all the stuff going into the spray tank that is not supported by unbiased research and is not a part of the herbicide manufacturer’s recommendation.

Next, properly use soil residual herbicides. We have come full circle on residuals. When the herbicide boom began back in the 1960s, many of the breakthrough herbicides were residuals. Residual herbicides have always been erratic, and that will not change.

As better postemergence herbicides were developed, we started backing up the residual herbicides with postemergence herbicides. We then went through a period where some of the better new herbicides were postemergence. As that happened, we began to move more away from residuals to “see the weed, spray the weed, and hopefully kill the weed” programs. This was partly due to the good postemergence herbicides we could mix and partly due to the erratic nature of residual herbicides.

Then, Roundup Ready came along and we went to 100 percent postemergence programs and life was good for 10 years.

Things work in cycles and now we are in a period when we tend to have more issues with postemergence herbicides, and we are back to needing combination programs. We are dependent upon soil residual herbicides to set the stage for postemergence programs.

In rice, there is no doubt that the best herbicide money is being spent on residual herbicides. I am a firm believer in getting at least two separate residual applications out before you see the first weed and most of time before you see any rice. I realize it is hard to spray a clean field again. That is part of the “see the weed, spray the weed” culture we have developed.

The problem is, in rice once we see barnyardgrass and sometimes other weeds, we wind up throwing the kitchen sink at the weeds and not killing them. The reluctance to invest $50 or so for two residual applications up front often costs $100 or more later.

I like the programs using a pre-emergence application of Command followed right after the first rain by a delayed pre-emergence application of some combination of Facet, Prowl, Bolero or more Command. Sharpen is turning out to be an interesting herbicide in rice, and I think we will see more of it integrated into both residual and postemergence programs.

I consider the two-shot residual approach a foundation program in rice now. Consultants and growers using it are having a much easier time. There is still the need for postemergence herbicides following this approach, but it is for clean up rather than for a fire fight or salvage program.

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