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Smith: time to burndown for next spring?

We haven’t even gotten the combines and pickers cleaned, and many are already asking about managing winter weeds. These requests are probably caused by flashbacks from last spring when weeds wouldn’t die. It has been my opinion that only in special circumstances will fall applications of residual herbicides be profitable.

We have come to realize if glyphosate-resistant ryegrass is present, fall applications are necessary. Jason Bond wrote a good article in Delta Farm Press on managing ryegrass a couple weeks back. But what about the other weeds such as horseweed, (the genus) Poa, primrose, and henbit?

My experience with fall applications for control of horseweed is they break about three weeks prior to planting time and new horseweed seedlings emerge. At this time, they are small and may be overlooked. This is also inside the plant-back window for dicamba and even if they are recognized, dicamba cannot be used for control. Our best control for horseweed has been an early burndown with dicamba plus a residual like Direx or Valor in late February.

The only failure we have noted with this recommendation was a few years back when we experienced a very cold and dry winter. The weeds simply were not growing enough for foliar uptake, and there was no soil moisture for root uptake. If we experience similar conditions in 2012, I would suggest waiting until early March and spiking the 8 ounces of dicamba with 1.5 pints of 2,4-D.

Even though horseweed and ryegrass are “our problem weeds,” it seems that henbit thicker than a green 1980 shag carpet has caused more problems and more failed burndown attempts over the past two years. In the spring of 2010, I thought, “Darn this henbit is bad this year, but hopefully we can get past it and back to more normal populations.”

After an even worse 2011 spring, I am beginning to think excessive henbit may be the norm. Some fields that were harvested early and bedded are already green with henbit. Each henbit plant is capable of producing 200,000 seeds, so it doesn’t take many to really increase the soil seed bank.

Growth chamber studies have shown that henbit can germinate in soil temperatures as low as 40 degrees, but most germination occurs in soils between 60 and 70. Field studies show that daytime tillage in the fall stimulates germination. Daytime tillage and moist soil at 60 degrees sounds like the Mid-South in the fall. Couple this with the potentially high soil seed bank from the past two years, and we may be set up for another henbit spring. For these reasons, I have encouraged fall herbicides to get a head start on henbit for next spring. If we can knock out this fall flush, I think we can manage the spring germinators with our normal burndown programs.

Paraquat or 2,4-D at 1.5 pints this fall will kill most of the henbit and early germinating primrose and horseweed. I don’t mind leaving some of the Poa on the beds to help reduce erosion during the winter. However, the 2,4-D will also kill some small Poa and reduce the growth for a while on what is left, and paraquat will take out the existing Poa as well as the henbit. Adding a residual herbicide such as Direx or Valor will prevent winter weed germination until the soil starts to warm next spring. This will also leave the bed bare and subject to more erosion.

By definition, a weed is a plant that interferes with human activity. If weeds did not interfere with our programs, we would never give them a second thought. There would be no Delta Farm Press articles, no herbicides applied and no wringing of hands or scratching of heads trying to determine the best management practice.

We know our current glyphosate-resistant horseweed biotype has a larger germination window than the biotype common to this area prior to 2003. It seems ryegrass is evolving similarly. This should not be too surprising. We select and breed crops by maturity and continually select those with the best cold germination test results for our early planting.

Weeds are a dynamic population in a very complex biological system and will adapt to survive under the conditions we provide. We have no research to demonstrate that henbit has evolved to germinate earlier, produce more seed, or tolerate more herbicides. But we do know henbit has been extremely hard to control with normal burndown programs over the past two years. This has prompted me to suggest a fall treatment to get a head start just might save some discouraging words next spring.

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