Don’t be overconfident in 2015 weed control program

Don’t be overconfident in 2015 weed control program

This has been a very frustrating spring. I thought last spring was very challenging, but it was a picnic compared to this one. This is particularly true with the cotton crop. The USDA had planting intentions at about 174,000 cotton acres in Tennessee. I think that was a good estimate at the time — but then the weather happened.

We had from about May 1 to May 15 to plant. Almost everyone used the first half of that window to plant corn and grain sorghum because it was getting late for those crops. As a result, a good deal of the cotton was planted in the last half of that window. Unfortunately, it turned off very wet with temperatures a couple nights in the low 40s in mid-May, which was devastating for a good deal of the cotton that had been newly planted.

It has rained seemingly every other day since then, giving no time to replant cotton.

It is now June 7, and it’s very clear that most of the ill-fated cotton will be planted back to soybeans later this month. This will make what would have been a very small cotton crop for our state even smaller. I am still hoping for 100,000 acres of cotton in Tennessee in 2015. This may be a high as more calls keep coming in on questions concerning replanting soybeans behind a failed cotton stand.

The lion’s share of what will be the regular soybean crop, the double-crop soybeans and the soybeans planted behind a failed cotton crop will likely be planted in middle June into early July. From a weed control standpoint this is a major concern because they will all have to be sprayed in a very short period of time.

Also, if it finally does warm up in June, Palmer amaranth will grow very quickly. Burndown upfront will be very important because some of these pigweeds will no doubt be large. Using the full rate of Gramoxone Inteon will be a key to starting clean. Moreover, a pre applied herbicide tankmixed in with the burndown will be very critical to help delay pigweed emergence to the point that a post application has a chance to be applied to Palmer amaranth no taller than 2 inches to 3 inches.

In LibertyLink soybeans, a Liberty application can control Palmer amaranth even up to  4 inches to 6 inches tall.

In Tennessee a good percentage of our double-crop soybeans are typically planted to LibertyLink varieties. Since LibertyLink, Roundup Ready and conventional varieties will likely be planted at the same time and in a short period of time, it will be easier this year to forget which herbicide trait soybean is planted in which field. Be sure to take the time to keep good records of what soybean variety and herbicide trait technology are planted in each field.

I have always felt the “Flag the Technology” program that started in Arkansas was a great idea. I know it has saved some mishaps from the wrong field being treated with the wrong herbicide. This year it may be even more likely to save a misapplication to a field.

Susceptible to herbicide burn

The cold and wet spell the early soybeans have gone through this year has left them more susceptible to post applied herbicide burn. The cuticles on the soybeans are very thin and in all our research, anywhere we sprayed a PPO herbicide — or even Liberty tankmixed with Dual or Warrant — we are seeing more burn on the soybeans than is typical. I often have been getting 25 to 35 percent burn from these applications. I would expect the soybeans to recover fine.

On a positive note, the same weather that has caused the soybeans to be more susceptible to post burn is also helping when trying to control taller Palmer amaranth. In most years in our research we can usually control a 2-inch-tall Palmer amaranth with PPO herbicides like Flexstar or Ultra Blazer applied at a pint per acre.

However, once the Palmer gets 3 inches tall, the control with those type herbicides becomes very sketchy and by 4 inches tall, forget it.

Over the past week, we have gotten good control of Palmer amaranth even as tall as 6 inches with Flexstar or Ultra Blazer.

Moreover, in LibertyLink soybeans, control with Liberty typically becomes inconsistent once Palmer amaranth reaches a height of 6 inches. So far, with the early soybeans we have seen great control with upwards of 10-inch-tall Palmer with Liberty.

I would not expect this to continue into mid-June. As the weather warms up I expect both the soybeans and Palmer amaranth to harden off, and we should start seeing less soybean burn from PPO herbicide applications as well as poor control on Palmer amaranth over 2 inches tall with these herbicides.

In other words, do not become overly confident at how large a pigweed can be controlled based on results from these earlier-planted soybeans.  The bulk of the soybeans will be planted from mid-June to early July and will no doubt go through heat and maybe some drought stress.  Palmer amaranth in these fields will need to be treated timely (less than 2 inches tall) with a PPO herbicide to have success controlling them.

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