As we get into mid-May, the die is pretty much cast on how successful our weed control will be this year.

How well did your burndown program work? Did your pre-applied (pre-emergence and or preplant) herbicide get activated?

For most folks in Tennessee, burndown applications were pretty successful. With the use of old standards like glyphosate, dicamba, Gramoxone Inteon and new herbicides like Sharpen, burndown success this year was quite high.

For additional information on the subject of weed resistance click here.

Unfortunately that was not the case on activating pre-applied herbicides. The dry stretch in mid-April did not activate some of the early preplant herbicide applications. I walked into several very weedy fields in late April that had Valor or Reflex applied that was not activated in time to provide residual control. Later rains did at least partially activate these herbicides, but that first Palmer pigweed flush had to be burned down just behind the planter.

Now that we have the post-applied herbicide applications coming up, I cannot stress enough the importance of scouting for weeds. Scouting for weeds was not important back when glyphosate controlled all weeds at virtually any height. One could essentially scout a field for weeds with a 120-foot boom at 18 miles per hour spraying glyphosate.

Now that we are trying to farm in the era of widespread glyphosate-resistant weeds, particularly Palmer amaranth, we must have eyes on the field to be able to effectively manage these weeds.

As some farmers and retailers found out in April with Palmer pigweed, we cannot assume a pre-applied herbicide is working, we have to know it is working. The only way to know for sure is to have someone scouting the field. Not only must it be scouted to see if the pre-applied has been activated, it must also be scouted to find out when the pre-applied will finally wear out.

When any herbicide is applied to the soil, the clock starts ticking on how long it will last. One never knows when the clock will stop because it depends on many factors including the herbicide used, the soil type and most importantly the amount of rainfall after application.

The indicator when the pre-applied herbicide has worn out is seeing the first flush of Palmer amaranth. The key to managing this weed is timeliness of the post application. The time from when the soil turns purple with Palmer emergence until the time the weeds are beyond the size that any herbicide will control it (4 inches to 6 inches tall) will vary on the date.

In April, it will take about four weeks for Palmer to reach this size. In mid-May or June as the days get longer, warmer and particularly if the soil is not dry, Palmer will only take about 10 to 14 days to reach 4 inches or taller. In practical terms for most farmers with thousands of acres of soybeans and cotton scattered across several counties, it will not be possible to spray every acre in just a few days. So having someone monitor when Palmer pigweed emerges is crucial.

Another option, of course, is to spray early post-emergence with a residual product over the cotton or soybeans before the first pre-emergence breaks. Products like Dual Magnum over-the-top of soybeans or cotton and Flexstar post-emergence in soybeans would be good choices for this option. Again, someone will need to monitor that these herbicides which have residual for Palmer pigweed get activated.

As many growers found out last year, there are no second chances with glyphosate-resistant Palmer amaranth. You either control it before it gets started or you live with it until harvest. Eyes on the field will be a crucial component on how well we manage Palmer amaranth in 2010 and beyond.

lsteckel@utk.edu