Prepare for Man vs. Weed
Resistant palmer amaranth has become a hot topic in the past several growing seasons. What started as a small trickle with a few palmer plants surviving here and there has now become a flood of surviving weeds across the field. In the past it has been a game of wait and see before you sprayed pigweeds. Once a good flush of palmers emerged you went across the field with an application of glyphosate, and “poof” they were gone in a week. The days of just spraying glyphosate to kill palmer amaranth are over. Driving across Kansas late last summer, I noticed many fields where the palmer had survived and would have surely been an issue at harvest. The battle of man versus weed has truly begun.
Palmer amaranth can be a relative easy weed to kill in grass crops where many broadleaf herbicides are labeled. The families of growth regulator and HPPD herbicides still work very well on this species. When it comes to broadleaf crops such as soybeans, the easy button gets hidden in the closet and is nowhere to be found when you need it. Palmer amaranth can still be controlled in soybeans, but a more progressive approach must be taken.
One of the best solutions to controlling palmer in a soybean crop starts with a strong foundation pre-emerge herbicide. If you start with a clean field and put a good residual herbicide down at planting time the scale is tipped in the right direction. There are many solid pre-emerge herbicides that can be used in soybeans, but always read the label and make sure the appropriate rates are used. Soil types can have adverse effects with some herbicides, and in some cases the soil type may restrict herbicides from being used. High pH and low organic matter are two of the biggest components of the soil that restrict the use of some herbicides.
Once you have down the groundwork of applying a solid pre-emerge, the next step is to scout the field and watch for newly emerging weeds. Once a new flush of weeds starts to emerge it is a good sign the residual pre-emerge is starting to run out. At this point it is a good idea to run a tank mix of another labeled residual herbicide and herbicide designed to kill emerged weeds. The postemerge herbicide could be glyphosate, provided the soybeans are Roundup Ready, or Liberty if the soybeans have the LibertyLink trait. There are several other herbicides labeled for postemerge treatments in soybeans that can provide knockdown of emerged weeds as well. This is especially important if glyphosate resistance is already suspected within a field.
Canopy closure is another important aspect in dealing with weed control. If there is known resistance to glyphosate in the field and the plan is going to soybeans, planting in narrow rows is a good option. By drilling instead of planting in 30-inch rows, the soybeans will close the crop canopy quicker, which will help to aid in weed control as well. This is not a cure all solution, but when coupled with a solid herbicide foundation it will help control the weeds better than glyphosate alone.