Late Season Weed Control in Sorghum

Post on August 20, 2024

Late Season Weed Control in Sorghum

Brent Bean, Sorghum Checkoff Director of Agronomy

Weed control in grain sorghum can be tough even in the best of years, but can be especially troublesome when conditions at planting and soon after emergence are challenging. This year, in many regions of the sorghum belt, growers had a difficult time working around rainfall events in getting their preplant or preemergence herbicide treatments applied. Or an abundance of rain caused the preemergence herbicide to be leached deeper into the soil profile, rendering it ineffective. Once weeds emerge, we are reliant on postemergence herbicides, which are limited compared to many other crops. These herbicides are also most effective on small weeds. If rainfall events prevent the timely application of these herbicides the result can be a weedy mess.

For grass control, the Double Team, iGrowth and Inzen technologies can certainly help on grass issues. However, broadleaf weeds, such as Palmer amaranth, kochia and morningglory can be particularly troublesome.

Here are some label points of some commonly used postemergence herbicide options as related to late application:

• Huskie FX - can be used up to flag leaf stage or 30 inches in height, whichever is first
• Atrazine - apply before sorghum exceeds 12 inches in height
• Peak - can be used between 5 and 30 inches in height and prior to heading
• 2,4-D - Do not apply from boot to early dough stage
• Dicamba - Anytime after soft dough stage is reached
• Aim - Do not apply past pre-boot stage, but may apply three days prior to harvest
• Glyphosate - May be used after physiological maturity is obtained
• Sodium Chlorate - Apply seven to 10 days pre harvest
  See labels for further instructions and pre-harvest intervals.

Although these products are labeled for use at later growth stages in sorghum, growers should not expect great weed control. Primarily because the weeds are going to be well past the stage of growth when they are most susceptible to a herbicide application. However, a late herbicide application can help prevent weed seed viability and improve harvest efficiency.

A recent late season sorghum weed control trial was conducted by Dr. Vipan Kumar, at the Kansas State Experiment Station near Hays, Kansas. In this trial, 10 treatments were compared for Palmer amaranth control applied when the sorghum was in the boot stage. At this time the Palmer amaranth plants were taller than the grain sorghum. Not surprisingly, none of the treatments provided excellent weed control 30 days after application. However, those treatments with dicamba provided approximately 70% control and certainly increased the harvestability of the sorghum while decreasing weed seed production. As indicated in the above label bullet points, dicamba may be applied to grain sorghum after the kernels have reached the soft dough stage.

Alternatively, a harvest-aid treatment can be applied a few days prior to harvest. The most common application is with glyphosate applied seven days prior to the anticipated harvest date. Some growers have added Aim to the glyphosate mix and reported lower percent moisture of the grain compared to a glyphosate alone treatment.