Pre-Emergence Weed Control


Weed control is critical for the success of any crop, and grain sorghum is not an exception. Unlike many other crops, the nonselective herbicides glyphosate and glufosinate cannot be used post-emergence to control weed escapes. In addition, research repeatedly has shown that maintaining weed control during the first 30 days after crop emergence is critical to protecting the yield potential of sorghum. For these reasons, the successful implementation of a pre-plant or pre-emergence weed control program is essential.  

Fortunately, products are available that do a good job of controlling the most common broadleaf weeds in sorghum and do a reasonably good job of controlling many annual grasses. Growers will get the best results when they use more than one active ingredient for pre-emergence weed control. When possible, use a mix of atrazine and one of the chloroacetamide herbicides:

- Bicep II Magnum, Bicep II Lite Magnum, Cinch ATZ, other generics
   Contain atrazine and S-metolachlor (some generics use metolachlor)
- Degree Xtra, Fultime NXT
   Contain atrazine and acetochlor
- Lumax EZ, Luxar EZ
   Contain a three-way mix of atrazine, S-metolachlor and mesotrione

All of these mixes require sorghum seed that is treated with the safener Concep III.  

If Palmer amaranth is the primary weed of concern, the two-way mixes of atrazine and either S-metolachlor or acetochlor normally will provide good control. Use the maximum rate for your soil type and rotation.  

Regardless which of these premixes is used, a grower should consider applying one-half to two-thirds of the treatment just after planting with the remainder to be applied early post-emergence. This helps protect against adverse weather conditions, such as not enough or too much rainfall, which may cause a single pre-emergence application to lose some of its effectiveness.  

If atrazine cannot be used because of soil type (only use in medium- and fine-textured soils) or other considerations, metolachlor (Dual), acetochlor (Warrant) or dimethenamid-P (Outlook) should be used pre-emergence. Then, in most cases, atrazine can be applied early post-emergence to provide additional residual control.  

Another non-atrazine pretreatment that has gained in popularity over the last few years isVerdict + Outlook. Verdict contains saflufenacil (Sharpen), which has good burndown activity as well as some soil residual. The product can be a good one to use if small weeds are present at planting. In addition to saflufenacil, Verdict contains dimethenamid-P. However, Verdict does not have enough dimethenamid-P to provide adequate residual control. For this reason, it is recommended to apply Verdict at 10 ounces plus an additional 10 ounces of Outlook. Check labels for soil-type restrictions.

Growers sometimes ask about the difference between S-metolachlor and metolachlor. These are slightly different forms (isomers) of the same molecule; however, S-metolachlor is more effective than metolachlor when applied at the same rate. When a label lists the active ingredient as metolachlor, it actually contains four different metolachlor isomers. In contrast, a label that lists the active ingredient as S-metolachlor will contain approximately 88 percent of the more active S-metolachlor isomer.  

Regarding formulations, growers also want to know the meaning of 'II' in Dual II Magnum and Bicep II Magnum. The 'II' indicates that the product includes the safener benoxacor. This safener has nothing to do with sorghum, but it increases the ability of corn to metabolize metolachlor and reduces the potential for plant injury. As mentioned earlier, sorghum seed must be treated with Concep III safener when using metolachlor, acetochlor or dimethenamid-P.  

All of the different environments and circumstances that determine if and how any given herbicide can be used cannot be discussed here. Consult the product label for the specific use of each herbicide.