Bill Williams, Daniel Stephenson and Donnie Miller

In Louisiana, most row crops are produced on “stale” seedbeds, which means the fields are prepared in the fall, allowing vegetation to cover the field over the winter and help prevent soil erosion. In the spring before planting, the farmer treats the field with herbicide to control the weeds. More than 20 winter weeds are commonly found in fields before planting, and most are easily managed with a herbicide treatment. But others, such as henbit, ryegrass and cutleaf evening primrose, can be difficult to manage. Henbit is becoming one of the most challenging weeds to control before planting corn, soybean and cotton in northeast Louisiana.

Historically, the herbicide glyphosate combined with 2,4-D has formed the backbone of preplant weed control programs in Louisiana. Unfortunately, combinations of glyphosate and 2,4-D are not effective at controlling henbit. Residual herbicides such as Goal (oxyfluorfen), Valor (flumioxazin) and Resolve (rimsulfuron) have been used to improve henbit control. In 2008 and 2009, however, there were problems with re-growth indicating poor initial weed control.
 
One theory on why henbit has become a greater problem was that the presence of rust, a fungal disease that develops on henbit, reduces herbicide efficacy. However, research indicated that glyphosate plus 2,4-D performed better on diseased henbit than on henbit without rust.

Another theory tested was that henbit is more difficult to control after it flowers. Results from experiments were mixed. Research showed that fall applications of herbicides prevented re-growth when applied to henbit less than 2 to 3 inches tall.

The effects of adjuvant systems and varying rates of glyphosate plus 2,4-D also were evaluated. Adding a surfactant appeared to help but not enough to control henbit. Additionally, reducing glyphosate and 2,4-D rates below 1 pound per acre reduced henbit control.

Still in 2009, glyphosate plus 2,4- D at 1 pound of each per acre resulted in less than 60 percent henbit control. While products such as Goal, Valor and Resolve provide excellent residual henbit control, they do not appear capable of controlling large henbit plants.

Poor henbit control is not a statewide problem yet, but it has increased from a few isolated cases in Tensas Parish in 2008 to widespread problems in Concordia, Tensas, Madison and East Carroll parishes in 2009 and in Catahoula Parish in 2010. The reason for increasing problems with henbit is still unknown. It is suspected that henbit populations are becoming more tolerant to glyphosate plus 2,4-D. At the Northeast Research Station, henbit control from glyphosate plus 2,4-D averaged 80 percent five years ago but less than 30 percent – and often zero – in 2010. Henbit samples from other parts of the state and outside the state are needed for further testing.

Without help from glyphosate plus 2,4-D, residual herbicides like Valor, Goal and Resolve will not consistently control large henbit plants. LSU AgCenter weed scientists are evaluating programs for managing henbit but have not been able to identify a suitable program capable of consistently controlling henbit and other troublesome weeds such as cutleaf evening primrose. For example, programs based on glyphosate plus the herbicide Firstshot appear to be working well for henbit but often do not control cutleaf evening primrose. At this time, AgCenter data suggest that 2,4-D rates should not be reduced below 1 pound per acre. This is especially true in areas were glyphosate plus 2,4-D and a residual herbicide still work.

In fields were henbit control has not been satisfactory, fall or early-winter herbicide programs will work best. Programs based on glyphosate co-applied with Firstshot and a residual herbicide in late December or January have performed well. The key to these programs is to make the applications while cutleaf evening primrose and henbit plants are small. 2,4-D can be used to clean up escaped primrose 30 days before planting corn, cotton or soybean. The herbicides Ignite (glufosinate) and paraquat are effective at cleaning up any henbit plants that escape before planting.

Programs other than traditional glyphosate- based ones, such as those based on Ignite and paraquat, are being considered, but information on other weeds is limited.