Optimizing grass weed control in soybeans

The main active ingredients available for pre-emergence application are pendimethalin (Prowl 400), pethoxamid (Successor 600), and DMTA-P (Isard, under a 120-day exemption). Other substances, such as metobromuron or clomazone, can supplement these programs, but their contribution is limited to the rates that can be used on soybeans. Furthermore, certain technical constraints must be taken into account. Pendimethalin and metobromuron may pose a risk of reduced selectivity on well-drained soils when rates exceed 1.5 L/ha. 

For pre-emergence application, the use of Isard at 0.7 L/ha is a standard treatment for major summer grasses and offers a broad spectrum of control against certain broadleaf weeds such as nightshade or ragweed. Increasing the dose to 0.9 L/ha improves the consistency of control. Combining DMTA-P with Atic Aqua or Proman at 1 L/ha enhances weed control and provides initial control of goosefoot and knotweed. In more complex weed situations, particularly where bindweed is present, the pre-emergence partner’s rate should be increased to 1.5 L/ha, except on highly permeable or heavily compacted soils. Replacing the Isard base with Successor 600 at 1.5 L/ha is possible within this strategy but appears slightly less effective against foxtail and barnyard grass. Adding Centium CS at 0.3 L/ha improves the consistency of control.
  

Soybeans, which provide little ground cover early in the growing season, tend to become overrun with grass weeds. A post-emergence treatment is therefore often necessary to control these new weeds. Imazamox (Pulsar 40 or Davai), in addition to its broad spectrum of activity against broadleaf weeds, effectively controls certain summer grasses, unlike bentazone. If control is insufficient, a follow-up treatment with a foliar grass herbicide remains possible and effective. It is recommended to rotate modes of action to limit the risk of resistance.

Certain situations remain particularly complex. Perennial grasses consistently require the use of foliar grass herbicides, as pre-emergence solutions and imazamox are insufficient. Regarding winter grasses, and particularly ryegrass, the development of herbicide resistance makes the effectiveness of foliar grass herbicides and imazamox unpredictable. Under these conditions, current registered pre-emergence programs, such as pendimethalin or pethoxamid used alone, are clearly insufficient. The choice should be a DMTA-P formulation at 0.9 L/ha, possibly supplemented with pendimethalin or metobromuron.