Was Dry Bean Weed Control Easier
in the Good Ol' Days?
It is not uncommon to hear growers complain that there are more weeds to contend with these days than 15 or 20 years ago. Dry bean producers and extension specialist across the central high plains appear to be seeing the same thing. It does seem to be more difficult to design a weed management program that provides acceptable weed control at a reasonable cost.
There are definitely new weed species that have become more prevalent. Good examples are toothed spurge (Euphorbia dentata), tall water hemp (Amaranthus rudis) and common water hemp (Amaranthus tuberculatus). These weeds are not controlled by many common weed management strategies and so their numbers have increased. This is called a weed shift and it can happen in two ways. This type of shift is called an inter-specific weed shift, which means there is a change in the composition of the weed community over time. The second kind of weed shift is called an intra-specific weed shift and this involves an increase in a subpopulation (called a biotype) of a weed species. These biotypes could be resistant to herbicides or could emerge later in the season, avoiding chemical and mechanical control. We are probably seeing the results of both types of weed shifts caused by applying similar selection pressures to weed populations over a long time period. This has resulted in the general observations that there are more weeds to deal with today than 15 or 20 years ago.
There are ways to reduce the selection pressure on weed populations since producers control decisions on crop rotations, cultural practices and herbicide programs. Each crop and cropping system can select for certain weed species so the more complex the rotation the less a single type of selection pressure will be applied. Tillage is an effective method of weed control, but the effects are temporary and may actually cause some weed species to germinate. The failure to control weeds during any part of a crop rotation can have significant long-term impacts. Seeds of many weed species can persist in the soil for 10 years or more making one failure a long-term headache. In Colorado and other western states, herbicide resistant weeds are very common. Field surveys indicate that 60% of kochia (Kochia scoparia) populations are resistant to Atrazine (photosynthesis inhibitor) or Raptor (amino acid inhibitor) or both. Producers need to select weed control strategies that combine or alternate herbicide modes of action in order to reduce the potential for intra-specific weed shifts. Herbicides with the same mode of action as Raptor can be used in a variety of crops so selecting alternative modes of action can be difficult.
Any strategy that makes the bean crop more competitive should improve weed control. Combining narrow row spacing with an adapted bean variety is one strategy to improve competitiveness. In this situation, variety selection is critical because narrow rows increase the potential for diseases like white mold. Inter-row ripping is another strategy that has improved bean competitiveness. Ripping improves root growth, reduces root diseases, increases water use efficacy, and improves nutrient utilization.
Producers should also remember that dry beans do not necessarily need to be weed free the entire growing season to provide acceptable net income. Field research examining weed competition in dry beans using time of removal experiments clearly indicates that dry bean yields will not be affected if fields are kept weed free for six weeks after planting. While this rule of thumb may apply to many weeds, most producers would say that for maximum bean quality hairy nightshade must be controlled for the entire growing season.
The difference between providing weed free conditions for six weeks versus weed free conditions for the entire growing season can be a significant amount of money and the extra cost may not increase net income. The best weed control program is one that provides sufficient weed control at the lowest cost and combines as many different control strategies as possible (chemical, mechanical, and cultural).
There have been very few recent changes in chemical weed control for dry beans. Raptor is the most recently registered herbicide for dry bean weed control. Raptor is closely related to Pursuit, but has considerably more grass activity and shorter rotational restrictions than Pursuit. Frontier was replaced by Outlook, a more concentrated formulation of the active isomer dimethenamid-p. Outlook remains the only herbicide that could be used as a layby treatment in dry beans. Layby is defined as a herbicide that is applied post-emergence (POST) to the crop, but pre-emergence (PRE) to the weed. Layby applications of Outlook could provide growers with a strategy to extend weed control later in the growing season. Season long weed control will require a combination of PPI or PRE herbicide applications combined with tillage and/or POST herbicide treatments. Cost per acre for this type of program could exceed $50/ac, while programs designed to provide six weeks of control would cost $17 to $27/ac. Producers need to have some idea about weed spectrum and severity when deciding where to plant dry beans and avoid fields that present a high risk for failure. Some options for chemical control are provided in Table 1.
Table 1: Some options for weed control in dry beans.
| Treatment |
Timing
|
Nightshade Control | General Comments |
| Sonalan + Eptam or Sonalan + Dual Mag or Sonalan + Outlook |
PPI
|
Would provide early but not late season nightshade control. | Programs would provide excellent early season weed control. |
| Eptam + ProwlEptam + Treflan |
PPI
|
Same | Same |
| Outlook (band) or Dual Magnum (band) + Cultivation |
PREPOST
|
Would protect the crop row from weed competition, and would provide early season nightshade control. | Least expensive program, requires good crop competition. |
| Outlook (band) or Dual Magnum (band)+ Cultivation + Outlook |
PREPOST LAYBY
|
Should provide extended nightshade control with layby application. | Band applications reduce herbicide costs, requires very clean cultivation, would not work well for kochia. |
| Sonalan + Eptam or Dual Mag or Outlook Raptor + Basagran |
PPI or
PREPOST |
Should provide season long control of hairy nightshade. | Expensive treatment that could include tillage before POST application. |
| Sonalan + Eptam or Dual Mag or Outlook Raptor + Basagran + Outlook and/or Select |
PPI or
PREPOST LAYBY POST |
Good option for fields with heavy nightshade pressure. | Very expensive treatment. Outlook should be applied no later than third trifoliolate. Should provide excellent grass, proso millet and sandbur control. |