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Herbicide timing in Clearfield lentils

Figure 1: Relative lentil yield showing the critical weed-free period Critical period of weed control in lentil. Weeds must be controlled from the five to the 10 node stage to prevent yield loss. Source: University of Saskatchewan.

The introduction of Clearfield (imidazolinone) lentil has widened weed control options beyond Sencor, trifluralin and Edge. However, the use of Odyssey, Odyssey DLX and Solo (IMI herbicides) in Clearfield lentils does not mean improved weed control is a slam-dunk. Weeds still need to be targeted at the proper crop growth stage in order to generate the highest yields. “As a general rule, controlling weeds early is a good practice, but if you go too early, later weeds can impact on yield in a crop like lentil, which isn’t very competitive with weeds,” explains Steve Shirtliffe, at the Department of Plant Sciences at the University of Saskatchewan. “Many growers are used to applying Sencor early for the best crop safety and weed control, but our research showed that very early timing isn’t the best for Clearfield lentils.”

Clearfield herbicides control both broadleaf and grassy weeds with a one-pass application. The timing of application differs from conventional herbicides such as Sencor. With conventional lentil, the timing for application of Sencor is in the one- to four- (above ground) node stage. Clearfield herbicides may be sprayed from the one- to six- (above ground) node stages. Some Clearfield herbicides are also designed to provide a residual effect and control flushing weeds for an extended period of time after application.

Those differences motivated Shirtliffe, and graduate student Leah Fedoruk to conduct two experiments at two locations in Saskatchewan, at Vanscoy and Saskatoon. They based their experimental design on the concept of Critical Period of Weed Control. This is the period of time that the crop must remain weed-free to avoid yield loss. 

For highest yield response in Clearfield lentil, apply herbicides at the five- to six-node stage.  Photo by Bruce Barker.

Longer weed-free period results in higher yields
The beginning of the critical period of weed control is an indication of when weed control needs to commence to ensure that unacceptable yield losses do not occur. Shirtliffe and Fedoruk determined that weed control should commence for lentil by the five- to six- node stage to avoid yield loss. Spraying earlier than this is okay but later flushes of weeds can emerge and cause yield reductions. Delaying spraying until after the five-to six-node stage resulted in lentil yield loss due to the competition from the weeds. As a result, the researchers determined that herbicide application at the five- to -six node was optimal.

However what was unknown was how long the lentils have to remain weed-free after herbicide application in order to prevent yield loss. This period in which the crop must remain weed-free in order to prevent yield loss is known as the critical period of weed control concept. Not surprisingly, the longer the crop remained free of weeds, the higher the lentil yields. What is interesting, though, is the very large impact that grassy and broadleaf weeds had on yields. For example, if the lentil crop was kept free of weeds only until the three-node above ground stage, yield was only 20 percent of the weed-free check. Generally, the treatments that remained free of weeds until the 10-node stage had similar yields to the weed-free control. Fedoruk explains in her research report that, presumably, weed growth after the 10-node stage will not affect lentil yield because either weed emergence has ceased or second flushing weeds that grow are too small to cause yield loss.

“What we were trying to see with this experiment was what effect late flushing weeds had on lentils. Lentil isn’t very competitive, and the crop canopy usually doesn’t close until around the 10-node stage, so second flushes that occur around the five- or six-node stage still appear to have an impact on yield,” explains Shirtliffe.  

Residual control can help
The second experiment compared application of Solo, Odyssey and Sencor/Poast at an early and late stage (two- or six-node stage) compared to a weed-free and weedy check. Solo does not have residual, Odyssey has some residual weed control, and Sencor has limited residual control. The results from both locations indicated that there was no clear application timing effect on lentil yield. However, the late herbicide application resulted in less weed biomass than the early application. Sencor/Poast was included as a comparison to the traditional approach to weed control, although producers would not use this method in Clearfield lentil.

Based on the results from 2006, the late Sencor + Poast application provided a high yield response at one location, but at the other location, yield was only 15 percent of the weed-free check. This result indicates that the time of emergence and weed pressure can impact on the Sencor/Poast control. However, when the 2006 and 2007 results were combined, Sencor + Poast resulted in better yields.

Overall, Shirtliffe and Fedoruk say that the best approach under normal to wet conditions is to control weeds in Clearfield lentils at the five- to six-node stage with Solo or Odyssey. This use of Odyssey with extended residual weed control in a late application provided weed-free conditions for the duration of the weed-free period. Odyssey and Odyssey DLX are registered for application from the one- to six-node stage. 

Solo herbicide, despite not having residual weed control activity, also performed well when applied at the six-node stage. “In a normal year, keeping the crop free of weeds, including the second flushes, until the 10 node stage will give the highest yield. After that stage, new emerging weeds don’t seem to matter.  So weed control around the five- to six-node stage is usually best, since Odyssey and Solo aren’t registered past those stages,” explains Shirtliffe.  “In a dry year, though, when weeds aren’t flushing, going a little earlier might help to reduce some of the weed competition. It depends on the year.”