Objectives
- Demonstrate the effects of pre-harvest herbicide and desiccant options for flax on seed and straw dry-down.
- Provide a forum for discussion on the potential advantages and disadvantages of the options evaluated with respect to both weed control and efficacy as a harvest aid.
Project Description
This project has demonstrated measurable benefits to using pre-harvest applications to enhance flax dry-down with some variation between the products evaluated but even greater differences in response across the environments under which they were tested
Indian Head was the wettest of the locations and actually received above-average precipitation during the 2021 growing season. While much of this precipitation came too late to truly benefit to the flax, it had a considerable effect on crop dry-down. Both glyphosate and diquat were effective in drying down seed and plant material but the specific nature of the responses to the two products differed. Diquat worked very quickly with striking differences noted as early as four days after application; however, under the wet conditions late in the season, a certain amount of re-growth occurred 14-21 days after application. Glyphosate was slower to take effect but still worked extremely well under the conditions encountered. Surprisingly, glyphosate was already having a visible effect at 4 DAA, albeit not nearly to the extent of diquat. By 14 DAA and, even more so at 21 DAA when the plots were harvested, visual differences between glyphosate and diquat had greatly diminished. With essentially no regrowth, glyphosate had actually dried both the seed and straw down to a greater extent than diquat; however, both products were highly effective in this regard. With the wet and cool finish to the season along with the lack of killing frost, the untreated control plots at Indian Head did not dry down well at all and were still green and wet when the crop was harvested 21 days after the treatment applications. This extended period of growth did result in a slight but significant yield advantage in the untreated control plots at Indian Head. It should be acknowledged that the treated plots, especially those treated with diquat, could have likely been harvested considerably earlier than they were at Indian Head.
At Yorkton, both glyphosate and diquat provided benefits in terms of improved seed and straw dry-down, but not to the extent observed at Indian Head. In particular, and attributable to the drier conditions and higher plant populations, the untreated control plots dried down much better at Yorkton than they did at Indian Head. While the visual ratings suggested that diquat may have started working more quickly, the later ratings and actual seed and straw moisture measurements revealed that it did not terminate the crop and dry it down to the extent achieved with glyphosate. It is likely that the weaker response to diquat at Yorkton was a function of both application timing and the environmental conditions after application. The treatments were applied on the morning of a hot, sunny day. Because diquat is activated by the sun, it is recommended to apply this product on cloudy days or in the evening to allow the herbicide to diffuse across plant surfaces prior to activation, thus ensuring more uniform and complete desiccation. It is also ideal to apply diquat when the longer-term weather outlook is for a warming trend and conditions will, in general, be conducive to drying. While the day after the treatment applications was hot (> 30 °C), much of the weather for two week period following the applications was relatively cool and wet. In contrast, at Indian Head, where diquat worked quite well, the treatments were applied late in the evening and at the end of cool, wet period with an extended stretch of warm, dry weather following the applications. Despite the weaker performance of diquat at Yorkton, it did provide significant seed and stem dry-down benefits in the end, just not necessarily as well as expected or to the extent of glyphosate.
Pre-harvest herbicides or crop desiccants are least likely to improve crop dry-down under hot, dry conditions where annual crops will often terminate and shed moisture reasonably well without being sprayed. Swift Current is in the dry Brown soil zone of Saskatchewan and, on average, is the driest of the regions where field trials were located. This was the sole location where we did not specifically measure benefits to the pre-harvest applications; however, high variability also limited our ability to do so. With less than 80% of normal precipitation and well-above normal temperatures, the conditions at Swift Current were not conducive to needing pre-harvest applications to assist with crop dry-down and this likely explains the lack of response to a large extent. The visual ratings confirmed that stem dry-down progressed steadily as the crop matured, regardless of variety or pre-harvest treatment and despite high variability.
In conclusion, this project has shown that whether or not a pre-harvest herbicide or desiccant application is likely to be beneficial will depend on the specific crop and environmental conditions leading up to and following application. Under low yielding, drought conditions with more dry weather in the forecast, the potential for realizing a benefit with respect to crop dry-down or harvestability is low, especially if it early in the fall with plenty of long days and time to complete harvest ahead. In contrast, if the weather is wet, stands are poor or uneven, and harvest will likely be delayed until late September or beyond, pre-harvest glyphosate or diquat can greatly accelerate crop dry-down leading to an earlier and easier harvest. Which of these two products is preferable will depend on several factors. Glyphosate has the advantages of being less expensive, providing excellent perennial weed control, and terminating the crop in a manner that regrowth will not occur even if harvest cannot be completed within a reasonable timeframe and wet conditions persist after the treatment applications. The disadvantage to glyphosate is that it often takes several weeks to thoroughly dry down physiologically mature crops and weeds and may not work consistently well if conditions are not conducive to herbicide uptake. In contrast, diquat, if used properly under favourable conditions, can rapidly dry down crop and weed material often allowing harvest to be completed within less than a week of application. The disadvantages to diquat are that it is generally more expensive, requires high solution volumes, will not provide control of perennial or grassy weeds, and, if wet weather persists after application, regrowth of both crop and weeds can occur while efficacy in general may be poor. If both perennial weed control and rapid crop dry-down are desired, there may be merit to utilizing both of these products with glyphosate applied first and following up with diquat in 5-7 days; however, this particular combination of treatments was not demonstrated.
