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 pre-harvest 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 and across the environments under which they were tested. Generally, pre-harvest herbicides or crop desiccants are least likely to improve crop dry down under warm, 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. Although Swift Current received above normal precipitation from May through July, August was extremely dry with only 3 mm of precipitation and temperatures that were well above average. This likely explains the lack of response to a large extent and the visual ratings confirmed that stem dry down progressed steadily as the crop matured, regardless of variety or pre-harvest treatment. In contrast, both Indian Head and Yorkton are in the Black soil zone where, on average, moisture tends to be much less limiting overall compared to the Brown soil zone. At these locations, total growing season precipitation was also well below normal, but the month of August was both cooler and wetter than Swift Current and the heavier soils also have much greater capacity to store water. At both Indian Head and Yorkton, we observed substantial differentiation in the stem dry down ratings between pre-harvest treatments and reductions in both seed and stem moisture content at harvest. Of the options evaluated, glyphosate applied alone was the least effective for drying the crop down but still worked quite well and provides excellent weed control benefits at a relatively low cost. Although doing so increases the cost of the pre-harvest application, tank-mixing glyphosate with saflufenacil accelerated stem colour change and led to lower stem moisture content at harvest than glyphosate applied alone at Indian Head. This treatment has similar weed control benefits to glyphosate on its own but, in addition to accelerating and potentially improving overall crop dry down, the tank-mix may also mitigate the risk of herbicide resistance development in certain weeds. Diquat was, by far, the most effective crop desiccant with respect to both the speed in which it took effect and the total level of seed and stem dry down achieved. The greatest downside to diquat is that is provides relatively little weed control, particularly for perennial weeds, and crop regrowth can occur under prolonged cool and wet conditions. Furthermore, applying diquat can make it more difficult to control perennials weeds later in the fall since it burns off top growth, thus removing the target for post-harvest herbicide applications intended to clean up fields for the following growing season. In terms of cost, diquat will generally be intermediate, more expensive than glyphosate applied alone but less than the glyphosate plus saflufenacil tank-mix. In conclusion, flax growers who are concerned about crop dry down and harvestability should consider pre-harvest herbicides or desiccants to help alleviate these issues. All of the options evaluated can potentially allow for earlier harvest, easier threshing, and perhaps an improved ability to chop straw if doing so is desired. The ideal product choice will depend on factors such as crop stage, location, specific environmental conditions, and the number and type of weeds that are present in the field. If the crop stage is relatively advanced, harvest in less than 10-14 days is expected, and there are few or no perennial weeds, diquat will allow for the earliest harvest and most thorough crop dry-down. In contrast, if perennial weeds are a concern, the crop is mostly close to the 75% boll colour change (as opposed to being well past this stage), and harvest can reasonably wait 10-14 days or longer, glyphosate or glyphosate plus saflufenacil might be more appropriate options. Doing so will require additional time and a larger monetary investment; however, growers who require both perennial weed control and the most rapid and thorough crop dry down possible might consider applying glyphosate at 75% boll colour change and following up with a diquat application after the glyphosate has been given sufficient time to be taken up by target weeds.
NOTE: Saflufenacil (Heat WG and Heat LQ) is lacking an MRL for China and the established MRL for the EU is too low for use as a desiccant in flax. The use of Heat WG and Heat LQ on a flax crop that is exported to the EU will result in the MRL being exceeded. As such, growers must refrain from using it without confirming with potential buyers that doing so will not limit opportunities to sell the treated crop.
