Pre-harvest herbicide and desiccation options for straight-combining canola: Effects on crop dry-down and seed quality

Project Summary

With improved genetic pod-shatter resistance and increasing producer confidence in the practice, straight-combining canola has become increasingly common in western Canada. Several agronomic questions have arisen with this dramatic increase in uptake, including whether pre-harvest herbicide or desiccant applications are necessary and which options are most effective.

Pre-harvest options were evaluated for both LibertyLink (glufosinate-tolerant) and Roundup Ready (glyphosate-tolerant) canola. Field trials were completed in 2017, 2018 and 2019 at four locations – Indian Head, Melfort and Scott in Saskatchewan and Melita in Manitoba. For LibertyLink canola, the pre-harvest options were glyphosate (890 grams of active ingredient per hectare), saflufenacil (50 g AI/ha), glyphosate plus saflufenacil (890 plus 50 g AI/ha), and diquat (40 g AI/ha). For Roundup Ready, the pre-harvest options were similar except glyphosate (applied alone) was replaced with glufosinate ammonium (408 g AI/ha) which, it should be noted, is not a currently registered pre-harvest option for canola.

Various data were collected but the most important were whole plant and seed moisture content at harvest time, seed size and per cent green seed.

Factors to consider when choosing an ideal pre-harvest option include:

• specific herbicide tolerance system
• relative crop dry-down benefits of
• the various options
• specific crop stage
• anticipated time until harvest
• weed control requirements
• cost

With high variability resulting from differences in environment (i.e. weather), timing of operations, and methods, the response data were not combined across site-years for analyses.

Results

Whole plant dry down is an important measurement, and in this study the effects of the treatments were not always consistent. Glyphosate applied alone reduced moisture content in LibertyLink (LL) canola 67 per cent of the time (from 29 to 24 per cent across all site-years) while glufosinate ammonium reduced plant moisture content in Roundup Ready (RR) canola 45 per cent of the time (from 28 to 24 per cent on average).

When saflufenacil is applied alone, it significantly reduced whole plant moisture content relative to the control at 33 per cent of the site-years, from approximately 29 to 27 per cent when averaged across all 12 site-years and both canola herbicide systems. (Results are given for saflufenacil alone in order to keep the effects of glyphosate and saflufenacil separate – for LL canola in particular – and the results are averaged across both RR and LL canola.)

When tank-mixed with glyphosate, the effects of saflufenacil on crop dry-down were similar to when this product was applied alone for RR canola and usually similar to glyphosate applied alone for LL canola. While it was relatively rare that saflufenacil plus glyphosate provided a measurable benefit over glyphosate applied alone in LL canola, this occasionally did occur (i.e. Indian Head 2019).

Diquat provided the most consistent dry-down benefits. It reduced plant moisture content at 83 per cent of the site-years, from 29 to 22 per cent when averaged across herbicide systems and site-years.

With regard to seed quality, impacts on seed size were infrequent and inconsistent. In cases where they did occur, it was presumably due to applying the treatments too early. Green seed was most commonly impacted by diquat – applying this product too early could result in dramatic increases in green seed.

Researchers note that individual results are likely to vary based on cultivar, crop stage and environmental factors such as soil moisture and weather leading up to and after applications.

Finally, while the researchers frequently observed benefits to spraying, they note that not applying a pre-harvest herbicide or desiccant should be considered a potentially viable option, especially for early seeded, reasonably uniform and weed-free fields where a hybrid with good pod shatter tolerance is being grown.