Ontario Grain Farmer - September 2021

22 HERBICIDES APPLIED TO soybeans pre-harvest are not a weed control cure-all, but they can help ensure a more efficient end to the growing season when weed pressure is high. Mike Cowbrough and Horst Bohner — weed control and soybean extension specialists with the Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA) — say applying specific herbicides days or weeks ahead of harvest can benefit fields planted with both conventional and glyphosate- tolerant varieties. They describe these benefits in a Field Crop News article, “Crop quality often increases because the harvest timing after the field Pre-harvest herbicides for soybeans BEST MANAGEMENT PRACTICES Matt McIntosh OVER 90 PER CENT OF THE PODS HAVE CHANGED COLOUR EVEN THOUGH SOME LEAVES REMAIN ATTACHED. THIS FIELD COULD NOW BE SPRAYED WITH A PRE-HARVEST HERBICIDE. PHOTO COURTESY OF MIKE COWBROUGH. has been sprayed is more predictable, and therefore harvest schedules can be better managed. Most importantly, a pre-harvest burndown will dry down weeds, making combining easier and cleaning up the field for winter wheat planting.” The drier plant matter more easily passes through the combine, lowering seed losses and reducing seed staining. Pre-harvest herbicides, do not speed up soybeanmaturation nor reduce dry-down times, but they will speed up harvest timing by one to three days since remaining leaves and green matter drop and dry more quickly. WHAT IS THE TARGET? Using the right product for the weeds present, and spraying at the right time, is critical. Perennial weeds start moving more energy to root systems as the weather cools. When applied at this time, glyphosate can similarly enter root systems at a greater rate, making the active ingredient an effective control for overwintering weeds such as Canada thistle, perennial sow thistle, dandelion, and others. Annual plants are a different beast. Though still effective, glyphosate’s comparatively slow speed of activity means it is often accompanied by more fast-acting products (such as carfentrazone in Aim, saflufenacil in Eragon LQ, and diquat Reglone). Cowbrough and Bohner say such tank-mixes are not ideal for combating perennials since fast ingredient activity can impede glyphosate from moving within the plant, thus reducing the effect of systemic activity. Not harvesting before designated interval periods is critical as well. The application to harvest interval for glyphosate, for example, is one week. Cowbrough and Bohner add Eragon LQ has a three-day interval, although the realities of weed dry-down time mean harvest readiness typically occurs at or after a week. Agronomy