Spraying in the rain is not advised. Product washes off before it gets absorbed. However, spraying just before a rain — allowing time to satisfy label requirements — can provide effective control, especially if rains are forecast for a few days and weeds are growing strong. Please read the Canola Encyclopedia chapter on weed management timing.
The rainfast requirement varies by product. Here are required intervals for each common product used in HT canola systems:
—Gylphosate-tolerant (Roundup Ready) canola. Check labels for specific glyphosate products. Roundup WeatherMax is rainfast within 15 minutes, which is the shortest interval of any HT canola product.
—Glufosinate-tolerant (Liberty Link) canola. With Liberty, rainfall within 4 hours may reduce control.
—IMI-tolerant (Clearfield) canola. Ares is 2 hours. Others are 3 hours. Tensile is 4 hours because of the clopyralid.
Two things to keep in mind:
1. Some pesticides (including herbicides) are limited to one application per season, even if washed off by rain. If efficacy is reduced due to spraying in the rain, options for second applications may be limited.
2. When dodging rains during weed management season, keep in mind that a sprayer sitting idle with product in the tank will increase the risk for product contamination. Read more.