How early is too early for weed spraying?

With an early spring in some parts of the Prairies, growers and agronomists have been asked “When is too early to spray weeds?” Spraying can start under the following conditions:

1. Weeds are growing. Winter annuals and perennials can look green but they may not be actively growing. Look for fresh lighter-green growth and signs that the plant is perking up — literally. Weeds still flattened by snow are probably still dormant.

2. Weeds are present. A few weeds here and there may not justify a really early spray, but the earlier that weeds emerge prior to the crop, the more competitive they will be against the crop and the more yield loss they will cause. It might be better to apply two lighter rates for two flushes than one big dose later.

3. Nights are warming. Wait until overnight lows are 5°C or higher.

4. Days are warm. If a warm day (10°C or higher) follows a warm night and it looks like conditions are trending this way.

5. It’s sunny. This will help herbicide uptake and efficacy.

In these conditions, an early spray is often the right choice. Winter annuals, once they start growing, can fairly quickly get to a size that makes them difficult to control. Herbicide rates may need to be bumped up anyway. Early spraying also stops these weeds from taking up valuable moisture and soil nutrients.

Even if seeding is two or three weeks away, an early pre-seed burn is recommended. A second pre-seed burn, if needed, is better than waiting to capture any new weeds or regrowth in crop.

Keep in mind that tank mixes are recommended at any time, especially with a pre-seed burnoff.

No adjuvant has been invented that will fix cold! You will often hear the promotion of “fully loaded” glyphosates but this is more of an American concept where glyphosate doesn’t have the full load of adjuvant. Pretty much all of the glyphosate sold in Canada comes with sufficient adjuvant — unless applying at less than 180 gae/acre. And most have similar “tallow amine” adjuvants to the Monsanto products, which have been shown to be the best performing adjuvants for glyphosate. The only thing an adjuvant such as LI700 will contribute above 180 gae/acre is drift retardant activity, done by evening out the droplet size. Note, there is such a thing as “over-surfactanting” a spray solution to the point that the spray is not retained on the leaves and simply runs off.

Further reading:

Use multiple modes in the pre-seed window
Big cleavers need pre-seed control :