A pre-seed burn down is often the first pass on a field. Some may overlook the importance of getting a strong burn-down, but in reality it can be a huge detriment if some components get overlooked. And if you’re going to do a job, you should do it well.
Why does a pre-seed burn-down matter? A crop that is being out-competed by weeds (they did have a head start, after all) is losing access to moisture, nutrients, and sunlight and your next sprayer pass may be too late to help the crop out. Are you willing to risk a drop in yield because of rushing ahead of the growing season? Here are some things to watch as you make that first sprayer pass:
Water Rates/Quality: Time is of the essence during seeding and oftentimes the first component of spraying to take a hit to save time is water rates. The recommendation for many products on the market is 5 gallons per acre and this should be stuck too as much as possible. With some products there is actually a call for 10 gallons per acre. Now, I rarely see anyone choose to go that high, but with some products (eg: contact pre-emerge herbicides like Aim from Nufarm, Heat from BASF, or bromoxynil-based products) there may be value in taking the time to haul that extra water. Get out in the field to determine the weed severity and identify if you are willing to sacrifice time or weed control — because that’s the trade-off.
Lastly, quality of water can hinder herbicide performance. Talk to your agronomist about getting your water tested, which products are susceptible to hydrolysis or tie-up due to high bicarbonate levels, high pH etc. and what options exist there to combat this. (Here’s a handy article discussing this.)
Weather/Seeding: The spring can be unpredictable and temperatures can drop well below the threshold for which metabolic activity of a plant needs to occur at for optimal weed control. If the weeds aren’t actively growing they will not take in the lethal dose of herbicide. After a lighter frost, say -1 degrees C for a couple hours, you can get away with going the next afternoon once temperatures have reach 10 to 12 degrees C for a couple of hours. For colder temperatures assess the damage of the weeds and adjust from there, in some situations you will need 24 hours or more to get those plants actively growing again, especially perennials. Lastly, I get asked every year how long to wait after spraying until seeding, in most instances 24 to 72 hrs is ideal, especially under cooler situations when there are perennials like dandelions involved. If your main targets are annual weeds then roughly 4 hours in good conditions is likely sufficient.
Two Modes of Action, Minimum: With the increasing risk of glyphosate resistance across the Prairies we are in danger of losing activity of one of the most effective herbicides ever produced. Make sure you are identifying a product that is registered to be applied in front of your intended crop and tank mixing it with glyphosate, this helps fight resistance and will increase efficacy of the application. Be sure you are using a healthy dose of glyphosate as well, the old 1/2 L equivalent (180gai/ac) of Roundup shouldn’t always be the default rate, be sure you know what weeds you are targeting and their growth stage so you can make the call as to whether a bigger dose is required to achieve the desired results.
Ensuring you have sized up the situation and taken these things into account (and others!) is going to be the key to having a strong pre-seed burn-down this spring.
Shane Thomas post an agronomy column every two weeks. Click here to see past articles!
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