Post-emergent Herbicides – Timing is Everything

Timing a post-emergent herbicide application is both art and science. If you wait until all the weeds emerge and all the flushes flush, you'll lose yield. If you go in too early and spray, your herbicide will run out of gas and give later-emerging weeds the opportunity to eat your yield.

About four weeks after crop emergence, the window closes for yield protection with post-emergent herbicides.

"There are really two issues here," says Trish Meyers, Agricore United ACE specialist in Saskatoon, Saskatchewan. "The first issue is timing the crop. The second is matching the crop stage to the grassy and broadleaf weed stages. It isn't always possible, but usually the best advice is to spray early. Weeds are easier to control when they're small."

Weed specialist Brian Hunt with Manitoba Agriculture in Carman says timing requires vigilance—don't spray too soon. "It may be too early if the most advanced weeds are only in the 2-leaf stage. An application at that stage would kill those weeds easily, but the weeds that emerge later could still reduce crop yield—depending on the crop stage and its vigor.

"It's a trade-off," Hunt says. "I would wait a week and make the herbicide application in the 2- to 4-leaf stage."

If weed stages aren't cooperating, a split application can work wonders. "Sure, go in twice, but don't exceed the recommended rate in total," he advises.

"A lot of top growers actually spray flax two or three times with reduced rates," he says. "They'll get the first weed flush with perhaps a one-third application rate then come back in two weeks with a two-thirds rate to get the broadleaf weeds. Flax isn't a very competitive crop."

Weed Spectrum

Meyers observes: "Often the producer wants to go in early to control wild oats, but doesn't have enough broadleaf weeds yet. During the wait, the wild oats grow. We recommend spraying out the wild oats first; then going back in later to spray the broadleaf weeds.

"It is very rare that the crop, the grassy weeds and the broadleaf weeds are synchronized so perfectly that the producer can spray once. Application timing is a real challenge.

"If grassy weeds are one-leaf stage ahead of the crop, they're much more competitive than if they're one-leaf stage behind the crop," says Meyers.

She adds one exception to the spray-early advice: "You really can't spray early if you're using 2, 4-D or MCPA. The cereal needs to be at the 3- to 4-leaf stage before you can go in to spray with those products. Every herbicide is different."

An economic threshold chart for spraying can help. Basically the chart helps identify a break-even point between weeds per square meter (or per square foot or yard) and spraying costs. Economic threshold charts for grassy weeds are available through provincial and state extension agriculture departments as well as many retails.

Wild Cards

Temperature is a wild card in the post-emergent application process. "You really don't want to spray certain products over 27 degrees C (80 degrees F). You have to watch that your humidity isn't too high and that the winds are down," says Meyers.

Your weed spectrum will determine your post-emergent product. "You want to be very sure you're spraying the right product for the right problem," advises Brian Hunt. "The chemistry you need to control weeds in the aster family like thistles and sunflowers is entirely different from what you'd use to control green smartweed, lady's thumb, redroot pigweed or the grasses. The only way to know your current weed spectrum is to scout."

Herbicide Lifespan Extended by Dry Conditions

Dry conditions lead to the worst cases of carryover. The microbes that normally break down the herbicide can't do their work without both moisture and warmth. If the window for breakdown is missed during the summer, the herbicide can be viable 12 months later when the new crop is being seeded.

The lifespan of some herbicides is closely linked to the soil pH level. In normal pH conditions, the breakdown is quick and predictable. In other pH conditions, it may be extended. Regardless of conditions, always check the individual product label for recropping restrictions.

If in doubt about carryover, request a lab to perform a plant bioassay.

Agricore United's Meyers says, "The lab will grow sensitive crops in your sample and let you know if you're at risk for carryover. If you're a direct seeder, get a sample 3 inches deep. If you're a conventional-till grower, take a 6-inch sample. The lab will require at least 4 pounds of soil. An untreated sample from one of your fields will be an asset for comparison."

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