Herbicide Uptake and Timing: Why They Matter

When choosing grassy weed herbicides, consider how time of application and site of uptake influences weed control

Most farmers seek out the herbicide that delivers the best control under all possible environmental and application situations. In seeking grassy weed herbicides that work the best, understanding competitive effects and how herbicides are taken in by the weed (the site of uptake) sheds light on how to get the best control in wheat.

Looking at time of application, Kirk Howatt, weed researcher and assistant professor in the Department of Plant Sciences at North Dakota State University at Fargo, says that weed competition depends on species and time of emergence. He says wild oats are much more competitive than green foxtail.

“Wild oats are a very early emerging weed. They grow quickly and early in the season under cool temperatures, much like wheat or tame oats,” explains Howatt. “Under moderate to heavy infestations, earlier control is better.”

Green foxtail, on the other hand, is less competitive early in the season, because it germinates later under warmer conditions.

NDSU research shows that time of removal – that is, the point at which wild oats are eliminated as a competitor in the developing wheat crop – is critical. Howatt says that if wild oats emerge at the same time as the crop and are removed at the three-leaf stage, yield loss in wheat ranges from eight to 15%. If the wild oats are left until the five-leaf stage, yield losses rise to 25 to 35%. And if delayed until the flag leaf stage, wheat yields are cut by 65 to 75%.

That’s a huge range in loss potential, related to application timing.

Howatt suggests that for moderate populations of wild oats, control by the third to fourth leaf is necessary to capture most of the wheat crop’s yield potential. Ideally, the earlier the better.

“Very few wild oat herbicides have soil activity, so that makes timing difficult. If you spray at the one-and-one-half to two-leaf stage with a herbicide that does not have soil activity, then you may have to go back later to control additional flushes and that can get very expensive,” he says.

Site of uptake can influence herbicide selection
That’s where understanding site of uptake can help farmers choose wild oat herbicides. Some products are absorbed only through the leaves, while a few products are absorbed through the leaves and through the roots.

“Certainly, the way a herbicide is taken up by the weed is important,” says weed scientist Eric Johnson with Agriculture and Agri-Food Canada at Scott, Saskatchewan. “With (grassy weed herbicides), a lot of the weed control comes primarily from contact with the meristems (growing points), especially the ACCase inhibitor herbicides.”

Johnson says that some grass herbicides do not translocate readily within the plant, and that is where differences in performance may emerge. He explains that these herbicides must hit the growing point of the weed to get the best control.

“Some herbicides have more limited translocation than others. That’s where application parameters become more important in control,” says Johnson. “If we are going to see a problem with ACCase inhibitor herbicides, we start to see it with low water volume and coarser droplets.”

Some ALS inhibitor herbicides have both foliar and root uptake. Everest and Assert are examples of products taken in by the leaves and also have soil activity, meaning they are taken up by the roots. Brent Petersen, Technical Service Representative with Arysta LifeScience, says that Everest doesn’t seem to be affected by environment and application parameters as much as ACCase inhibitor products. He says that flexibility comes from the dual sites of uptake, which pays off in years where growing conditions are not conducive to rapid weed growth or when good conditions cause multiple flushes.

“If a herbicide has several sites of uptake, it will be able to perform under a wider range of application or environmental conditions,” says Petersen. “We’ve seen that where several flushes of wild oats and green foxtail can be controlled by the soil activity and that can increase your application flexibility.”

Greater application flexibility with multiple sites of uptake
In research trials at AAFC Saskatoon, Research Scientist Tom Wolf along with Eric Johnson at the Scott Research Farm investigated how droplet size and water volume affected grassy weed herbicide performance.

Three water volumes of four, eight and 12 gallons per acre were applied through medium, coarse and very coarse droplet nozzles. The researchers were trying to find out where weed control drops off when increasing droplet sizes (to reduce drift) and while lowering water volumes (to increase application efficiencies).

The researchers found that products with dual soil and leaf uptake were better able to perform under a wider range of application variables. In the trials, ALS inhibitor products had consistent and high wild oat control at all water volumes and droplet sizes tested.

In the first year of trials, rain moved the ALS inhibitor herbicides into the root zone. Coarse sprays and lower water volumes did not affect ALS inhibitor weed control at full label rates. “Is it the result of soil uptake? We didn’t assess that, and lots of other factors could be involved,” says Wolf. “We do know, though, that ALS inhibitor herbicides produce a different response to application parameters, if soil type and crop rotation allow a producer to use them.”

Wolf notes that without rainfall soon after application, the ALS inhibitor results weren’t so pronounced in the second year of trials.

Several ACCase inhibitor herbicides were also included in the research. Both Assure II (quizalofop-p-ethyl) and Discover provided very good oat control at recommended label rates and control did not tend to vary with water volume for medium and coarse sprays. For Assure II, application in a very coarse spray resulted in reduced oat control at all three water volumes, with the greatest loss at the lowest water volume.

Discover had decreased control when moving to very coarse sprays at the lower water volumes. At the highest water volume and very coarse droplets, weed control was still acceptable but tended to be more variable than it was with the finer sprays.

One possible reason for the reduced control with ACCase inhibitor herbicides at very coarse droplets and lower water volumes is that targeting the plants becomes more difficult. With the vertical orientation of the grassy weed, getting larger droplets to stick is harder to achieve.

Petersen says that dual herbicides with both foliar and root uptake can help increase weed control and allow better performance under tougher application parameters. However, he says farmers should still focus on good weed coverage. Poor spray patterns and poor coverage may still result in poor weed control, even with dual herbicides. “We are seeing a lot of high clearance sprayers using coarser sprays and lower water volumes with good results,” he says, “but you need to be careful. We need to better understand how coarse droplets affect soil activity and root uptake.”

The NDSU Weed Science web site has links to more information, including the annual weed control guide, with information outlining wild oat herbicides and their recommended use and timing. Go to www.ag.ndsu.nodak.edu/weeds.