Preemergence herbicides with residual activity are used routinely in most crops, including corn, grain sorghum, and soybeans. They have not, however, commonly been used in wheat.
There are five ALS-inhibiting herbicides labeled for preplant or preemergence use in wheat: Amber, Finesse, Maverick, Olympus, and Pre-Pare. The newest product in the preemergence wheat market is Zidua. Zidua has a different mode of action than the other preemergence herbicides in wheat and should only be applied as a delayed preemergence or early postemergence treatment.
In wheat, preemergence herbicides are often used in no-till situations where they can be tankmixed with glyphosate during burndown applications just prior to or at planting. The addition of one of the preemergence herbicides at the time of the final burndown application can give residual control or suppression of susceptible broadleaf and grass weeds.
Preemergence treatments in wheat can be inconsistent in effectiveness. They require rainfall to be activated. If weeds emerge before the herbicide is activated, control may be poor, especially the grasses. But when there is enough rain to activate the herbicide before weeds emerge, control or suppression can be good. However, all else being equal, most of the herbicides labeled for preemergence applications will be most consistent when applied as fall postemerge treatments. The one exception is Zidua, which has minimal activity on emerged weeds.
The labels of these herbicides differ somewhat in what is allowed with a preemergence application.
Finesse allows for a higher use rate when applied preemergence than when applied as a postemergence treatment. This can provide for good season-long control of susceptible broadleaf weeds, unless they are ALS resistant. However, it does not allow for a follow-up postemergence treatment later with Finesse, although a follow-up treatment with Olympus or PowerFlex is allowed.
With Amber, the top-end of the range of rates allowed is a little higher for preemergence applications than with postemergence applications. As with Finesse, if Amber is used at the higher rates preemergence, producers cannot come back later in the season with another application of Amber, although a followup treatment with Olympus or PowerFlex is allowed.
With Olympus, the allowable rate as a preemergence application is 0.6 oz/acre, which is lower than the rate allowed if Olympus is used as a postemergence treatment. However, producers are allowed to follow up later with another 0.6 oz/acre of Olympus if needed.
Maverick has a single standard rate for all application timings. Pre-Pare is marketed primarily in the northern plains and has not provided very good preemergence cheatgrass control in research at K-State.
Zidua has a different mode of action than the other preemergence wheat herbicides and may be especially helpful to manage ALS-resistant weeds. Zidua is very effective for control of Italian ryegrass, but can also provide suppression of winter annual brome species and some broadleaf weeds. Zidua should not be applied until 80% of the germinating wheat seedlings have a shoot at least ½ inch long. It can also be applied early postemergence to wheat, but primarily has preemergence activity and generally will not control emerged weeds.
Application rates of Zidua range from 0.7 to 2 oz/acre depending on application timing and soil texture. Zidua should not be applied preplant to wheat and wheat should not be seeded more than 1.5 inches deep prior to a delayed preemergence application. Zidua can cause some temporary stunting of wheat if soils are excessively wet during the early seedling stages of wheat development.
Dallas Peterson, Weed Management Specialist