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 herbicides labeled for preplant or preemergence use in wheat: Amber, Finesse, Maverick, Olympus, and Pre-Pare. Each of these products is an ALS-inhibiting herbicide with residual activity.
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 labeled sulfonylurea herbicides at the time of the final burndown application can give residual control of susceptible broadleaf weeds and suppression of cheatgrass species without having to make a separate application later in the season.
Preemergence treatments in wheat can be inconsistent in effectiveness. They require rainfall to be activated. If weeds and cheatgrasses 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 and cheatgrasses emerge, control or suppression can be good. When used as a preemergence application, Finesse or Amber probably provide the best chance of providing cheatgrass suppression. All else being equal, each of the herbicides labeled for preemergence applications will work best as fall postemerge treatments.
The labels of these herbicides differ somewhat in what is allowed with a preemergence application.
Finesse allows for a higher use rate when applied as a 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 followup 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 as a 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.