Rice Weed Control 101

Delayed Pre "Trick" Programs For Rice Weed Control

Andy Kendig, State Extension Weed Specialist

Printed/Posted with permission of Mid America Farmer Grower

You may have heard about low-cost "delayed pre" weed control treatments with Prowl, Bolero or Facet. This is a very special application that is unique to rice. Delayed preemergence has a fairly strict definition: It is a 7 to 10 day delay in the application of a residual type herbicide. DURING THAT DELAY A SIGNIFICANT RAINFALL OR FLUSH MUST HAVE OCCURRED AND SEED MUST HAVE IMBIBED WATER. If the seed has not imbibed water, Prowl and Bolero can cause significant damage to the rice as it germinates and emerges. Basically the mechanism is if the seed imbibes only water it's ok, but if it imbibes water which contains freshly applied Prowl or Bolero the herbicide will harm the small, fragile plant. Facet is not harmful in a true preemergence application, but is promoted for delayed PRE application because it appears to work better in this timing.

Delayed PRE has one other important consideration. There shouldn't be much grass (or it should be VERY small). Prowl has very little activity on emerged grass, Bolero can kill barely emerged grass and Facet can kill grass with about 1 leaf and 1 leaf only. If the grass has any significant size you should tank mix a propanil based product. In fact, Stam increases the flexibility and this is why Stam plus these residual herbicides are more popular than true delayed PRE programs.

Compared to typical rice herbicides, Prowl is a bargain and is probably the most popular delayed PRE herbicide. A common weed control program uses a stale seed bed with Prowl + Roundup applied after planting, but before rice emergence. The only limitations are that broadleaf weeds will be released and that Prowl can fail is there is too much rainfall or flushing or if the flood establishment is delayed. You will have to make another application for broadleaf weeds; however a number of low-cost herbicides are available.

Bolero delayed PRE is a unique weed control program. Bolero requires wet conditions for optimal activity. But there are a few Missouri growers who use the Bolero delayed pre, flush regularly and achieve good weed control. Bolero is a very good sprangletop herbicide, controls most other grasses and also controls a number of aquatic weeds.

We have had good barnyardgrass control from Facet in true PRE-immediate post -plant applications as well as delayed pre applications. The major limitation with a Facet delayed pre program is sprangletop. Facet has virtually no activity on this weed.

Finally, to really confuse things- it's popular to mix delayed PRE herbicides together in any possible combination. Facet or Bolero mixed with Prowl gives you a good herbicide if it's dry and a good herbicide if it's wet. Prowl or Bolero with Facet will take care of Facet's sprangletop weakness. Facet with Bolero or Prowl can extend the residual weed control.

Delayed PRE programs are very timing sensitive. They are also complicated by the fact that you have to wait not just for a rain or flush, but THEN YOU HAVE TO WAIT FOR THE GROUND TO DRY if you plan to use ground equipment. In reality, you should really plan on having delayed PRE herbicides applied by air. Since you're probably saving herbicide costs on a delayed PRE program you can afford to use an airplane. As with all rice weed control programs it is always saves money in the long run to do things right up front- even if you spend a bit more.

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