A common question each year is: “What herbicides can I use on late-emerging weeds or weeds that were not controlled in earlier applications?” This can often be difficult to answer because of the limited number of products available for use in the late growing season.
I rarely recommend that growers use a reduced-rate herbicide program or increase a herbicide rate to the higher end of the labeled rate. Reduced-rate programs are useful, but oftentimes cost more money because more applications are needed over a growing season. Reduced-rate programs often have late-season weed problems that cost more to control. The new herbicides labeled in the past five to 10 years are not as rate reactive as older herbicides; therefore, increasing the rate may not increase weed control, but it will increase costs.
Making the right decision requires knowing the field’s history. Weed spectrums in a given field normally do not drastically change from one year to the next. It is a good idea to keep records of weeds in a field from year-to-year. If a problem occurred in a field one year, the following year a herbicide program can be selected to manage that problem early and not let it develop into a major problem. The best late-season weed control is to implement a preventative early season herbicide program.
Limited Late-Season Herbicide Options
All herbicides should be applied early in the growing season at the correct application timing and at the correct application rate. In these economically strapped times, it is tempting to wait to apply 2,4-D for the cost effectiveness of this herbicide. This is a viable option, and one I often recommend.
However, if adverse weather conditions occur, as was the case in 2004, the short 2,4-D application window may be missed. If this occurs, there are few options for the producer, and the cost of the products increases substantially. Most herbicides have a window of application from emergence to late-tillering or panicle initiation.
Permit is a herbicide that can be applied up to 48 days prior to harvest; however, Permit is not as effective on large broadleaf weeds compared with an early season application, but it can be used to suppress seedpod production of hemp sesbania and Indian jointvetch.
Storm and Ultra Blazer are labeled up to the early boot stage of rice, but weed size is important when applying these herbicides late season. The grass herbicides are labeled up to late tillering for Ricestar HT and 60 days prior to harvest for Clincher. Clincher has been successful at controlling grass weeds late season; however, it can be inconsistent if it is relied on to control large grasses. Do not apply Clincher or Ricestar HT late season if the weeds are under drought stress. I recommend a shallow flood at the time of application to obtain adequate control.
Uniform Stand, Surface Irrigation And Permanent Flood
Production practices can also help in controlling weeds before they become a problem and, in many cases, may be more economical. Start the season with a clean, well prepared seedbed. This can be accomplished by tillage or by an effective stale seedbed burndown program. If planting stale seedbed, the first application of a burndown herbicide should be four to six weeks prior to planting and may require a second application near planting. Plant the correct amount of seed as recommended by the LSU AgCenter to establish a uniform rice stand to outcompete the weeds for water, nutrients and sunlight.
A uniform stand can be effective in controlling weeds, such as ducksalad and the perennial grasses. Surface irrigate the field as needed to prevent drought stress and establish the permanent flood as soon as the rice is large enough to survive in the water.
Late-season weed control can be achieved, but it is oftentimes expensive and inconsistent. Research at the LSU AgCenter’s Rice Research Station has shown that weed control in the first three to four weeks after emergence is the most important time to achieve increased yields and prevent late-season weed problems.
Dr. Eric Webster is a weed scientist with LSU AgCenter. Contact Webster at firstname.lastname@example.org.