As most of you have heard by now, there is a new herbicide from Dupont that controls emerged Johnsongrass and pigweed in corn that is up to three feet tall. This material has been available in the rest of the United States for several years, however, California requires additional studies to be run which are taking two extra years to complete. This past January new legislation (AB771) was passed which allows interim registration of environmentally and economically desirable compounds while certain of these tests are still being conducted.
The new law has cleared the way for the California Department of Pesticide Regulation (CDPR) to work on their review of the product, which they are currently doing. CDPR has promised that the review will be done by June 1, this season. Once the review is completed and the material gets a clean bill of health, there is a 30 day posting period to allow time for public comment before the product can be sold. I have submitted economic and environmental justification for the director of CDPR to consider allowing sale of the product during the posting period because of the severity of the Johnsongrass problem in our area. So, assuming that no problems turn up in the review (none are expected) the latest we should get Accent is July 1, but it could be June 1 if they agree to allow sale during posting, and possibly even sooner if they were to finish the review ahead of schedule.
To help in your planning process for this season, here is a brief summary of how to use this product.
Accent works by stopping the weeds from making proteins needed for growth. Johnsongrass smaller than about 12 inches will stop growing and eventually die. Taller Johnsongrass does not usually die but stays the same size while the corn continues to grow. Corn plants have the ability to break down the herbicide before it causes damage. If the corn is under stress, this could interfere with that ability. Don't apply Accent to corn that is stressed from too much or too little water, prolonged cold temperature, or has been cultivated less than 10 days prior to the application.
Accent is very effective at controlling both seedling and rhizome Johnsongrass. In my trials, Johnsongrass rhizomes have not resprouted even though the top growth was three feet tall when sprayed and remained alive but not growing throughout the rest of the season. Many weeds besides Johnsongrass may be controlled by Accent but most must be quite small for best results. Pigweed less than four inches is readily killed, but tall pigweed may resprout from side branches. Watergrass is less susceptible and may recover in weak corn stands unless cultivated. Lambsquarters and velvetleaf are not controlled.
Accent may be used in combination with Banvel or Buctril but not with 2,4-D.
Accent should always be used with cultivation. Not only does this ensure the best weed control and corn yield, but reliance on only one of any kind of pesticide increases the chances of weeds (or insects or diseases) becoming tolerant to the material. Using cultivation to kill most of the weeds means fewer weeds need to be killed by the herbicide, and any escapes that are not in the row will be destroyed. Cultivation also improves control of hard-to-kill weed species and reduces competition from weeds that are stunted but not dead. Cultivation may be done either when the corn is small with Accent applied at three feet, or by treating the corn with Accent while it is very small and cultivating at layby.
If Johnsongrass populations are very high, it is important to reduce the weed competition early. One way to do this is to treat the corn with Accent while it is very small (about six inches high) and cultivating at 12 to 18 inches. Broadcast applications of Accent over small corn without cultivation may result in poorer weed control because tiny weeds sheltered under the corn leaves escape the spray and grow up to produce seed. In the worst case, if there is rain or other moisture soon after treatment, a new flush of seedlings will result and, if not cultivated, will result in an infestation nearly as severe as if not treated at all.
Another alternative is to apply Accent as a band in the corn row and later cultivate out the middles. This has the obvious advantage of saving at half to two thirds the cost of the material. Offset nozzles on either side of the corn plants should work better than a single nozzle over the row because it prevents corn leaves from shielding weeds and also keeps the material out of the whorl, avoiding the potential for injury. If only one nozzle is used over each row, it becomes especially important for the cultivation to throw soil around the base of the corn to bury small escaped weeds in the row.
The advantages of applying Accent early and cultivating later are:
Disadvantages of early application are:
Where Johnsongrass is the only weed problem, another option is to cultivate when the corn is small (about 12 inches) and follow with an Accent treatment with drop nozzles when the corn is two to three feet high. An early cultivation is essential to minimize early weed competition and also to clear out between the rows to prevent weeds from interfering with the drop nozzles. In several local trials, we added Comite to the Accent plus crop oil concentrate and applied the combination through drop nozzles. This treatment gave excellent control of both mites and weeds. Applying the miticide with drop nozzles at three feet gave better mite control and a 1.4 ton yield advantage over applying the miticide at two feet. Registration of this combination is currently being discussed; consult your chemical supplier for registration status before making this treatment. Extreme care must be taken to keep the spray material out of the whorl of the plant to avoid injury. Be especially careful of low areas in the field where the corn may be shorter than the drop nozzles.
I have had good results from early cultivation followed by a late Accent treatment even when the Johnsongrass was very heavy when both water and fertilizer were more than sufficient. In other trials where the Johnsongrass was especially dense, the corn was visibly stressed because of competition from heavy weed pressure remaining in the row. Treating late has worked very well where Johnsongrass was the only weed. Other weeds may be too large for effective control if treatment is delayed until the corn is three feet tall.
The label recommends that oats not be planted for eight months, but does not prohibit this rotation, so growers following Accent with oats do so at their own risk. I have observed no injury in any of my trials where the Accent has been followed by oats or wheat. I have had only one instance where Accent has been followed by alfalfa and no injury was observed.
Johnsongrass does not significantly reduce overall tonnage in corn silage unless the weed pressure is extremely high. In most cases, the Johnsongrass substitutes for the corn on a one-for-one basis. In some cases, apparent fields may actually go down when the Johnsongrass is controlled because the Johnsongrass can be wetter than the corn at harvest. Of course, there is a big difference in quality. On silage where Johnsongrass, left uncontrolled, would have been 35% to 50% of the total biomass (this is about average for a field with bad Johnsongrass), the percent ADF (acid detergent fiber) of the silage was 32.3 where the plots had been cultivated at eight inches then treated with Accent at three feet, compared to 39.4 in the plots that had only received cultivation. Protein in the same study was 9.5% for the weedy plots compared to 8.6% in the Accent treated plots. Johnsongrass usually has a higher protein content than corn, however the poorer fiber levels more than offset any feeding advantage.
I want to acknowledge the many growers, custom applicators, custom harvesters, pest control advisors and chemical company representatives who cooperated in the trials on which this information is based. Thank you all your help.
November 5, 1999