March 27^th, 2017-

Agronomy Notes are back this week after a brief intermission to stay in front of the cold related issues affecting greenhouses around the state. We’re going to jump ahead in the season just a little and discuss the specifics of weed management for growers considering the use of the herbicides sulfentrazone (Spartan 4F or Spartan Charge) and clomazone (Command 3ME). Given the wide range of common and troublesome weed species found in North Carolina tobacco production (Table 1), these two materials offer the greatest window for control.

Table 1. The Ten Most Common and Troublesome Weeds in North Carolina Tobacco Production*.

We often receive a number of questions related to herbicide application programs and how these programs play into weed management from a much bigger picture. Most commonly these questions revolve around the two materials previously referenced. Without hesitation, the responses given to producers are 1.) what crops are being produced in your rotation, 2.) what are the most common soil types on your farm, and 3.) how will you be using these materials? All three factors should weigh heavily in the decisions made.

First, in terms of crop rotation, cotton tends to be the major one of concern as there is an 18 month plant back interval listed on Spartan 4F and Spartan Charge¹ labels. This point should be strongly considered as sulfentrazone carry over into cotton can result in significant early-season injury. In addition, applications of clomazone can carry over into small grains planted behind tobacco. In this situation, it is advisable to reduce the application rate of Command 3ME from 2.0 pts/acre to 1.5 pts/acre. Reduced application rates certainly open the door to reduced weed control; however, we have not observed this to be a large factor playing into successful weed control where grass and Ragweed populations are reasonable in on-farm studies.

Second, soil type (and organic matter content) will largely determine the appropriate application rate for sulfentrazone. As a rule of thumb the coarser the soil type, the lower the application rate. The opposite is true for finer soil textures, such as those more commonly found in the Piedmont and Burley growing regions. In more practical terms, many producers will apply 4.0 to 5.0 fl. oz. Spartan 4F per acre in the Coastal Plain and, perhaps, as much as 6.0 fl. oz. per acre in the Old Belt. If Spartan Charge is to be applied, then rates MUST be adjusted accordingly to account for the difference in formulation. Table One presents commonly used rates of Spartan 4F and the corresponding application rate of Spartan Charge for the convenience of the reader.

Table 1. Conversion Table for Rate of Spartan 4F and Spartan Charge.

Third, and just as importantly as points one and two, producers should strongly consider the method in which these herbicides will be applied. As another general rule of thumb, lower application rates are recommended for PPI applications than for PRE-T applications. The reason for this consideration is that a PPI application places the herbicide(s) within the bed, thus increasing seedling exposure after transplanting. Alternatively, with a PRE-T application, herbicides are placed on top of the beds and require rainfall, irrigation, or cultivation for activation. The “barrier” created with a PRE-T application allows for plant growth to occur with little to no exposure to said herbicides. Of course, there are certain additional factors that must be considered when choosing a PPI vs. PRE-T application. For one, a PRE-T application can require an additional trip across the field. It is recommended that beds be made and knocked down to transplanting height prior to herbicide application. This practices reduces herbicide movement during transplanting and greatly improves in-row weed control. Some producers have implements designed to knock beds down and apply herbicides simultaneously, which improves efficiency. Another factor that must be considered is weed pressure…weed pressure in the sense of germination occurring between bedding and transplanting. Pre-plant incorporated applications can certainly keep weed populations to a minimum during this time; however, PRE-T applications may not—particularly if rainfall during this period is not sufficient for activation. If weed germination occurs between bedding and transplanting (and herbicide application has not occurred), an application of Spartan Charge + Command 3ME could prove beneficial. In addition to sulfentrazone, Spartan Charge contains carfentrazone (the active ingredient in Aim EC). Carfentrazone is a contact herbicide that offers some post-emergence suppression of pigweed and morningglory, if a timely application is made and weeds are smaller than about four inches in height. Carfentrazone has little to no residual activity, so the window of control will be very small and spray tanks must be thoroughly cleaned before applying other pesticides after transplanting.

Last season, producers did an excellent job with weed control—specifically with regards to managing weed seed contamination. I’m confident that there are a number of factors playing into such promising results; however, the fact remains that producers should NOT scale back the efforts used for weed suppression. Simply stated, one season of reduced efforts can result in many seasons of increased efforts. Producers are still encouraged to utilize integrated management strategies such as a strong pre-transplanting program, timely cultivation, layby herbicide application, and hand-weeding to stay in front of these issues. Should you have any questions regarding appropriate application rates for the herbicides referenced in this post, please do not hesitate to contact your local Cooperative Extension Agent. Finally, be sure to check back with us in a few weeks to see what we’re thinking for layby herbicides and how they can be used to provide a boost to weed control programs.

¹ Cotton may be planted after 12 months where Spartan Charge was applied at rates 8 fl oz/acre or less and meets the following conditions:

-medium and fine soils

-pH <7.2

-Rainfall or irrigation must exceed 15” after application of Spartan Charge to rotate cotton