From the Field-Agronomy Notes
March 9th, 2016
It’s never too early to start thinking about a weed management plan. Coming out of a year like 2015, with less than stellar crop yields and prices, it is easy for growers to think about cutting back on input costs. Herbicide applications or reduced rates tend to be one of the first areas for consideration among growers. However, it is strongly advised to stay the course and not give weeds a break; especially the more problematic species like Palmer amaranth, nutsedge, and some grass species. For tobacco production there are few herbicide options after transplanting, so early season weed management will set the stage for the rest of the season.
With fields being worked several weeks ahead of transplanting, a pretransplant-incorporated (PPI) application will buy time after bedding. This is advantageous for many reasons, specifically during a cool, wet period that may delay getting into the field for transplanting. One important point to remember with a PPI application is the need for uniform incorporation. In a cool, wet environment (as we have seen in the past couple of years) herbicide injury can be more severe when materials are not properly incorporated, as the plant roots are very slow to grow out of treated soil or plants are too small to tolerate excessive exposure to some compounds. Prowl (pendimethalin) for example, is commonly used PPI and is essentially immobile in soil; therefore it is unlikely that spring rains will remove this material from the root zone to allow for improved plant growth. The result is uneven and delayed plant growth across the field. Furthermore, Prowl is subject to photodegradation if left on the soil surface, which proves the need for good incorporation to ensure the herbicide is in the weed seed germination zone. Most weeds germinate within the top 1 inch of the soil profile. On the contrary, incorporation that is too deep or a non-uniform application can promote crop injury.
As we discussed in our winter production meetings, a tank-mix of Spartan (sulfentrazone) plus Command (clomazone) pretransplant (PRE-T) is a good combination to provide excellent control of most broadleaf weeds and grasses that are typical in tobacco fields. This application is made directly to the soil surface, so beds should be formed and knocked down prior to application. It is important not to remove additional soil from the bed when transplanting; as this can potentially provide inconsistent weed control by removing the herbicide from the row ridge. As previously mentioned, some level of moisture is required for herbicidal activation; be it from existing soil moisture, rainfall, or irrigation. In any case, if moisture is limiting 5 to 7 days after treatment, a light cultivation may help in the activation of the herbicide and control any germinated weeds.
Throughout the season, it is important to stay vigilant and be on the lookout for any weed escapes—which are those note controlled by cultural or chemical practices. Timely cultivation through layby is key; however, there is a lot of time between the last cultivation and final harvest which creates an environment favorable to multiple weed flushes. In these cases, hand-weeding should be utilized prior to seed set. Hand-weeding is the last line of defense late in the season, but should be accomplished in a timely fashion to prevent weed seed distribution and contribution back to the weed seed bank. Weed removal can be accomplished with very little added production cost when suitable weed suppression has been realized in early and mid-season. In addition, this will also aid in harvest efficiency and reduced weed seed contamination in cured leaf. Along with crop rotation, herbicide selection, timely cultivation, and hand-weeding round out a sound weed management program.
As the season progresses, we will revisit this issue and hopefully provide some helpful considerations along the way. In the meantime, the 2016 NCSU Tobacco Production Guide and Ag Chemical Manual list the various herbicide options and application rates. As always, do not hesitate to contact us with any questions or comments.