Herbicides are only part of a total weed management program that should include crop rotation, early stalk and root destruction, and cultivation. Total reliance on herbicides is costly, less effective, environmentally detrimental, and unsound weed management. A rapidly growing tobacco crop aids weed control by shading beds and row middles. Weed problems are much worse when crop growth is restricted because of disease problems, fertilizer injury, or chemical injury. Therefore, it is important to follow practices that promote healthy tobacco roots: crop rotation, disease control, fertilizer application during or within 10 days after transplanting, proper pesticide usage, and liming.
Herbicides can reduce the number of cultivations needed to produce a profitable, high-quality crop. However, properly timed cultivations are still an important weed and crop management tool.
Cultivation helps manage weeds not controlled effectively by herbicides. It also can improve weed control with soil-surface-applied herbicides, such as Command and Spartan, in dry periods soon after transplanting. However, excessive and deep cultivation can decrease the effectiveness of surface-applied herbicides by removing them from row-middles. Extend weed control with these herbicides by limiting deep cultivation to lay-by time.
Cultivation is also a good crop management tool. For example, building a high row ridge improves drainage, which aids disease management and decreases drowning. Cultivation also improves aeration and water penetration by decreasing crusting. However, excessive cultivation increases leaching of potassium and nitrogen, injures root systems, increases leaf scald in hot weather, spreads tobacco mosaic virus, and contributes to soil erosion.
Timing of Herbicides
Pretransplant-incorporated herbicides offer several advantages. Growers can tank-mix them with other chemicals to save one or more trips across the field, and rainfall isn’t as essential for activity with them as it is for surface-applied herbicides. In addition, when poor field conditions delay transplanting, pretransplant-incorporated herbicides help prevent weed growth that may start in the freshly prepared soil.
The most important disadvantage is crop injury. Prowl, Tillam, and Devrinol have the potential to limit root growth and cause slow early-season growth (stunting). Stunting is most likely during cool, wet springs. Poor incorporation, applying high rates, and tank-mixing two or more of these herbicides increase the chance of root injury.
Several herbicides are labeled for PPI applications in tobacco and form an integral part of an overall weed control program. Refer to specific herbicide recommendations for rates and special application considerations.
Surface Applied Before Transplanting (PRE-T)
Command and Spartan are labeled for soil-surface application before transplanting in addition to the more traditional pretransplant-incorporated method. This method is common in other crops but new to tobacco.
When applying herbicides PRE-T, apply other chemicals, including insecticides, nematicides, and fumigants, in the usual manner before bedding. Before transplanting, knock down the beds to transplanting height and apply the herbicides to the soil surface. For best results, knock down the beds as close to the time of transplanting as possible (keeping in mind the 12-hour worker reentry restriction on the Spartan and Command labels). Do not knock off additional soil during transplanting.
Herbicides applied to the soil surface depend on water to move into the soil where weed seeds germinate. Therefore, the PRE-T application method fits well in irrigated situations. If rainfall does not occur within 3 to 5 days, a light cultivation may aid in activating the herbicide. Lack of rainfall early in the season can result in reduced weed control when herbicides are applied to the soil surface.
Overtop Within 7 Days of Transplanting
Command and Devrinol are labeled for application overtop of tobacco within 7 days after transplanting. This method provides weed control similar to PRE-T application and offers the flexibility of application after transplanting. Application at transplanting is usually preferable to waiting up to 7 days because it saves a trip through the field and the herbicide is in place before weed seedlings emerge.
In fields with high row ridges, previously applied herbicides are moved along with treated soil from between the rows onto the row ridge. This justifies lay-by applications of herbicide to row middles in fields with a history of severe grass problems.
Lay-by applications help extend grass control when a short-lived herbicide such as Tillam is used. Also, a lay-by application of Devrinol or Prowl following the earlier soil-incorporated Tillam will extend grass control, and crop injury will be less than when a tank mix of Tillam and Devrinol or Prowl is used.
Poast can be applied to actively growing grasses in newly transplanted tobacco up to 42 days before harvest. Application rates vary from 1 to 1.5 pints per acre, depending upon the size of grass weeds. Grasses must be fully covered by spray to ensure control. Add 2 pints of crop oil concentrate or 1 pint of Dash HC spray adjuvant according to label directions. Apply Poast overtop (OT) or directed in a band.
Poast may be desirable in many of the same situations mentioned in the above discussion of herbicide applications at lay-by. The main difference between Poast and other grass herbicides labeled for use on tobacco is that it is applied to actively growing grass weeds after emergence (see label for maximum height of weeds controlled). This allows growers to delay grass herbicide application until grass populations are known, or to provide control of grasses after other measures have failed.
Post-Directed After Layby or First Harvest
Aim can be applied using a shielded sprayer or hooded sprayer to emerged, actively growing weeds in the row middles prior to layby. Aim can also be applied after first harvest when nozzles are directed underneath the crop canopy. Damage can result if spray solution contacts the tobacco plant. Do not apply when conditions favor drift. Refer to the Aim label for specific recommendations regarding application precautions in tobacco.