Introduction

Weed control has always been a major focus of tobacco farmers; however, within recent years this issue has gained increasing attention for a number of reasons. With a growing emphasis on export markets and meeting the demands of those markets, now more than ever it is critical that producers take appropriate action to ensure that certain quality standards are met. While there are a relatively small number of herbicides (seven) labeled for use in tobacco production, and even fewer approved for mid or late season application, there are management practices that can implemented that have great benefit

Weed Control Issues

Season long weed control is the desire of every farmer, regardless of location or commodity; however, such expectations are often difficult to meet. Tobacco production is no exception to this concept but often times it is held to a higher standard by producers and industry personnel. The tobacco plant itself and the management practices imposed during its production are extremely beneficial from a weed control standpoint; alternatively, some of these practices can also make weed control more difficult than in other crops.

The first point of consideration is that the tobacco plant itself grows extremely fast under conducive environmental conditions. During the first two to four weeks after transplanting the plant experiences pronounced lateral growth that covers the vast majority of a raised plant bed. It is during this time that smaller, newly germinated weeds can be shaded and outcompeted for sunlight, water, and essential nutrients. The downside to this factor is that coverage is reduced for the few herbicides that can be broadcast applied over the top of tobacco plants at this stage. These herbicides (Command, Devrinol, and Poast) require adequate soil or weed coverage (as is the case with Poast) to ensure maximum effectiveness. The issue of sufficient coverage is made greater when plant size is considered later in the growing season. Furthermore, as the season progresses, fewer products are labeled with this application method.

The second point of consideration is the length of the growing season. In most cases, flue-cured tobacco will be in the field for five to five-and-a-half months, which is an extremely long period of time to guarantee complete weed suppression from any control practice. In addition, as the harvesting portion of the season progresses, leaves are removed from the plant which opens the crop canopy and allows greater light penetration. It is during this time that many weed species grow uninhibited due to the breakdown and/or leaching of herbicide materials and the impractical nature of cultivation at this stage of crop growth. Weeds and weed seed can contaminate harvested as well as cured leaf, thus reducing the quality and value of US tobacco. There are no labeled herbicide options when this occurs late in the growing season.

Points of Consideration for Improving Weed Control

Despite the many weed related issues experienced in tobacco production, exceptional late season control can be realized. What follows are brief points that growers might consider.

Early season weed control is key to late season suppression. Simply stated, the less weed pressure experienced at the beginning of the season, the less pressure typically experienced later in the season. There are a number of recommended practices that can be implemented early in the growing season, all of which, when used in combination, will aid in weed suppression.

o   Deep Tillage-In current times, deep tillage is not a common practice due to its increased cost and the implementation of conservation plans on many farming operations. However, research in multiple crops, including tobacco, has demonstrated that weed populations can decrease when deep tillage is utilized during field preparation. Deep tillage inverts the top 6-10 inches of the soil profile, thus burying weed seed at a depth that germination is not possible.

o   Pre-transplanting Herbicides-Most herbicides labeled for use in tobacco production have pre-emergence (PRE) control of a broad range of weed species. It is recommended that producers explore these options when making production related decisions. The two most commonly used PRE herbicides in tobacco production are Spartan (sulfentrazone) and Command (clomazone). Spartan provides excellent control of nutsedge, morningglory, and amaranth (Pigweed) species while Command will control most grasses and ragweed species. It is estimated that 60% of the tobacco acreage in North Carolina receives both of these herbicides. For additional early season herbicide information please see the North Carolina State University Flue-Cured Tobacco Production Guide.

o   Cultivation-Tobacco is one of the few crops produced in the United States that still receives post-planting cultivation. Cultivation is utilized for a number of reasons, weed control is one. Most tobacco producers will cultivate tobacco two to four times between transplanting and layby, but one must exercise caution in cultivating too many times or too late in the season as serious root injury can decrease crop yield and provide infection points for many soil borne diseases and organisms. Cultivation will primarily remove and/or damage weeds between rows; however, significant soil movement from between row to in-row will assist in control as well.

Mid to late season weed suppression can be more difficult to address, but is not impossible. Both preventative and curative measures do exist, and must be considered as a component of an entire weed control system.

o   Herbicide application at layby-of the seven labeled herbicides for tobacco production, four have are approved for a layby application (Prowl, Devrinol, Poast, and Aim). Each of these herbicides has serious considerations that must be addressed prior to their application, but reasonable control of certain weed species can be expected when correct application occurs. Prowl (pendimethalin) and Devrinol (napropamide) are PRE herbicides and must be band applied to the row middles and incorporated after application. There is concern over Prowl carryover into subsequent small grain crops and this issue is amplified when flumetralin based sucker control materials (Prime+, Flupro, and Drexalin Plus) are used. Poast (sethoxydim) is a post-emergence (POST) grass herbicide; therefore, weeds must be emerged for adequate control. Poast can be applied in a broadcast application overtop of tobacco plants; however, coverage can be reduced due to impedance from tobacco plants, as was discussed in the previous section. Furthermore, Poast has a 42 pre-harvest interval (PHI) which must be considered as well. Like Poast, the herbicide Aim (carfentrazone) has no residual activity and must be used in a POST application. Aim is a contact burner type herbicide and must be applied with a hooded sprayer at layby or in a post-directed application after first harvest. When using Aim, ensure that it does not come into contact with tobacco as it will cause significant crop injury. Aim will provide excellent control of morningglory and amaranth species; however, application of Aim must occur before amaranth weeds are greater than four inches tall or plants will not be killed. A summary of mid to late season labeled herbicides, application rates, and use patterns can be found in Table 1.

o   Hand weeding-Historically, there has been a rather large amount of hand labor needed for tobacco production, and while this need has greatly diminished in recent decades, many producers still utilize physical labor for topping, suckering, harvesting, and processing leaf both before and after curing. Work crews can be utilized to manually remove overgrown weeds from fields and this activity can occur simultaneously with other tasks. It is critical that hand weeding occur in a timely manner because once weed seed develop they can be distributed by hand removal. To prevent weed dispersal it is ideal that weeds are pulled from the soil before seed set occurs. It is also recommended that once removed from the soil, weeds are then removed from the field. In rare situations, weeds such as grass and amaranth species have been known to “re-root” when left on the soil surface.

Table 1. Mid to Late Season Herbicides Labeled for Use in Tobacco Production.

Conclusions

Season long weed control in tobacco production can be a challenge due to the long growing season; however, there are numerous practices that can be implemented through an integrated approach.