Weed Seed Contamination in US Produced Flue-Cured Tobacco Leaf
The seed of noxious weed species has recently been discovered in cured tobacco leaves destined for export from the United States. The tobacco industry was contacted in early 2013 and notification was given that prior to export, weed seed were found in processed tobacco leaves by the China Inspection and Quarantine (CIQ) office. The CIQ is the equivalent of the USDA-APHIS for the Ministry of Agriculture of the People’s Republic of China. Once weed seed were found, grow out studies were conducted by CIQ representatives. From these grow out studies two things were determined: 1.) that weed seed were indeed viable due to germination and 2.) the identification of weed species that were present. A large number of various weeds were correctly identified, and of these, six were determined to be specific to the flue-cured tobacco producing regions of the southeastern United States. Those weed species are as follows: foxtail, crabgrass, palmer amaranth, bahia grass, goose grass, and ragweed.
It is widely believed by tobacco and weed science specialists that weed seed contamination largely occurs in the presence of mechanical harvesters. During mechanical harvest, entire weed plants are pulled into harvesting heads both in-row and at row ends in field borders where harvesters maneuver prior to entering harvest rows. The weeds are then inadvertently mixed with green tobacco leaves and are ultimately sent to curing facilities for post-harvest processing and curing. Prior to curing, the stalks of weeds are removed to prevent NTRM contamination; however, the weed seed is often left behind where it remains on cured leaves as a contaminant.
Currently, the CIQ has not rejected shipments of tobacco from the United States to China; however, measures to do so exist. It is extremely critical that this issue is addressed at all levels of production to ensure the sustainability of the United States tobacco industry as a whole.
Points to Consider Regarding Contamination
- Weed species reaching maturity can distribute seed as a means of reproduction and propagation. For weed species that are taller than tobacco plants (Palmer amaranth), seed can fall onto tobacco leaves where the sticky leaf surface can retain them during harvesting, curing, and processing. Contamination from these species can occur during harvest as well, when green leaf and weeds are intermingled. For weed species shorter than tobacco plants (grass and ragweed) seed contamination is likely to occur during harvest.
- Removal of entire weed plants from green leaf prior to curing does not remove 100% of the weed seed. The surface chemistry of green tobacco leaves is extremely sticky and gummy, and as a result weed seed can remain adhered to the surface of leaves. This is of particular concern for seed of Palmer amaranth due to its extremely small size and weight.
- The high temperatures (165-170°F) reached during the stem drying phase of curing are not sufficient to reduce seed viability. The stem drying phase of the curing process is a very brief period (1-2 days), where the relative humidity of the curing environment is often ≤15%. This brief exposure to extreme temperatures is not long enough to kill the seed. Furthermore, the low humidity (≤15%) of the stem drying phase is not high enough to promote seed germination, which would promote seed/seedling death upon exposure to high curing temperatures.
- Weed seed is not removed during primary processing (de-stemming/blending). Once lamina is removed from the mid-rib of the tobacco leaf, weed seed can still remain intact. This is evident because weed seed were discovered post-primary processing.
To combat the issue at hand, an action plan was created by University faculty with tobacco interests, the North Carolina Department of Agriculture and Consumer Services, the United States Department of Agriculture, and the entire United States Tobacco Industry. That plan is as follows:
- Train/educate county extension personnel regarding contamination and control measures
- Include weed seed contamination as an educational component of the US Tobacco GAP Program
- Include the issue as a portion of the US Tobacco GAP Manual
- Include weed seed contamination as a sub-section of the “Managing Weeds” chapter in the NCSU tobacco production guide
- Discuss contamination at annual production meetings
- Review, with producers, cultural and chemical controls that can be implemented on-farm
On-farm Measures for Weed Control and the Prevention of Contamination
Many agronomic management practices exist to control weed growth and reduce weed seed contamination in cured tobacco. These strategies are considered to be Best Management Practices and are to be implemented as a part of an entire weed management program.
- Consider deep tillage when preparing fields for tobacco production. Deep tillage with a moldboard plow will invert the top six to eight inches of the soil profile and bury weed seed at a depth that germination cannot occur. It is advisable to leave the soil profile intact for at least 36 months post-deep tillage to reduce seed viability. Growers are encouraged to use minimum-tillage practices, as cropping systems permit, in growing seasons following deep tillage. Preliminary research from North Carolina State University indicates that deep tillage in tobacco production can reduce entire weed populations by as much as 50% when compared to shallow tillage systems.
- Consider using herbicides labeled for tobacco production as a component of a weed management program. Refer to the current NCSU Tobacco Production Guide for a comprehensive list of approved herbicides, recommended application methods, and application rates.
- Palmer amaranth is of specific concern for cropping systems in the flue-cured tobacco production areas of the United States. There are currently three herbicides (Aim, Prowl, and Spartan) registered for use in tobacco production that provide control of this weed. Of these three, Spartan (sulfentrazone) is likely the best option for season-long control, reduced risk of carryover into subsequent crops, and ease of application. Spartan can be applied in a pre-plant incorporated (PPI) application or a pre-transplanting soil (Pre-Trans) application. The risk of Spartan injury to tobacco is greatly reduced when a pre-transplanting soil application is used. It is advisable to remove the row ridges prior to application in order to prevent treated soil from being displaced to row middles as transplanting occurs. Preliminary research from NCSU indicates that adding Spartan to a weed control program can reduce weed populations by as much as 80% compared to programs where it is not used. Spartan will also provide exceptional control of nutsedge and morningglory weed species.
- For grass species it is recommended to consider the use of Command (clomazone) as a pre-emergence herbicide or Poast (sethoxydim) as a post-emergence herbicide. Command will also provide excellent control of ragweed and can be applied as a PPI, Pre-Trans, or even overtop up to 10 days following transplanting. Poast will provide excellent control of many grass species post-emergence, but application timing must be considered as this product has a 42 day PHI.
- Spartan and Command can be applied PPI or Pre-Trans as a tank mix. This allows for a single pass application and makes management more time efficient. It should also be noted that the crop injury from Spartan is greatly reduced when it is tank mixed with Command. Residual activity and efficacy are not inhibited in this application.
- Hand weeding is also an effective practice to ensure adequate weed control. Labor crews are often used for topping and suckering, and as these crews move through fields it is advisable that they physically remove large weeds from production areas. Hand weeding must occur in a timely manner to ensure that weed seed is not manually distributed in fields. The removal of weeds from the field following removal from the soil is desirable because regrowth/re-rooting can occur under certain conditions.
- Maintain adequate weed control in field borders. As previously mentioned, large weeds can enter mechanical harvesters as they are transported through field borders. Additional concerns are raised once natural weed seed dispersal mechanisms are considered. Weed seed can move from field borders into production areas if control measures are not properly implemented.
In summary, weed seed contamination has been observed in tobacco exported from the United States. An action plan has been created and grower education initiatives have already been implemented through a number of resources. This issue must be addressed in current times to ensure the future of the tobacco industry in the United States.