T. David Reed, extension agronomist for tobacco at the Virginia Tech Agricultural Research and Extension Centers, provided a closer look at the agronomic practices impacting nicotine concentration of FC tobacco. He referred to a 2019 study by Henry, Vann and Lewis, which suggested that proposed regulations mandating lower nicotine concentrations in tobacco products would likely require changes in tobacco production while maintaining yield and quality.
In standard FC tobacco production practices, Reed pointed out, a population of 13,600 plants per hectare to 16,100 plants per hectare is typical. The topping height is between 18 leaves to 22 leaves per plant, and the total number of leaves amounts to around 296,000 per hectare.
Topping is a standard production practice, and its timing impacts yield and sucker control. When topping is delayed past the early flower stage, a yield loss of 17 kg per hectare per day may occur. With late topping, the cured leaves are thinner and less bodied.
Regarding fertilization, nitrogen is the most responsive nutrient in terms of tobacco yield and quality. The nitrogen fertilization rate is determined based on soil texture and depth to a clay layer as well as field history. The recommended nitrogen rate is 67 kg to 90 kg per hectare but can be as high as 112 kg/ha.
While tobacco is relatively drought tolerant, it is responsive to rainfall and irrigation, with too much rain being a more common occurrence. Unlike other crops, such as certain grains or soybeans, tobacco is not as dependent on timely rainfall to produce an adequate yield. Dry conditions paired with high temperatures can impact cured leaf quality and leaf chemistry.
According to Reed, the number of harvests or primings has decreased in recent years. For most growers, three harvests are typical; some harvest four times. The time from topping to harvest can range from 8 weeks to 14 weeks or more on a given farm.
Tobacco growers, Reed emphasized, choose their agronomic practices in order to maximize their yield potential of high-quality, marketable tobacco. Leaf chemistry, sugar or nicotine content are not part of their consideration.
Reed quoted three studies by Caleb Hinkle that investigated plant population, topping time and topping height of low-nicotine FC production practices in field trials in 2019 and 2020 as well as a collaborative Coresta study of the low-nicotine tobacco agronomic production practices task force. Modified production practices with low-nicotine FC varieties, he concluded, did not consistently reduce nicotine to the proposed target levels. While plant population, topping height and nitrogen fertilization rate had minimum impact, delayed topping had a significant effect, with no topping having the greatest impact on nicotine. Not topping tobacco, Reed stressed, is not commercially viable in the U.S. The growing season was a major factor on nicotine levels. Both yield and leaf quality, as currently measured, were significantly lower with low-nicotine flue-cured varieties. Leaf texture and body were altered, which impacts the handling of the cured leaf.