Proposed regulations mandating lower nicotine concentrations in tobacco (Nicotiana tabacum L.) products will likely require changes in tobacco production to reduce nicotine while maintaining yield and quality. The agronomic practices used for tobacco production have a significant impact on the synthesis and accumulation of nicotine in flue‐cured tobacco. Nicotine is the primary alkaloid in flue‐cured tobacco and is one of the main reasons for its commercial production. Most agronomic practices that improve plant health and yield have a positive effect on nicotine production and accumulation. Some of the most important factors that affect nicotine concentrations are N fertilization, planting density, topping practices, sucker control, and harvesting practices. The amount of N available to the plant has a substantial effect on nicotine, as N is a primary component of the nicotine molecule. Factors leading to higher N uptake lead to higher nicotine concentrations. Plant and leaf densities within the field also have a significant effect on nicotine, where increasing densities leads to lower nicotine concentrations. Flowering and sucker production are both significant sinks of energy and other resources. Eliminating the inflorescence via topping and controlling suckers lead to higher nicotine concentrations. In fact, substantial nicotine synthesis and accumulation occurs in the days and weeks following topping. This comprehensive review discusses the agronomic factors affecting alkaloid production in flue‐cured tobacco, and how these factors can be adjusted to manipulate the ultimate nicotine concentration.Core Ideas Proposed regulations may require lower tobacco nicotine concentrations. Production practices and timing significantly influence nicotine and leaf quality. Nitrogen fertility, crop density, growth regulation, and harvesting are paramount. Low density, high N, and increased maturity enhance nicotine accumulation. Flowers and axillary shoots are sinks that limit foliar nicotine concentrations.

Palmer amaranth (Amaranthus palmeri S. Wats.) is a highly competitive weed that can be difficult to manage in many cropping systems. Research to date has not quantified the growth and development of A. palmeri in a manner that allows direct comparisons across cropping systems. Research was conducted to compare the growth, development, and seed production of A. palmeri when competing with corn (Zea mays L.), cotton (Gossypium hirsutum L.), peanut (Arachis hypogaea L.), and soybean [Glycine max (L.) Merr.] when emerging with crops or emerging three weeks after crops emerge. Regardless of when A. palmeri emerged, seed production was greatest and similar in cotton and peanut and exceeded that of corn and soybean; seed production in soybean exceeded that of corn. However, seed production was approximately 10-fold greater when A. palmeri emerged with crops compared with emergence three weeks later. These results illustrate the importance of controlling weeds during the first three weeks of the season relative to contributions of A. palmeri to the weed seed bank and is the first report comparing seed production in presence of these crops in a manner allowing a statistical comparison of seed production and highlighting the importance of crop sequence for seed bank management.

Deriving crop information from remotely sensed data is an important strategy for precision agriculture. Small unmanned aerial systems (UAS) have emerged in recent years as a versatile remote sensing tool that can provide precisely-timed, fine-grained data for informing management responses to intra-field crop variability (e.g., nutrient status and pest damage). UAS sensors with high spectral resolution used to compute informative vegetation indices, however, are practically limited by high cost and data dimensionality. This research extends spectral analysis for remote crop monitoring to investigate the relationship between crop health and 3D canopy structure using low-cost UAS equipped with consumer-grade RGB cameras. We used flue-cured tobacco as a case study due to its known sensitivity to fertility variation and nutrient-specific symptomology. Fertilizer treatments were applied to induce plant health variability in a 0.5 ha field of flue-cured tobacco. Multi-view stereo images from three UAS surveys collected during crop development were processed into orthoimages used to compute a visible band spectral index and photogrammetric point clouds using Structure from Motion (SfM). Plant structural metrics were then computed from detailed high resolution canopy surface models (0.05 m resolution) interpolated from the photogrammetric point clouds. The UAS surveys were complimented by nutrient status measurements obtained from plant tissues. The relationships between foliar nitrogen (N), phosphorus (P), potassium (K), and boron (B) concentrations and the UAS-derived metrics were assessed using multiple linear regression. Symptoms of N and K deficiencies were well captured and differentiated by the structural metrics. The strongest relationship observed was between canopy shape and N foliar concentration (adj. $r^2$ = 0.59, increasing to adj. $r^2$ = 0.81 when combined with the spectral index). B foliar concentration was consistently better predicted by canopy structure with a maximum adj. $r^2$ = 0.41 observed at the latest growth stage surveyed. Overall, combining information about canopy structure and spectral reflectance increased model fit for all measured nutrients compared to spectral alone. These results suggest that an important relationship exists between relative canopy shape and crop health that can be leveraged to improve the usefulness of low cost UAS for precision agriculture.

With the passing of the 2014 US farm bill, there is more interest in industrial hemp (Cannabis sativa L. < 0.3% total tetrahydrocannabinol [THC]) grown for cannabinoid production. However, production recommendations that outline fertilizer requirements of cannabidiol (CBD) hemp do not exist. Our primary objective was to identify nitrogen (N) and potassium (K) fertilizer rates for maximizing biomass and CBD yield. A secondary objective was to identify the relationships between N and K rates and total THC and total CBD concentrations. Fertilizer rates from 0 to 224 kg N and 0 to 185 kg K ha⁻¹ were tested separately at four general locations in North Carolina. Two locations were used in 2019 and 2020, while the other two were used only in 2020. Dry weight yield was predicted to increase linearly from 1822 to 3384 kg biomass ha⁻¹ as N rate increased from 0 to 86.8 kg ha⁻¹. Nitrogen rates above 86.8 kg ha⁻¹ were not predicted to increase biomass. Likewise, as N rate increased from 0 to 84.2 kg N ha⁻¹, CBD yield was predicted to increase linearly from 204 to 389 kg CBD ha⁻¹. Additional N was not predicted to increase CBD yield. The CBD and THC concentrations showed a slight bell‐shaped response curve over increasing N rates, ranging from 11.33% to 12.11% and 0.473% to 0.509%, respectively. Potassium application did not affect yield nor CBD and THC concentrations. Results from this work indicate that N is a more limiting factor than K for maximizing CBD hemp biomass production.

Research was conducted in 2012 and 2013 to determine the effect of nitrogen application timing and method on the yield, quality, and leaf chemistry of flue-cured tobacco. Liquid urea–ammonium–nitrate supplied 100% of the total nitrogen and was applied in differing combinations of rates, timings, and methods. Nitrogen rates above, at, and below recommendation for specific locations were split into two, three, or four application timings depending on treatment. Nitrogen application was initiated at transplanting and concluded before or at topping. Applications of nitrogen were either soil applied at each interval or were soil applied until topping where application occurred over top to promote stalk rundown. Leaf tissue samples were collected at layby and topping to evaluate total nitrogen content throughout the growing season. Composite cured leaf tissue samples from all four stalk positions were analyzed for total alkaloid and reducing-sugar content. Yield data were collected and leaf quality was determined according to U.S. Department of Agriculture grade. Crop value per hectare was determined using a combination of yield and quality. Data were subjected to analysis of variance using the PROC GLM procedure in SAS ver. 9.4. Treatment means were separated using Fisher's Protected LSD at P ≤ 0.05. Soil-applied treatments yielded higher than stalk rundown treatments but contained lower nitrogen content at topping and chlorophyl level (SPAD) measurements. Leaf quality was similar when nitrogen was applied as a stalk rundown at above recommended rates and when soil applied at recommended rates. Leaf value and total alkaloid content were not affected by treatments; however, sugar content was highest when less nitrogen was applied early in the season. Leaf nitrogen was higher at layby when a larger percentage of nitrogen was applied earlier in the season. Rainfall played a significant role in observed results and producers should exercise similar treatments with caution.