Nitrogen management for industrial hemp fiber production

Industrial hemp (Figure 1) is one of the oldest and most widely cultivated plants across the globe (James et al., 2023). In the United States, however, hemp production was restricted for decades under the Federal Marijuana Tax Act of 1937, which classified hemp as a “Schedule I Controlled Substance.” (Ekren et al., 2023). Interest in hemp production was renewed with the passage of the 2014 farm bill, which allowed states to establish hemp research and pilot programs (James et al., 2023). The development of cultivars with low Δ-9 tetrahydrocannabinol (THC) concentrations further distinguished industrial hemp from marijuana, which typically contains 3 to 15% THC. In 2021, the USDA finalized regulations requiring compliance with THC limits, including both Δ-9 THC and tetrahydrocannabinolic acid (THCA) (USDA-AMS, 2021). 

Industrial hemp is a short-day, annual herbaceous plant with many commercial applications. Fiber hemp, in particular, is valued by the textile and fiber industries because of its high cellulose content. In spite of its numerous uses, there is still limiting information on the appropriate nutrient management program for optimizing growth and fiber yields of industrial hemp. 

Nitrogen (N) is the most limiting nutrient influencing crop production, including industrial hemp. This is because N is crucial for growth and other essential metabolic processes that contribute to the synthesis and transfer of energy in plants (Laleh et al., 2021).  Growers have been faced with the issue of establishing an optimal N rate for advancing industrial hemp fiber production. The leaves of industrial hemp are very sensitive to N supply, and studies have observed that hemp usually amasses high N demand in the initial growth phase for emergence and plant height optimization (Papastylianou et al., 2018). 

While studies by Ekren et al. (2023) have observed a 48, 65, and 57% increase in plant height, stem yield, and fiber yield, respectively, when raising N rates from 0 – 250 lb N/ac, the effect of N on industrial hemp growth and fiber productivity has been quite complex and multidirectional. 

Researchers have observed that the effect of N on industrial hemp depends on rainfall intensity and distribution, temperature, soil type, location, cultivar type, and the primary purpose of production (fiber, flower, or seeds). 

Studies that evaluate the response of commonly grown fiber-type industrial hemp cultivars to N rate under southeastern U.S. conditions are very limited. The objective of this study was to evaluate six N rates on two hemp cultivars under Florida conditions. 

How was it done?

A three-year (2021–2023) study was conducted at the University of Florida’s West Florida Research and Education Center (WFREC) in Jay, FL. The area is characterized by a humid subtropical climate. The soil type was sandy loam, possessing good drainage and nutrient retention. Weather data was obtained from the Florida Automated Weather Network for 2021–2023 (Figure 2).  A factorial in a randomized complete block design with four replications was adopted for this study. The study evaluated six N rates (0, 50, 100, 150, 200, and 250 lb N/ac) on two industrial hemp cultivars (NWG-2730 and IH Williams).  

The field was disked and subjected to strip tillage prior to planting. Industrial hemp was seeded each year using a six-row cone planter to a spacing of 7.5 inches. The N fertilizer treatment was applied using urea, and to maximize then use efficiency, application was done in two splits. The first split was a uniform application of 50 lb N/ac at two weeks after planting, while the remainder of each N rate was applied at four weeks after planting, respectively. Other agronomic management practices like base fertilization, irrigation, and weeding were conducted as required to maximize treatment effect on the evaluated parameters. Data was collected on aboveground biomass and stem biomass. The holistic process for this evaluation is presented in Figure 3. 

What do the results tell us?

The aboveground biomass and stem biomass are two industrial hemp fiber yield indicators. Studies have observed that the higher the aboveground and stem biomass weight, the greater the resulting fiber yield. From the results, we discovered that total biomass and stem biomass were affected by the year × cultivar interaction, with no cultivar effect in 2021 but much high yield with cultivar IH Williams compared with cultivar NWG-2730 in 2022 (Figures 4 and 5). Although there was an initial increase in aboveground biomass, there was no significant increase beyond 100 lb N/ac. In a similar vein, there was no significant increase in stem biomass after 50 lb N/ac. Stem and total biomass were more with fertilizer N applied at 150 lb/ac compared with 0 lb/ac.

Further, the results showed that aboveground biomass and stem biomass were 38 and 41% higher, respectively, in cultivar IH Williams than cultivar NWG-2730. This was due to the high-yielding genetic capacity of IH Williams as observed in its higher plant height and stem diameters (Nwosu et al., 2026). Although IH Williams is a dual-purpose industrial hemp cultivar, it is primarily grown for fiber production, while NWG-2730 is primarily grown for seeds. We observed a similar response in the coupling effect of N rate × cultivar on aboveground biomass and stem biomass, which is consistent with observations in Vermont (Darby & Sullivan, 2024) and Oregon (Visković et al., 2023). 

Our results indicate that IH Williams could result in more fiber yield than NWG-2730 under Florida conditions, and the optimal N rate for aboveground biomass and stem biomass, respectively in industrial hemp was not dependent on the cultivar type. 

However, the significant interaction of N rate × year (with the highest peak in 2023) denotes that the effect of N rate on aboveground biomass and stem biomass was dependent on the yearly weather patterns (Figure 2).

Conclusions

Considering the findings from this study, we can conclude that the performance of the incremental changes in N rates on the two commonly cultivated industrial hemp cultivars evaluated are similar. The study also concludes that cultivar IH Williams will give more fiber yields than NWG-2730, while the latter will do better for seed yield. The study also concludes that weather patterns play a crucial role in dictating the level and direction of influence of N rates on industrial hemp. The study concludes that generic N recommendations that do not consider the hemp cultivar type, location, and prevailing environmental conditions (rainfall, temperature, and soil type), should be avoided.