Hemp textiles could replace their cotton counterparts in the future—at least, that would be the case if ecological considerations were taken into account. Scientists at the Leibniz Institute of Agricultural Engineering and Bioeconomy have investigated water productivity for fiber hemp as a measure of water use efficiency and found that the results from the dry year of 2018 show that hemp plants use water about six times more efficiently for biomass production than cotton. The study was recently published in the journal Water.
Hemp (Cannabis sativa L.) is promising in this regard. Once a globally widespread plant for fiber production, hemp was almost completely replaced by cotton and later by synthetic fibers in the last century. However, cotton cultivation is associated with high water requirements of at least 750 mm of rainfall or irrigation water per growing season, frequently resulting in soil salinization and intensive pesticide use. Hemp, on the other hand, is a high-yielding multipurpose plant that is comparatively undemanding in terms of plant protection and fertilization.
The water use of hemp plants during growth has been little researched to date. ATB scientists therefore conducted a field trial to investigate the water productivity of two hemp varieties. The varieties 'Santhica 27' and 'Ivory,' adapted to Central European site conditions, were selected based on the results of the MultiHemp project completed in 2017. The studies were conducted at the ATB research site in Marquardt, northwest of Potsdam – one of the driest growing areas in Germany with an average annual rainfall of 579 mm. The groundwater table is 10 m below ground level, and the sandy soil has only limited storage capacity for water and nutrients. The 2018 growing season was extremely dry, with only seven days from May to September receiving light rainfall of 0,7 to 19,6 mm. To ensure the trial's success after germination, the plants were irrigated during the emergence phase.
For the study, in addition to meteorological data, soil water availability, transpiration and volumetric water content, leaf area index, and plant physiological parameters such as gas exchange, photosynthesis performance, and responses to environmental influences were regularly recorded. For the first time, the scientists used a novel measurement setup to determine canopy penetration, i.e., the amount of precipitation penetrating the leaf canopy.
The calculation of water productivity—that is, the relationship between water demand and dry matter buildup—included all components of water inflow via air and soil used for plant growth. The total biomass yield and the fiber-containing bast content were determined immediately after harvest in September.
The results show that the average water productivity of industrial hemp, at 2,4 kg dry matter per cubic meter of used water, is six times higher than that of cotton (0,4 kg/m3).
"We see that hemp offers great potential for cultivation in relatively dry locations. This makes fiber hemp an attractive option as an environmentally friendly alternative to cotton, thanks in part to its lower plant protection requirements," says Dr. Hans-Jörg Gusovius, fiber plant expert at ATB. "The THC-free cannabis strains used here, which were specifically bred for European growing conditions, are also suitable for cultivation in relatively dry locations."
"The results of our study reflect the conditions for production at a single location in 2018," summarizes Dr. Katrin Drastig, hydrogeologist at ATB. Considerable research is still needed to further improve agricultural water use productivity at the farm level in light of the expected climatic scenarios. "We need much more data to derive models for specific regions, management practices, and crop species from individual results. This would enable us to provide farmers with decision support for optimizing water use," the scientist summarizes.
