The Effects of Irrigation, Topping, and Interrow Spacing on the Yield and Quality of Hemp (Cannabis sativa L.) Fibers in Temperate Climatic Conditions

The study was aimed at determining the most suitable irrigation schedule program for hemp grown for fibers by using evaporation from the free water surface (Eo), measured by a Class A pan and related crop plant coefficient (Kc). The experiment, carried out in Vojvodina, a northern part of the Republic of Serbia, included three irrigation regimes: I1, I2, and I3 corresponding, respectively, to daily evaporation from an open water surface (Eo), two interrow spacings: RS1 (12.5 cm) and RS2 (25 cm), and topped (T) and not topped (NT) variants. The Kc values used for the calculation of daily evapotranspiration (ETd) were 0.42 for April and May and 1.00 (I1), 0.80 (I2), and 0.60 (I3) from June to the harvest. In addition, the nonirrigated (Io) control variant was also included in the trial. The dioecious fiber hemp variety Marina was used for the trials. Irrigation was carried out by a drip irrigation system and was scheduled based on the water budget method. It started when readily available water (RAW) in the soil layer of 0.4 m was completely depleted by the plants. In the first year of the study, irrigation did not influence the yield of hemp fiber, but in the second one, the best result was achieved for the irrigation variant I1. Interrow spacing did not have a statistically significant effect on both the hemp fiber yield and the quality. The topping of plants significantly decreased the yield of hemp, and it is not recommended when the hemp is cultivated for fibers. The quality of the hemp fibers was not influenced by irrigation, inter-row spacing, and topping. The rate of hemp evapotranspiration was in an interval of 312 mm (ETa) to 520 mm (ETm). The highest values of IWUE and ETWUE in both years and the bark yield in the first year were detected for the I3 irrigation variant, which suggested that the crop plant coefficient (Kc) of 0.6 could be recommended for the correction of Eo values in the calculation of the daily evapotranspiration of hemp (ETd) from June to August. The yield response factor (Ky) with values of 0.22 and 0.60 for the total growing season reveals that, in rainy years, hemp for fiber production could be grown without irrigation in the temperate climate of the Vojvodina region. However, such years are rare. Without doubt, irrigation of fiber hemp is recommended in the mentioned region in order to obtain high yields of good quality product