Growth and development of fibre hemp (Cannabis sativa L.) : A thesis submitted in partial fulfilment of the requirement for the Degree of Master of Agricultural Science at Lincoln University

Abstract

This research aimed to understand the growth and development of hemp (Cannabis sativa L.) crops and determine their impact on hemp fibre quality. An experiment was sown using four cultivars, ‘Ferimon’, ‘Kompolti’, ‘Futura 75’ and ‘Carmagnola’, on three dates (22/10/2020 (SD1), 11/11/2020 (SD2) and 09/12/2020 (SD3)) in Canterbury, New Zealand. The duration of important phenophases was quantified using temperature and photoperiod. Components of dry matter (DM) growth patterns were calculated and explained by differences in light interception, canopy architecture and radiation use efficiency (RUE). Secondary fibre (F2) production was related to key physiological drivers to determine optimum harvest stage.

The facultative photoperiod response meant the duration of the vegetative phase, from emergence to flowering, was described in thermal time (Tt), and was progressively longer (P<0.001) for plants sown earlier. Among cultivars, ‘Carmagnola’ and ‘Futura 75’ spent longer in this phase than ‘Ferimon’ and had the lowest Tt requirement to 50% male and 50% female flowering. The phyllochron, of ‘Ferimon’ was shortest (P<0.001), at 21.4 °Cd/leaf, compared with ‘Carmagnola’ and ‘Kompolti’ at 24.3 °Cd/leaf and 23.6 °Cd/leaf (± 0.359), respectively. A sowing date effect (P<0.001), indicated SD1 had the longest phyllochron of 28.1 °Cd/leaf (± 0.222), followed by SD2 with 23.7 °Cd/leaf and SD3 with 17.5 °Cd/leaf. This was unexpected but could not be explained by an investigation of cardinal temperatures, so may have resulted from differing plant populations because SD3 had a reduced emergence (P<0.001) with a plant density of 37.8 plants/m2 (± 5.11).ii Total and stem DM yield progression followed a sigmoid curve and showed the optimum sowing dates to maximise stem yield were SD1 and SD2. SD2 had a shorter lag phase and therefore reached maximum stem production sooner, and therefore accumulated the same yield as SD1 but in a shorter time. Results also suggested ‘Carmagnola’ and ‘Futura 75’ are suitable cultivars for a fibre crop, with yields of 11.4±0.61 (‘Carmagnola’ SD2) and 10.9±0.61 t DM/ha (‘Futura 75’ SD1). Selecting optimum harvest times is important because a drop in stem DM yield can occur when plants partition assimilates to flower production. Flowering was also identified as a guide for optimum harvest time for earlier but not later sown crops.

Time in the vegetative phase was closely linked to DM production, and this was driven by the amount of light interception. Stem yields were positively influenced by increasing total light interception among both sowing dates (<0.001) and cultivars (<0.001). The faster rate of canopy development meant that SD2 was able to accumulate the same maximum DM as SD1. Canopy extinction coefficient (k) differed among cultivars, ‘Kompolti’ at 0.503 (± 0.027), and ‘Futura 75’ with 0.585 (± 0.035) had more droopy leaves than ‘Ferimon’ with a k value of 1.17 (±0.073). RUE was influenced by temperature and cultivar, with a mean of 0.915 g DM/MJ total radiation/m2 (±0.065).

Plants took 849 °Cd (± 9.5) to produce F2 in SD1, but only 752 °Cd (P=0.029) in SD2 and SD3. In contrast, stem yields at F2 initiation were not different for SD1 and SD2, with an average yield of 4.11 t DM/ha (± 0.144), while SD3 had lower (P = 0.011) stem yields with 1.87 t DM/ha. Therefore, these data suggest that SD2 was the optimum sowing date for high yield with nil F2. Fibre quality was quantified in relation to accumulated Tt, total DM and stem DM. The rate of F2 increase, against these influences has been defined for limited periods of time, from F2 initiation, with a 5% increase in F2 height per t DM/ha (± 1.5%). This was This thesis recommends sowing dates earlier than mid-November, at high plant populations (over 90 plants/m2) and the use of crops with later flowering dates to increase dry matter yields and minimize secondary fibre production. Overall, this study improves understanding of hemp crop physiology and provides an important resource for future hemp production in Canterbury.