Guayule Seed Production, Harvesting and Processing

Bedane, Guta (2007). Guayule Seed Production, Harvesting and Processing PhD Thesis, School of Land, Crop and Food Sciences, The University of Queensland.

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Author Bedane, Guta
Thesis Title Guayule Seed Production, Harvesting and Processing
School, Centre or Institute School of Land, Crop and Food Sciences
Institution The University of Queensland
Publication date 2007-08
Thesis type PhD Thesis
Supervisor Gupta, Madan L.
George, Doug
Subjects 300000 Agricultural, Veterinary and Environmental Sciences
Abstract/Summary Guayule (Parthenium argentatum Gray) is a source of high quality low-allergenic natural rubber. The effort to commercialise this plant has intensified in the last decade. That guayule can survive extreme environmental conditions makes it more attractive to farmers, especially under conditions of moisture stress. Recent research has indicated that guayule grows well in Australia producing high rubber and resin yield. Efficient seed production technology resulting in high quality seed is vital to full commercialisation of this crop. The main objective of this research was to investigate production, harvesting and processing of guayule seed. This included investigation on flowering nature of guayule, determination of optimum seed maturity, effect of plant population on seed yield and quality and the development of harvesting and seed processing technology. Guayule was established in the field at four plant populations. A seed harvester was developed and tested over two production seasons. A seed threshing and cleaning machine was also developed and evaluated. Flowering behaviour of guayule was studied over three years with the objective of determining when peak flowering occurs and to investigate factors affecting flower initiation and pattern of flowering. The results indicated that photoperiod is the likely main trigger for flower initiation with soil moisture and temperature also having some influence. With rainfall as the only source of moisture, flowering peaks occurred in mid-spring (October) and in Summer (February) in Southeast Queensland, Australia. After initial flowering, the flowering pattern was mainly affected by soil moisture. High summer temperatures reduced the number of flowers. The optimum harvest maturity and indicators of maturity for guayule were investigated by harvesting seed sequentially after flowering and comparing seed quality parameters. Heat units expressed as growing degree days after flowering were calculated and related to seed development stages and quality. Seed quality was assessed by germination test, capitulum dry mass, 1000-seed mass and percentage of filled seeds. Results indicated that guayule seed can safely be harvested at about 329 growing degree-days (GDD) or 28 days after flowering under south-east Queensland climatic conditions. At this date seed quality was better and seedhead colour was comparable to cinnamon (code 165C) on the Royal Horticultural Society (R.H.S) standard colour chart. Of all the parameters, GDD, 1000-seed mass and percentage of filled seeds provided more rapid and reliable measures of optimum seed maturity. The effect of plant population on seed yield and quality was studied by planting guayule at 4444, 8300, 12500 and 25000 plants/ha. Data were collected at 16 and 28 months after planting. Seed was harvested manually multiple times over 4 weeks each year following the main flowering period in spring. Harvested seed was threshed and clean seed yield was compared among different plant populations. Seed quality was also compared in terms of 1000-seed mass and seed size. Lowest plant population of 4444 plants/ha provided the highest yield at 28 months but produced the lowest yield at 16 months from planting when the plants had not yet reached full size to compensate for wider spacing. However, at both ages this treatment produced heavier and larger seeds. The difference in yield or seed quality between plant populations ranging from 8300 to 25000 plants/ha was not significant. Overall results of the study demonstrated that seed yield and seed size, which is important in direct seeding, can be affected by plant population. A single-row guayule seed harvester was developed after investigating different methods of seed dislodgement and collection. The harvester consisted of a seed dislodgement mechanism that removes seeds by vibration of spring steel rods. An axial flow fan placed directly over the vibrating rods catches the seed before it falls to the ground. The seed is then conveyed by the air stream through an inflatable canvas tube to a collection bin. The design and selection of the seed catching system was based on terminal velocity of the unthreshed seed. Vibration frequencies ranging from 9.9 to 14.5 Hz and amplitude from 4 to 5.5 cm were used to test the harvester in the field. Harvested seed was analysed for percentages of clean seed in the harvested material, seed loss and immature seeds harvested. The performance of the machine was evaluated based on harvesting capacity and efficiency. Guayule seed harvested by the machine ranged from 1.73 to 7.18 kg/ha and harvest efficiency varied from 77 to 91%. The percentage of immature seeds removed from the crop during mechanical harvesting was as low as 0.1%. Although there was a trend for better efficiency from higher vibration frequencies, the variation was not statistically significant. Nearly 21% of clean seed was obtained in the harvested material which was much higher than that reported by other researchers. The harvester can be converted into a multiple row machine to increase its capacity. A guayule seed processing system consisting of a belt thresher, a vibrating screen separator and horizontal air grading unit was developed and evaluated. The machine efficiency was up to 77% with little seed injury. Seed purity reached 98% and germination 76%. Of the 3 grades of seed collected from the machine, up to 63.5% of the seed processed was first grade with 1000-seed mass achieving 764 mg; this is highly desirable for direct seeding. The processing capacity of the threshing unit can easily be upsized by increasing belt width. Threshing clearance and belt speed affected seed quality and grade as well as overall efficiency of the machine. The greatest efficiency was obtained with a combination of clearance between 0.75 and 1 mm and belt speed of 9 m/s.

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