Direct seeded guayule grown in Arizona under furrow and subsurface drip irrigation

Abstract. Guayule (Parthenium argentatum, Gray) commercialization depends on economical plant production. Establishment costs can be reduced significantly with direct seeding instead of transplanting. However, available irrigation water management strategies are entirely based on transplanted guayule data. Since direct-seeded and transplanted guayule plants develop different root structures, they are also likely to have different water use and soil water management requirements. The objective of this study was to evaluate and compare production parameters (biomass, rubber content, resin content and yields) of guayule irrigated with five different irrigation rates (six treatments) including five with subsurface drip irrigation (SDI) levels with 50%, 75%, 100%, 125% and 150% replacement of estimated soil water depletion (SWD) and denoted as D50, D75, D100, D125 and D150, respectively. There was also one treatment (100% replacement of SWD) grown with furrow irrigation (denoted as F100). Calculations were made in a root zone soil water balance model. The experiment was repeated in two fields: one at The University of Arizona, Maricopa Agricultural Center (MAC) in Maricopa, Arizona with sandy loam soil and the other in the Bridgestone Americas Guayule Research Farm, Eloy, Arizona with a clay soil. The experiment consisted of 18 plots (6 treatments x 3 Replicates). Each plot had 6 beds at MAC (8 beds at Eloy). Each bed was 1.02 m wide and 75 m long. The experiment was based on a split plot design with location as the main plot. The sub-plots (irrigation treatment) were arranged in a randomized complete block design, where the two fields were divided into three blocks and the six treatments were randomly distributed inside each block. The three replicates of each treatment were irrigated together via one main pipe and one irrigation station, which consisted of a solenoid valve, a flow meter, a pressure regulator, a pressure gauge and an air vent. Also, they were connected at the end via a flush pipe that ended with a flush valve and a pressure gauge. The experiment was initiated on April 20, 2018 at MAC and on April 17, 2018 in Eloy. In both locations, guayule variety AZ-2 was direct-seeded on raised beds spaced 1.02 m apart (20-30 cm on top and 15-20 cm high). A 4-row planter was used to plant the seeds, one row per bed, at a rate of 46 seed/m. After planting, sprinkler systems were installed to irrigate the two fields during the first two weeks. The total water applied through the sprinkler system was 296 mm at MAC and 317 mm in Eloy. Prior to initiating treatments, all SDI plots received equal amounts (a total of 887 mm for MAC and 954 mm for Eloy). Treatments started in late-July and early-August at MAC and Eloy, respectively. Total irrigation applied to D100 and F100 were 1543 mm and 1571 mm, respectively, for MAC and 1512 mm and 1470 mm, respectively, in Eloy.  Whole plant samples were harvested in late March 2019 (11 months after planting). Results indicate that rubber and resin content decreased with increasing water application rate. Biomass and rubber and resin yields were higher at MAC than Eloy for all treatments except the F100, which had higher biomass and resin yield in Eloy. The highest rubber and resin yields for MAC were in the D75 treatment, which were not significantly different from the rest of the SDI treatment but D50. The change in biomass and rubber and resin yields with the water application depth (rate) was similar to the change in NDVI and TGI vegetation indices. Guayule basal crop coefficient was calculated for the first year of growth. In general, MAC sandy loam soil had higher rubber and resin yields than the heavy clay soil in Eloy.