LAUSR

Abstract Crop establishment costs of guayule (Parthenium argentatum A. Gray), a perennial desert shrub that produces natural rubber, can be significantly reduced using direct seeding rather than the traditional practice of transplanting greenhouse-grown seedlings. However, information regarding the irrigation application, crop evapotranspiration (ETc), and crop coefficients (Kc) for managing direct-seeded guayule crops has not been provided. In this study, guayule was direct-seeded in Apr. 2018 in fields at two location in Arizona; Maricopa, on a sandy loam soil and Eloy, on a clay soil, and harvested 23–24 months later in 2020. At each location, five irrigation rates were applied with subsurface drip irrigation (SDI) ranging from 50 to 150 % replacement of ETc (denoted as D50 to D150 treatments), respectively. A 6th treatment using furrow irrigation at 100 % ETc replacement (F100) was included. Treatments were replicated three times. The ETc was estimated for the first 74–84 days of crop establishment and thereafter, actual ETc (ETc act) was determined weekly-biweekly for the D100 and F100 treatments using a soil water balance. The objectives were to evaluate the responses in dry biomass (DB), rubber yield (RY), and resin (ReY) yield to water application rate, develop irrigation management criteria for the two soil types, and determine the ETc and crop coefficients for the 100 % treatments. The total irrigation applied to treatments ranged from 1830−1910 mm to 5090–5470 and averaged 3590 and 3320 mm for the 100 % SDI (D100) and furrow (F100) treatments at Maricopa and Eloy, respectively. The summed estimated ETc plus ETc act for the D100 and F100 treatments were 3663 and 3506 mm at Maricopa, respectively and 3428 and 3320 at Eloy, respectively. Average measured mid-season Kc in the 1st year varied from 1.20 to 1.26. Average measured mid-season Kc in the 2nd year were higher for D100 (≈1.30) than for F100 (≈1.23). Adjusted to the standard climate proposed in FAO56, mid-season Kc are 1.24 for D100 and 1.17 for F100 in the 2nd year. Average DB at Eloy (28.6 Mg ha−1) was not significantly higher than at Maricopa (24.0 Mg ha−1). However, RY and ReY were both significantly higher at Maricopa. At each location, rubber content was significantly higher for the F100 and the two lowest SDI rates than for other treatments. The highest mean RY and ReY were achieved with D100 at Maricopa and D75 at Eloy. These two also had significantly greater water productivity (WP; DB, RY, and ReY per unit of total water applied) than those at higher SDI rates and the F100 treatments. RY and ReY and their WP were generally higher for D100 than F100 in the sandy loam but not in the clay soil. For direct-seeded guayule in clay soils, furrow irrigation should be considered due to the lower rubber content and higher costs associated with SDI.