Guayule Germination and Growth under Subsurface Gravity Drip and Furrow Irrigation in Arizona

Guayule (Parthenium argentatum, A. Gray) is a perennial shrub, native to the desert of northcentral Mexico and southwestern Texas, which produces high quality natural rubber. The annual water requirement under flood irrigation is approximately 732 mm (2.4 acre-ft/year), which is within the annual water allocation depth for many central Arizona farmers. However, under the uncertainty of water supply in some areas, growers might be forced to cut irrigations or adjust their cropping practices. When flood irrigation is used, guayule germination requires about 380 mm of water, which can be reduced when a more efficient irrigation system is adopted. The objective of this study was to compare guayule germination and growth, as well as yield and water productivity under furrow irrigation (FI) and subsurface gravity drip (SGD) irrigation. A direct field-seeded guayule irrigation study was initiated in May 2022 on a 1.5-ha field at the University of Arizona, Maricopa Agricultural Center farm, in Maricopa, Arizona. The field study consisted of six plots: three 100 m x 18.3 m plots were under SGD and three 100 m x, respectively) 8.1 m plots were under FI. The experiment included one flood and one SGD treatments (denoted as F2.5 and D2.5, respectively), which were supposed to receive a predetermined irrigation amount (IA) of 762 mm (2.5 acre-ft/year) but they received 803 mm and 713 mm, respectively. Two other SGD treatments were included in the experiment, denoted as D2.0 and D1.5, which received IA of 615 mm (2.0 acre-ft/year) and 517 mm (1.7 acre-ft/year), respectively. Results indicated that SGD reduced water use during germination by 38%, with IA of 223 mm compared to 360 mm applied under FI, while providing a good crop stand with a slight increase in density (10 plants m^-2) compared to FI (8 plants m^-2). Moreover, SGD improved guayule yield and water productivity with D2.0 treatment being the best in terms of WP-DBY (1.56 kg m^-3), followed by D2.5 (1.25 kg m^-3), then D1.5 (1.13 kg/m³). There was no difference in WP-RY among SGD treatments (~0,04 kg m^-3) nor in WP-ReY between D2.5 and D1.5 (0.09 kg m^-3). The average for the three furrow plots was the lowest (WP-DBY = 0.83 kg m^-3, WP-RY = 0.02 kg m^-3, WP-ReY = 0.06 kg m^-3) despite receiving the highest total irrigation amount [TWA] (941 mm). Rubber content (R, %) and resin content (Re, %) were slightly higher for D2.0 and D1.5 which received less water. Overall, using D2.0 treatment with 615 mm IA could save 20% of water while providing 37% more DBY, 21% more RY, and 47% more ReY than FI, which received 941 mm of water (irrigation + precipitation).