Emergence and early growth of multiple herbicide–resistant and -susceptible late watergrass (Echinochloa phyllopogon)

Abstract

Abstract Late watergrass is a competitive weed of rice that is well adapted to both aerobic and anaerobic environments. Cultural controls such as a stale-seedbed and alternating from wet- to dry-seeding have been proposed as management options. However, the effects of these systems on its emergence and early growth are unknown. The objective of this study was to modify a previously developed population-based threshold model (PBTM) to predict emergence and early growth under field conditions. In 2013, a series of experiments were conducted at the California Rice Experiment Station (CRES) in Biggs, CA, to evaluate emergence and early growth of multiple herbicide–resistant and -susceptible late watergrass at four burial depths (0.5, 2, 4, and 6 cm) under three irrigation regimes: continuously flooded (CF), daily flush (DF), and intermittent flush (IF). Resistant plants emerged at a significantly higher rate under the IF treatment (P < 0.05). Both biotypes showed decreasing emergence with increasing depth, and no plants emerged from the 4- or 6-cm depths in the CF treatment. Using the Gompertz growth curve, resistant plants had greater predicted growth rates (k), lower predicted maximum heights (h max ), and a shorter time to predicted maximum growth rate (t m ) than susceptible plants under the CF and DF treatments. Under the IF treatment, the susceptible plants had greater k, lower h max , and shorter time to predicted t m . Information about burial depth and irrigation was incorporated into a previously developed PBTM for late watergrass, and validated at the CRES in a field with a susceptible late watergrass population in 2013 and 2014, under two irrigation systems, CF and IF. Model fit was best in the CF treatments (average Akaike information criteria [AIC] = 199.05) compared to the IF treatments (average AIC = 208.6).