Potential reduction of non-imidazolinone rice grain yield by imidazolinone soil residual activity

Background  Imidazolinone (IMI) soil residual activity can carryover and affect non-tolerant rice growth and grain yield.

Objective  This study aimed to determine the IMI soil residual concentration that can potentially reduce non-IMI rice plant height (PH) and grain yield (GY).

Methods  Two field experiments in a randomized complete block design were performed to estimate the D₁₀ value (herbicide rate that reduces the rice plant height or grain yield by 10%). The factorial arrangement comprised: A) non-IMI rice (Provisia^TM, IRGA 417, and IRGA 424); B) rates of imazapyr + imazapic: 0 to 0.25x (x = 147 + 49 g ai ha^-1) applied 28 days before non-tolerant rice sowing. IMI concentration analyses were performed through high-performance liquid chromatography coupled to tandem mass spectrometry (HPLC-MS/MS) using soil samples from the 0-20 cm depth on the day of rice sowing. The data (concentration of imazapyr, imazapic and imazapyr + imazapic in the soil) were analyzed via linear regression. The means of imazapyr + imazapic were used to fit PH and GY by a nonlinear mixed-effects model.

Results  Imazapyr + imazapic soil concentration ranged from 0 to 10.08 μg kg^-1 (Year 1) and 0 to 20.11 μg kg^-1 (Year 2). D₁₀ values for PH were 5.20, 4.03, and 4.07 μg kg^-1, while D₁₀ values for GY were 4.55, 2.76, and 3.84 μg kg^-1 for Provisia^TM, IRGA 417, and IRGA 424, respectively.

Conclusions  The soil concentration of imidazolinones at rice sowing that can reduce grain yield is 2.8 to 4.5 μg kg^-1, close to the field applied doses of 3–6%, depending on various environmental and soil factors.

Carryover; Clearfield^®; Imazapyr; Imazapic; HPLC-MS/MS