Title: Effect of intermittent irrigation on rice productivity and weed suppression Authors
|Anders, Merle -|
Submitted to: Proceedings of Southern Weed Science Society
Publication Type: Abstract Only
Publication Acceptance Date: January 28, 2014
Publication Date: N/A
Technical Abstract: Certain indica rice lines from Asia have been shown to suppress barnyardgrass (Echinochloa crus-galli) effectively in conventional flood-irrigated (FLI) production systems, but their weed suppression potential in “intermittent flood-irrigated” (IFI) production systems is unknown. IFI systems (aka “alternating wetting and drying (AWD)”) have been gaining popularity internationally and in the USA, due to the potential water savings. In these studies, weed-suppression potential of indica and commercial tropical japonica rice cultivars was compared under FLI and IFI production systems. Rice was drill-seeded on April 30, 2012 and 2013 into 3-m-long plots with 9 rows spaced 18 cm apart at a depth of 1.9 cm. Weed suppression by PI 312777 and Rondo (indicas), XP753 (commercial hybrid), Katy, Cheniere, and Bengal (commercial cultivars), and STG06L-35-061 (derived from PI 312777 x tropical japonica crosses) was evaluated under weedy and weed-free conditions. Plots were flush-irrigated as necessary after rice planting to maintain healthy crop plants prior to establishing the permanent flood. Immediately before application of the permanent flood, 110 kg/ha nitrogen was applied as urea to all plots. FLI plots were subsequently maintained fully flooded to a depth of ~10 cm for the remainder of the crop season, whereas IFI plots were allowed to deplete their irrigation water until the soil volumetric water content (VWC; as determined using transmission line oscillator [TLO] probes that measure the dielectric permittivity of soil) in the driest of the replicate plots had been reduced to a target level of 25% to 30% (average over 0-11 cm depth). At this VWC, the soil surface had a dry appearance, but the rice plants appeared to be healthy and not moisture-stressed. All IFI plots were then re-irrigated to a depth of 10 cm. These re-flooding procedures were repeated intermittently as needed throughout the growing season. The experimental design was a split, split plot, with irrigation method as the main plot, cultivar as sub plot, weed level (“weed-infested” or “weed-free”) as sub sub plots, and four replications. All weeds in the weed-free plots were killed by post-emergence spray applications of commercially recommended rates of quinclorac + propanil followed by fenoxaprop (2012) or quinclorac + cyhalofop (2013), supplemented by hand-weeding. The weed-infested plots were sprayed post-emergence with sub-recommended rates of 2.2 kg/ha (1/2 X) and 1.1 kg/ha (1/4 X) propanil, respectively, in 2012 and 2013, which were intended to lightly suppress, but not kill the weeds. Due to warm temperatures and plentiful rainfall after the 2012 planting, pre-flood barnyardgrass densities in weed-infested plots were quite high, averaging ~182 plants m-2. In 2013, pre-flood barnyardgrass densities in these plots were ~134 plants m-2, but due to an extended period of unseasonably cold temperatures after planting, the rice plants emerged six days later than in 2012. Thus, early season weed competition in the weed-infested plots was severe in both years, resulting in midseason weed control ratings ranging from 9 to 31% in 2012 and 5 to 9% in 2013 . Midseason barnyardgrass biomass averaged ~15% less in IFI compared to conventional FLI plots, and was similar in the FLI plots in both years (1827 and 1969 g m-2 in 2012 and 2013, respectively). In weed-free plots, the conventional FLI rice yields averaged 7666 and 8451 kg ha-1 in 2012 and 2013, respectively, and yields averaged ~25% less in the IFI than in FLI plots in both years. However, yields of XP753, Rondo, and PI 312777 were statistically similar in the two systems, whereas yields of the non-suppressive commercial cultivars were substantially lower (Bengal, 44%; Cheniere, 35%, and Katy, 32%) under IFI than FLI. Yield reduction in barnyardgrass-infested plots was usually >90% in both years From these limited results, it is clear that weeds can dramatically reduce production of suppressive and non-suppressive rice cultivars in both IFI and FLI systems, particularly if high weed population densities or delayed crop emergence provide a competitive advantage to the weeds during the initial stages of crop establishment. Under weed-free conditions, yields of the weed-suppressive indicas and the hybrid were relatively less affected by the IFI conditions than were Bengal and the other commercial cultivars, suggesting that the weed-suppressive cultivars may be relatively better adapted to IFI systems. Re-flooding at higher VWC levels might be a means to prevent some of the yield loss attributed to the IFI system in this study.