Title: Do weed-suppressive rice lines maintain weed control at reduced planting densities? Authors
Submitted to: Weed Science Society of America Meeting Abstracts
Publication Type: Abstract Only
Publication Acceptance Date: January 15, 2009
Publication Date: February 1, 2009
Citation: Gealy, D.R., Yan, W. 2009. Do weed-suppressive rice lines maintain weed control at reduced planting densities?. Weed Science Society of America Meeting Abstracts. Vol 49: No 143. Technical Abstract: Weed interference and control have long been major costs to U.S. rice production systems and are even more challenging and unpredictable in reduced-herbicide-input systems. Although conventional cultivars typically suppress weeds very little, indica lines and commercial hybrid cultivars have frequently suppressed barnyardgrass (Echinochloa crus-galli) and other grasses substantially at traditional planting rates. We hypothesized that cultivars with proven natural weed suppression or high tillering traits would maintain superior weed suppression capabilities, even at low planting densities. Thus, a field study was undertaken to determine the degree to which reduced planting rates used in commercial hybrid rice production would affect natural weed suppression of conventional and weed suppressive cultivars. The experimental design was a split plot with five cultivars by three planting densities as the main plots, two weed levels (weed-free; or weedy, with barnyardgrass) as the sub plot, and four replications. Cultivars included the non-suppressive conventional long-grain, ‘Wells’, the weed-suppressive or high-tillering Asian indicas, ‘PI312777’, ‘4484-1693’ (‘Rondo’), and ‘4612’, and the commercial hybrid ‘XL723’. Rice planting densities of 160, 320, and 480 seeds/m2 were established. The lowest was similar to that used for hybrid cultivars and the highest was approximately the maximum recommended for drill seeding of traditional cultivars in Arkansas. Uniform barnyardgrass stands were established in weedy plots using the natural infestation in the soil supplemented with a broadcast seeding at planting. Experiments were planted May 7, 2007 and May 19, 2008 at Stuttgart, AR. Weed-free plots were sprayed post-emergence with propanil + quinclorac (4.4 + 0.275 kg ai/ha) and hand-weeded to prevent weed growth. Weedy plots were sprayed post-emergence with a 1/4X rate of propanil (1.1 kg ai/ha; to provide mild stunting of barnyardgrass) and with 0.55 kg ai/ha bentazon to kill broadleaf weeds. Weed control was rated visually at midseason; 100% indicated complete weed control and 0% indicated no weed control relative to a weedy plot without rice. Rough rice grain yield and other biological variables for rice were expressed as a percentage of weed-free rice plots. Biological variables for weeds were expressed relative to their levels in weedy plots with no rice present. Over all planting densities and years, maximum yield potential in weed-free plots was greatest for XL723, averaging 9860 kg/ha, and was lowest for Wells, averaging 8120 kg/ha. Generally, tillering density potential was lowest for Wells and highest for PI 312777, which averaged 31% more than Wells over the two years. In 2007, XL723 had greater natural weed suppression and maintained greater yield performance in weedy plots (as percent of weed-free) than all other cultivars at all planting densities, and weed suppression and grain yields also increased with planting density. In 2008, these trends continued, except that the cultivar responses were usually indistinguishable. Overall, weed control ratings ranged from 38% (4484-1693 at 160 seeds/m2) to 73% (XL723 at 480 seeds/m2) in 2007 and from 31% (Wells at 160 seeds/m2) to 64% (XL723 at 480 seeds/m2) in 2008. Similarly, rice yield (as percent of weed-free) ranged from 39% (4484-1693 at 160 seeds/m2) to 82% (XL723 at 480 seeds/m2) in 2007. In 2008 however, the yields followed a somewhat unexpected pattern, ranging from 26% for XL723 at 160 seeds/m2 to 83% for Wells at 480 seeds/m2. Although somewhat variable for the two years, PI 312777 appeared to maintain relatively stable suppression of weeds and yield production under weed pressure compared to Wells, which was among the cultivars most sensitive to planting density. This observation is consistent with both the proven allelopathic activity of PI 312777 and its high tillering potential. Overall, these results indicate that at low planting densities, even the high tillering or allelopathic lines did not consistently provide weed suppression or maintain high yields in the presence of uncontrolled weed pressure. However, PI 312777 and XL723 performed relatively well compared to the conventional cultivar, Wells, which tended to lose weed suppression activity dramatically at low planting densities. Moderate to high planting densities are likely to remain necessary for consistent weed suppression in reduced-herbicide-input systems.