The effects of herbicide application timing on Echinochloa densities in 2016 were only significant for T5; however, Echinochloa density was generally greater in plots with deeper-seeded rice. In 2017, Glyphosate alone reduced Echinochloa density by 30%, 31%, and 73% in 1.3 cm, 2.5 cm, and 5.1 cm planting depths, respectively, while glyphosate f.b. pendimethalin reduced Echinochloa density by 58%, 66%, and 80%, across the same depths. All other treatments reduced Echinochloa density by 87% or more. Treatment timing only effected Echinochloa density in T3 and T4 in 2017. L. fusca densities were lower than those of Echinochloa in either year . Treatment effects on L. fusca density were only apparent at 1.3 cm rice planting depth either year, with T4 having the highest density of 37 L. fusca plants m-2 in 2016, and 48 plants m-2 in 2017. T4 was also the only treatment with significant timing effects on L. fusca density, with lower density at greater rice planting depth either year. Sedge densities in 2016 were affected by herbicide treatment at each rice planting depth. Glyphosate alone reduced sedge density from UTC by 40%, 23%, and 86% in 1.3 cm, 2.5 cm, and 5.1 cm rice plantings, respectively, while glyphosate f.b. pendimethalin reduced sedge density by 74%, 45%, and 86% over the same planting depths. Treatments with POST herbicides reduced sedge density the most at any rice planting depth, with an average 95% reduction.Maximum air temperature at seeding was 21.8°C in 2016, and increased to greater than 30°C by the time of rice emergence . In 2017, the maximum air temperature at day of seeding was27.5°, however, the following day saw 15.2 mm of rainfall, growing rack and maximum temperature fell to 19.4°C, remaining below 25°C for several days. Rice began to emerge from 1.3 cm planting depths seven days after planting in 2016, and 8 DAP in 2017 .
Planting rice deeper than 1.3 cm delayed stand emergence similarly in both years; emergence for rice planted to 2.5 cm and 5.1 cm was delayed by three and four days, respectively. Time to 50% heading was also delayed by 1-2 days for rice planted to 2.5 and 5.1 cm. Applying glyphosate just as rice was beginning to emerge did not result in any observable crop injury in either year. In 2016, overall rice stand establishment was not affected by herbicide treatment or planting depth, although stands generally decreased with planting depth, averaging 178, 119, and 101 plants m-2 at 1.3 cm, 2.5 cm, and 5.1 cm depth, respectively, averaged across herbicide treatments. Untreated plots in 2017 were exceptionally weedy; therefore, the rice stand was impossible to estimate for UTC plots at 2.5 cm and 5.1 cm planting depths. Nevertheless, there were no stand differences among treated plots at any given seeding depth in 2017. Planting depth did affect rice stands in 2017, however. Stands in treated plots decreased by an average 89% and 96%, at 2.5 cm and 5.1 cm depths, respectively. Rice tiller density was significantly affected by herbicide treatment and planting depth in both years . Across planting depths in 2016, tiller density was 1.6 times greater than in UTC for glyphosate alone and glyphosate f.b. pendimethalin , increasing to 2.4 times greater than UTC with T5. Tillering in 2016 decreased by an average of 19% in deeper plantings. In 2017 tiller density was greatest with T5 at 1.3 cm depth, and lowest in UTC plots at 5.1 cm depth. Compared to 1.3 cm planting depth, tiller density in treated plots decreased by 60% and 56% at 2.5 cm and 5.1 cm depths, respectively in 2017.Rice plant heights were affected by herbicide treatment in both years , however, no planting depth effects were observed in 2016. In 2016, plant height was generally higher in T3, T4, and T5, averaging 95 cm, whereas plants in UTC, T1, and T2 averaged 87 cm.
In 2017 rice heights decreased as planting depth increased. Plant heights in 2017 were greatest in T3, T4, and T5, averaging a combined 93 cm, 91 cm, and 85 cm at 1.3 cm, 2.5 cm, and 5.1 cm planting depths, respectively. Yield components were largely unaffected by herbicide treatment or planting depth in 2016 , however, in 2017 differences in panicle grain yield, number of florets, and unfilled florets were apparent. In 2017 there were no harvestable panicles in UTC plots seeded at 2.5 cm and 5.1 cm planting depths, or in T1 plots seeded at the 5.1 cm depth. In either year, panicle grain yields were generally higher in less-weedy plots, particularly in plots with POST herbicides . Planting depth effects on panicle yield were likewise only apparent in weedier plots . Thousand-grain weights were lower UTC plots either year, although there were no differences among treated plots or planting depths. In both years, florets per panicle were greater in less-weedy plots, particularly with T3, T4, and T5. Florets per panicle in less-weedy plots also increased as planting depth increased. Floret filling appeared to be little affected by plot weediness or planting depth either year, and observed differences in unfilled florets were inconsistent. Both florets per panicle and unfilled florets were generally greater in 2017 than in 2016. Rice yield was significantly affected by herbicide treatment in both years , but was less influenced by planting depth in 2016 than in 2017. In either year, yields were generally greater in less-weedy plots. In 2016, yields in plots treated with glyphosate alone were 2.4-fold, 3.6- fold, and 1.7-fold greater than UTC in 1.3 cm, 2.5 cm, and 5.1 cm plantings, respectively, while yields in plots treated with glyphosate f.b. pendimethalin increased 2.9-fold, 4.4-fold, and2.6-fold over UTC, at the same planting depths.
In 2017, yields were generally higher in plots that received POST herbicides , though yields decreased as planting depth increased. Additionally, in 2017 yields in plots planted to 2.5 cm and 5.1 cm depths, and treated with T3, T4, and T5 decreased from those at the 1.3 cm planting depth by 48%, 28%, and 24%, and by 67%, 72%, and 54%, respectively.The aim of this study was to assess the feasibility of combining a stale seedbed with deep rice seeding depth, as a means to accommodate a non-selective weed burndown treatment without delaying planting. If implemented correctly, this post plant-burndown method may provide a novel cultural tool for combatting herbicide resistance in rice. Deep-seeding of rice sufficiently delayed stand emergence to allow a PPB of glyphosate without injuring rice stands. However, burn down timing effects on weed density varied by year. In 2016, Echinochloa control with glyphosate PPB alone was reduced at deeper rice plantings. Given that PPB treatments were timed to rice emergence, we expected to see greater Echinochloa control as PPB application was delayed in deeper-seeded plots. However, in 2017 delaying PPB by 5 days in the 5.1 cm planting depth plots did reduce Echinochloa density considerably, even though Echinochloa pressure was far greater that year. It is possible that the added PPB treatment delay in 2017 afforded more time for grasses to emerge and be controlled with the treatment. As Echinochloa plants were not reduced 100% by glyphosate PPB alone in any depth or year, it is evident that Echinochloa emergence is nonsynchronous at the study site, which is in agreement with previous studies . Nonsynchronous emergence may provide some insight into the inconsistent effects of PPB treatment delay with greater rice planting depth. It is also interesting that in both years, Echinochloa densities in T3 through T5 were higher with increasing rice planting depth. It is likely that reduced rice stands in these plots resulted in concomitant reduced competition from rice, potentially allowing more Echinochloa seedlings to establish . In addition, delayed flooding at 2.5 cm and 5.1 cm planting depths may also have allowed later-emerging weeds to avoid flooding suppression. Echinochloa pressure was considerably higher in 2017 than in 2016, which had significant effects on the relative competitiveness of L. fusca and sedges. Grasses in general are the most competitive weeds in DS rice , hydroponic rack system but Echinochloa tend to emerge earlier and more vigorously than sedges and L. fusca , and can easily dominate fields where control measures are inadequate.
In either year, high Echinochloa densities in UTC, T1, and T2 plots effectively suppressed L. fusca, accounting for discrepancies between visual control estimates at 20 DAP, and weed density counts at 60 DAP. However, L. fusca was more competitive in T3 and T4 at 1.3 cm planting depth, reflecting reduced Echinochloa density, and the lack of an effective POST herbicide for L. fusca in those treatments. L. fusca can become a dominant species when Echinochloa and sedges are suppressed in DS rice systems . Delaying PPB application at 2.5 and 5.1 cm depths in T3 and T4 appeared to enhance L. fusca control, however, therefore PPB treatments afforded by planting rice deeper can aid in L. fusca management efforts, particularly in fields where L. fusca resistance to cyhalofop may a problem . Planting rice deeper than 1.3 cm delayed stand emergence by several days, although the differences between 2.5 cm and 5.1 cm planting depths were minor. This is not surprising, as rice seedlings elongate quickly in soil once seed reserves are mobilized . In a related study of California rice cultivars, below-soil seedling elongation for the most vigorous cultivars increased markedly after 6 DAP , resulting in reduced emergence delays as planting depth increased. In either year, stand establishment tended to decrease with greater planting depth, however, stand establishment with deeper seeding was much lower in 2017, as several days of cooler weather coincided with planting in 2017. Colder temperatures can reduce seedling vigor and slow elongation in heavy soil. A related study found that lower-vigor California rice cultivars continued to emerge at low rates after 21 DAP . It is therefore possible that cool weather just after planting in 2017 slowed emergence of deeper-seeded rice, resulting in final rice stands somewhat larger than those measured at 20 DAP. In WS systems, rice is typically seeded at 170 – 200 kg ha-1 to overcome seed loss due to wind or predation. Drilling seed at a higher rate may likewise overcome stand and tillering loss from deeper planting in staledrill systems. In either year, rice tiller density was reduced by a lesser degree than stand density, by either treatment or depth. Tillers per plant would be expected to increase as stand density decreases , reaching up to 5-6 tillers per plant with California cultivars. However, comparing tiller and stand densities for deeper plantings in 2017 suggests up to ten tillers per plant by 60 DAP, which seems unlikely and further suggests a weather-induced delay of rice emergence, as noted above. Ultimately, although tiller density in treated plots decreased at depths greater than 1.3 cm, planting-depth effects seem to diminish between 2.5 cm and 5.1 cm depths, in accordance with a related study on depth effects on California rice .Glyphosate alone and glyphosate f.b. pendimethalin provided sufficient weed control to limit yield reductions due to weed competition to 23 – 65% in 2016, however, in 2017 yield reductions in those treatments were up to 100%. Planting rice deeper than 1.3 cm did not have an effect on yields in 2016, but yields were reduced with increased planting depth in 2017. Yield decreases in 2017 were greater than tillering decreases, suggesting that tiller die-off in deeper plantings reduced final panicle density that year. As panicle yields and 1000-grain weights were consistent across years for the less-weedy plots, it is apparent that planting depth does not affect grain quantity or 1000-grain weight. It is interesting that both florets per panicle and unfilled florets were both higher overall in 2017, resulting in similar filled grains per panicle in both years. Higher temperatures can play a role in increasing florets per panicle , while cooler nighttime weather during anthesis can cause sterility in rice , yet there were no such phenomena in 2017 to explain the elevated florets per panicle or percentage of unfilled florets.Rice [Oryza sativa L.] is grown on about 200 000 ha in northern California . Due to its hot, dry summer climate, the CA rice environment is especially conducive to competitive grass weeds, particularly Echinochloa species. For this reason, CA rice is predominantly water-seeded, in order to suppress the growth and emergence of grasses . In water-seeding, pregerminated rice seed is broadcast by aircraft into permanently flooded basins, where the seed settles on the soil surface and pegs-down roots.