M-209 T50 was affected by planting depth in 2019, at 79 DAP and 80 DAP for 3 cm and 6 cm planting depths, respectively. Full-season plant heights were not affected by PPB treatment, cultivar or planting depth either year .In 2018 there were no harvestable panicles or yield component analysis in UTC plots. Rice yield components in weed-free plots were significantly affected by cultivar in 2018 ; all measured yield components were greater for M-209 at either planting depth. M-209 averaged 26 more florets per panicle , and 19 more filled grains than M-206 in 2018. Percent unfilled grains was 1.6-fold greater in M-209 . Although not significant, panicle yield, 1000-grain weight, total florets per panicle, and filled florets per panicle were generally greater for rice seeded at the 6 cm planting depth for both cultivars in 2018. In 2019 yield components were significantly reduced in UTC plots. Averaged across planting depth and cultivar, panicle grain yield, florets per panicle, filled grains per panicle, and 1000-grain weight decreased 47%, 31%, 37%, and 15%, respectively . M-209 planted at the 6 cm depth had the greatest reductions in yield components in 2019 UTC plots, with nearly 30% lower 1000-grain weight and 50% blanking when seeded to that depth. In 2019 weed-free plots, cultivar differences in yield components varied. Averaged across planting depths, M-206 had greater panicle grain yield and 1000-grain weight , vertical grow racks while M-209 had greater florets per panicle . M-209 blanking percentage averaged 1.7-fold greater than M-206 in 2019.Weed group composition in untreated plots did not vary between years, however weed group densities varied greatly.
Aquatic broad leaves and algae were not observed in either year, which is in agreement with previous research on DS rice using early-season flush irrigation, orfull-season alternate wetting and drying . As aquatic weeds and algae can inhibit rice growth via shading and physical barrier in water-seeded systems, the potential suppressive benefit of early-season flushing in DS rice is clear. This can be a useful component of the stale-drill method as a measure for herbicide resistance management, as algae and broad leaf suppression can occur without any additional resistance selection pressure. Grass densities were high in both study years, however Echinochloa densities were lower in 2019, allowing L. fusca and sedges to become more competitive. It is interesting that all three major weed groups were present in roughly equal numbers early in 2019 , yet Echinochloa and sedge densities decreased dramatically by 45 DAP, while L. fusca densities increased. In 2018, extreme relative density allowed Echinochloa grasses to easily out-compete other weeds and rice in UTC plots, whereas in 2019 reduced Echinochloa density allowed L. fusca to out-compete other weeds. Emergence of L. fusca is discontinuous throughout the season , and the lack of suppressive competition -particularly from Echinochloa spp.- appeared to allow later-emerging L. fusca to freely establish.Drilling rice seed at 3cm and 6 cm depths delayed rice emergence and successfully permitted the use of a postplant burndown herbicide treatment just as rice was beginning to emerge. Using flush-irrigation to prime weed seed resulted in timed emergence of the majority of observed grasses, and glyphosate use alone reduced combined grasses by more than 50% in both study years .
Although L. fusca emergence appeared to continue after PPB treatment, this was not observed with Echinochloa or sedges. Echinochloa spp. emergence is also known to be discontinuous in rice systems , however our results suggest that shallow flushing may have inhibited Echinochloa emergence from heavy soil as it dried and crusted over. Previous research showed a weed control benefit to applying pendimethalin at rice emergence, as a part of a PPB treatment . As there were no differences in further weed control between the subsequent treatments in the present study , it appears that there were no added late-season benefits of applying pendimethalin or clomazone at the 3-leaf stage. Using the stale-drill method with PPB can achieve the dual cultural-chemical effects suppressing aquatic species and shifting the weed spectrum, as well as allowing novel modes of action to be used to control grasses and sedges. Both of these effects could reduce the spread of herbicide resistance, if stale-drill is used in rotation with other rice establishment methods.Previous research found that M-206 emergence in the field was delayed by up to three extra days by planting to 5.1 cm, compared to 1.3 cm . However, emergence in that study was likely slowed by unseasonably cool temperatures immediately following planting. A related greenhouse study comparing the relative vigor four California cultivars found that M-206 planted to 5.1 cm and 6.4 cm had time to 50% emergence of 9.4 and 14.0 days, respectively. In contrast, M-209 had T50 of 8.3 and 9.4 days at the same respective planting depths . Based on these prior findings, we expected shallower-sown rice to emerge earlier, and expected M-209 to emerge before M-206. However, although there were minor differences in emerged seedling length, we found no differences in emergence date between cultivars or planting depths. The soil at the study site is a Vertisol, characterized by shrinking and cracking as it dries .
We observed that soil cracking in hot weather after the initial flushing event followed the lines of furrows left by the seeding drill. This cracking likely exposed elongating seedlings to light and oxygen, hastening emergence . Taken together, our findings support the hypothesis that California cultivars have sufficient vigor to emerge rapidly and evenly from these depths, however the increased vigor of M-209 may provide an emergence advantage if planted in cooler than normal conditions. The stand reductions at 6 cm planting depth observed in 2018 were not repeated in 2019. It is possible that physical or allelopathic effects of the much higher weed density that year inhibited some rice from establishing. The relative competitiveness of rice and Echinochloa spp. is well documented, and recent research suggests that root exudates from E. crus-galli and E. colona may have inhibitory effects on rice germination and emergence. Alternatively, growing degree day accumulation was more rapid in 2019 due to the later planting date, which may have minimized stand reductions due to deeper seeding. Nevertheless, increased tillering in M-206 compensated for stand reductions in 2018. In a related study we found increased tillering with increasing soil crown depth in these cultivars , although the opposite has been observed previously in small-seeded cereals . In addition, M-209 planted at a depth of 6 cm reached heading later than the more shallow seeding in 2019, which resulted in fewer filled grains at time of harvest.Applying glyphosate to just-emerged rice resulted in tip die-back, but no other symptoms developed. Glyphosate is a systemic herbicide, and needs to be translocated to the crown of a graminid species in order to fatally inhibit the 5-enolpyruvlyshikimate-3-phosphate synthase enzyme . Although the emerged rice seedlings were green at the time of PPB application, the lack of secondary symptoms could be evidence that seedlings were not yet translocating, and therefore still using seed reserves for growth. In addition, glyphosate in solution is anionic, and readily binds to clay soil particles, especially in lower pH soils such as found in rice systems . It is also possible that soil particles attached to the coleoptile below the emerged leaf may have protected the rice somehwat by binding glyphosate molecules. Al-Khatib et al. found that foliar uptake of glyphosate bound to particles of silt loam with 6.6 pH was less than 1% each in alfalfa and pea , and 3% in grape , compared to roughly 50% uptake of aqueous glyphosate in the same species. Grains with starchy reserves such as rice tend to tolerate anaerobic environments well , and this fact is certainly a major factor in the ability of rice organs to elongate vigorously through heavy soils or floodwaters. Alpi and Beevers found that a vigorous japonica cultivar was able to continue coleoptile elongation for up to two weeks before exhausting seed reserves. The cellular machinery in rice that is optimized for fermentative anaerobiosis also appears to provide an emergence benefit for rice grown aerobically in deep soil. Rapid and even stand emergence is key to timing a PPB treatment in deep-sown rice, and good field scouting is essential to determining emergence.
Although application of a non-selective herbicide directly to emerged rice seedlings would not be recommended, growing tables the continued reliance of just-emerged seedlings on seed reserves can provide a fail-safe against application of a normally-lethal herbicide as a burndown treatment.Differential cultivar responses to seasonal variability were apparent in this study. Trials were planted later in 2019, for a 129-day growing season versus 139 days in 2018. Although both cultivars have nominal season durations of about 140 days , M-209 is slower to mature than M-206, reaching 50% heading about six days later in traditional WS rice culture. M- 209 reached 50% heading eight days later than M-206 in 2018, but only 3-4 days later in 2019, reflecting a T50 shift of 3-4 DAP in 2019 . As the rice was planted later in the summer, growing degree day accumulation would be expected to be greater early in the season, resulting in panicle initiation occurring earlier than expected in M-209. This possibility, along with the shortened season duration, may explain why M-209 appeared to have higher sensitivity to seasonal fluctuations than M-206.We find that the present study serves as a successful proof-of-concept for the stale-drill method as a new strategy for rice production and weed management. This work agrees with previous studies that suggested that California semidwarf rice cultivars possessed suitable vigor to emerge evenly from seeding depths up to 6 cm, and that a PPB application of a non-selective herbicide could be safely administered to emerging rice without causing sustained crop injury. Aquatic weeds were suppressed by water management, and the PPB treatment reduced overall weed density by more than 50% in both years, regardless of planting depth or cultivar used. Variability in observed effects of deeper planting on rice growth and development do not support planting rice deeper than 3 cm, however. Although the two cultivars used in this study have varying levels of observed responses to seasonal variability, adequate field preparation, irrigation management, variety selection, and scouting can help to ensure healthy and economically competitive stands. In order to validate our confidence in this method, field-scale trials analyzing the logistical and economic parameters of implementing this program across soil and climate types are necessary. In addition, further refinements to herbicide programs emphasizing reduced input costs, and the potential of reducing seedbanks of weedy rice and herbicide-resistant weeds, would help to more adequately assess the flexibility and potential utility of this method.Glyphosate-resistant crops, also known as “Roundup Ready” , have become an important part of cropping systems in the United States. In 2004, approximately 13 percent of corn, 85 percent of soybean, and 60 percent of cotton acreage was occupied by RR varieties. Alfalfa is the nation’s third most important crop in economic value, and it occupies more than 22 million acres in the United States . It is considered the premier forage crop. It is the primary feed for dairy production, and is commonly fed to beef cattle, sheep, and horses. Alfalfa is also used for green chop and silage in many areas. California is the leading producer of alfalfa hay in the United States, followed by Wisconsin, South Dakota, Minnesota, and Idaho. Roundup Ready technology has been successfully incorporated into alfalfa and is scheduled for commercial release in 2005. This publication reviews the important attributes and issues pertaining to RR technology as applied to alfalfa and the potential impacts of this technology on production systems and markets.Roundup is a broad-spectrum herbicide that kills a wide range of plants. It is not normally applied directly to crops. The RR technology incorporates genetic resistance to glyphosate into crop plants by inserting a single bacterial gene that modifies 5-enolpyruvylshikimate-3-phosphate synthase, an enzyme essential for plant growth. Monsanto has used this technology to develop several RR crops . Specifically, these new varieties will allow glyphosate to be applied over the top of the entire crop to control a wide spectrum of annual and perennial weeds commonly found in alfalfa. Several of these weeds, especially perennials, are difficult to control using conventional herbicides or non-herbicide weed control methods.