If growers are to adopt alternative irrigation systems, understanding potential shifts in weed species’ composition will be critical to weed management. It is well documented that weed community composition can affect yields. The critical period of watergrass competition for rice in California is the first 30 d after planting, and yields can be reduced by as much as 59% when watergrass is uncontrolled . However, critical periods of competition for other weed species are not known, and differences in composition between early- and late-season weed communities and the impacts of late-season competition on rice yields remain to be seen. Using two alternative irrigation systems adapted for California rice, the primary objectives of this research were: to determine weed community composition in rice under different irrigation systems; to determine whether there are differences between early and late weed communities within a system; and to quantify differences in yields between irrigation systems in both the presence and absence of weed competition.Field preparation was standard for the California rice growing region and consisted of chiseling twice, followed by disking twice, to prepare a level seedbed . In the water-seeded alternate wet and dry and water-seeded control conditions, fertilizer was banded in by drill in strips before seeding. Fertilizer applications in the drill-seeded alternate wet and dry treatment were broadcast approximately 1 mo after planting . In all years, ebb flow tray nitrogen was applied at a rate of 171 kg ha−1 . Drilled nitrogen was applied as urea , and broadcast N was applied as ammonium sulfate .
Phosphorous was applied as triple superphosphate at a rate of 86 kg ha−1 in 2012 and at a rate of 45 kg ha−1 in 2013 and 2014. Potassium was applied as potassium chloride at a rate of 25 kg ha−1 in 2013 and 2014 only. The WS-AWD and WS-Control fields were broadcast seeded onto dry soil at a seeding rate of 168 kg ha−1 . The DS-AWD field was seeded to a depth of 2 cm at a rate of 112 kg ha−1 into dry soil. Rice seed for all treatments was pretreated with a 1 h soak in 2.5% NaClO solution to prevent infection with Bakanae disease [Gibberella fujikuroi Wollenw.]. Plots in all irrigation treatments across all years were seeded with M-206, a Calrose medium-grain rice variety widely grown in the region. The three main plot irrigation treatments were the DS-AWD, WS-AWD, and WS-Control. The DS-AWD treatment was initially flush irrigated for rice emergence and then flush irrigated once more when soil volumetric water content reached 35% .Immediately after the N fertilizer application at approximately 1 mo after planting, the DS-AWD was flooded to 10 cm above the soil surface, and water was held at that level to allow for N uptake. The WS-AWD and WS-Control plots were flooded to 10 cm above the soil surface within 24 h of broadcast seeding. The WS-AWD plot remained flooded until canopy closure of the rice, at which point water flowing into the system was shut off, and the standing water was allowed to recede into the soil. Canopy closure of the rice was determined to be when photosynthetic photon flux density reached or fell below 800 μmol m−2 s −1 , which is approximately where subcanopy PPFD stabilized. PPFD was measured every other day using a line quantum sensor at 15.2 cm above the soil surface, which was below the rice canopy.
Canopy closure was determined to be at 47, 49, and 54 d after seeding in 2012, 2013, and 2014, respectively. After being drained at canopy closure, both the WS-AWD and the DS-AWD treatment plots were flush irrigated again when soil VWC reached 35% . The WS-Control plot remained flooded until 1 mo before harvest, when it was drained to allow harvesting equipment onto the field . Soil VWC for irrigation purposes was measured at hourly intervals in each plot using EM5B data loggers and 10HS soil moisture sensors . The 35% VWC was determined using the average of the three replicates for each treatment. Further management details can be found in LaHue et al. .In 2012 there were only minor differences between irrigation systems in the weed counts taken at 20, 40, and 60 DAS. There were no significant differences in population densities of watergrass species, small flower umbrella sedge, and rice field bulrush between irrigation systems across all counts. Our results confirm previous research that showed watergrass plasticity and ability to germinate and emerge under both aerobic and anaerobic soil environments . There were three weed species with differences among irrigation treatments: redstem, ducksalad, and sprangletop. For redstem, there was an interaction between irrigation systems and count timing . Redstem was not present in any system at 20 DAS, but at both 40 and 60 DAS, the redstem density was greater in the WS-AWD than in the other two irrigation systems . Density was greater in the WS-Control system than in the DS-AWD system. The high redstem population in the water-seeded systems is consistent with earlier research showing redstem emergence under water-seeded but not under dry-seeded systems . Ducksalad density was greatest in the WS-Control and WS-AWD systems, irrespective of count timing . Sprangletop density was greatest in the DS-AWD system across all counts , though the difference was only significantly greater than the density in the WS-AWD system. These results are not surprising, since sprangletop emergence is reported to occur only under aerobic conditions in California .
Since it emerged in both the WS-AWD and WS-Control systems, further investigation of the species is warranted to elucidate whether water depth may affect emergence under flooded conditions, allowing the species to emerge under a shallow flood. Both species of sprangletop found in California, bearded sprangletop and Mexican sprangletop [Leptochloa fusca Kunth ssp. uninervia N. Snow], emerged from rice flooded to depths of 5 cm in Valencia, Spain . In Turkey, bearded sprangletop emerges at greater numbers and at a faster rate under flooded conditions than under dry conditions . Differences between weed counts at 20, 40, and 60 DAS indicate that certain species are emerging at different timings throughout the rice-growing season. Redstem did not emerge until 40 DAS across all irrigation systems. Sprangletop emerged by 20 DAS in the DS-AWD system, but did not emerge in the two water-seeded systems until 40 DAS. All other weed species emerged in significant numbers by 20 DAS, and then plant density was reduced by 40 and 60 DAS, presumably through competition for light as the canopy closed . RC and RDW. There were no significant interactions between irrigation system and years for either RC at canopy closure or RDW at harvest for all weed species and rice; therefore, only main effects are presented . RC of small flower umbrella sedge, flood drain tray watergrass species, and ricefield bulrush increased across systems from 2013 to 2014 , though the increase in rice field bulrush was not highly significant . The RC of rice also increased across all systems from 2013 to 2014. This increase may be due to the decrease in RC of ducksalad in 2014, since all other weed species increased in RC in 2014. In water-seeded Arkansas rice, ducksalad decreased yields by about 21% when germinating with rice . The decrease in RC of ducksalad in 2014 may be due to competition with other weed species, particularly watergrass, which had the greatest increase in RC of all weed species. There was a negative correlation between watergrass RC and ducksalad RC in 2013, but the relationship did not hold in 2014 . Thus, it is difficult to say with certainty why ducksalad cover decreased in 2014. Redstem and sprangletop RC were the same across years. At canopy closure the WS-Control and WS-AWD were dominated primarily by ducksalad and watergrass species, but both sedges were also present in small quantities . Sprangletop and redstem were present, but differences between systems were not significant . The only difference in weed composition between the two water-seeded systems at canopy closure was in the small flower umbrella sedge cover, which was significantly greater in the WS-AWD compared with the WS-Control. The weed species composition of the DS-AWD at canopy closure was significantly different from the composition of the water-seeded systems. It was dominated by watergrass species, and the only other species present was sprangletop . RDW of all weed species did not vary across years. There were only two species that were significantly different across irrigation systems: small flower umbrella sedge and watergrass species .
The RDW of small flower umbrella sedge was greatest in the WS-AWD, which was consistent with its RC at canopy closure. Ducksalad was not present at harvest, presumably because it had completed its life cycle and decomposed, although no information on longevity of this species is recorded in the literature. In Arkansas wet-seeded rice, Smith found that ducksalad matured by approximately 8 wk after seeding. In the DS-AWD system at harvest, the RDW of rice was 3% . In comparison, the WS-Control and WS-AWD systems had rice RDW measures of 72 and 77%, respectively. The differences in firequencies of weed species in the DS-AWD and the water-seeded systems corresponded to the differences in RC and RDW . Frequency of small flower umbrella sedge varied between WS-AWD and WSControl . The percentage contribution of small flower umbrella sedge to the dissimilarity between the irrigation systems was the greatest of all weed species at every measurement point, except at canopy closure assessment in 2013. Analysis of the two systems over time showed that although the firequency of small flower umbrella sedge was similar in the WS-AWD and WS-Control at canopy closure in 2013, the firequency of the species was consistently greater in the WS-AWD at all other assessment points . Small flower umbrella sedge cover was greatest in the WS-AWD treatment , and the relativecover of small flower increased in 2014 over 2013 . The relative dry weight of small flower umbrella sedge was greater in the WS-AWD than in the other treatments in both 2013 and 2014. Both the initial germinable seedbank assessment in 2012 and the plant density counts at 20 DAS in 2012 indicate similar germinable populations of small flower umbrella sedge in the WS-Control and WS-AWD irrigation systems. The increased density in the WS-AWD system at 40 and 60 DAS and the increased cover and biomass in both 2013 and 2014 may indicate that the drain at canopy closure affects small flower umbrella sedge germination or competitive ability. Small flower umbrella sedge germination is best under flooded conditions, though it appears to germinate well under saturated soil conditions as well . Preliminary evidence suggests that small flower umbrella sedge has a biphasic emergence pattern , and the relative growth rate of plants emerging under the second germination flush may be greater under the drier conditions of the WS-AWD. The irrigation system was shut off and the water was allowed to recede into the soil beginning at 47 DAS in the WS-AWD system in 2012. In 2013 and 2014 this occurred at 49 DAS and 54 DAS, respectively. Weed density counts were taken 1 wk before the irrigation shutoff in 2012, and weed relative cover ratings were taken 1 d before irrigation shutoff in both 2013 and 2014. Thus, it is possible that the increase in small flower umbrella sedge in the WS-AWD system may be unrelated to the irrigation system and was an artifact of greater population density in 2012. This could be related to the lower ducksalad density in the WS-AWD system that same year. Ducksalad may have a suppressive effect on small flower umbrella sedge, given that it quickly covers the canopy, blocking out light, which small flower umbrella sedge requires for germination . The two weeds had a similar density at the beginning of the experiment but small flower increased as the experiment continued .Rice relative cover increased from 2013 to 2014 over all treatments, yet the increase in 2014 at canopy closure did not correlate with an increase in rice biomass at harvest in 2014. This response confirms earlier research in California that showed competition with late watergrass after the critical period of competition further decreased rice yields . It is significant to note that despite statistically similar initial populations of watergrass species in all fields , rice cover and biomass were lowest in the DS-AWD compared with the water-seeded treatments, either indicating that the watergrass species are more competitive against rice under anaerobic conditions or confirming that rice is less competitive with weeds under anaerobic environments .