Air temperature (Ta) is commonly used for modeling rice phenology. However, since the growing point of rice is under water during the vegetative and the early part of the reproductive period, water temperature (Tw) is likely to have a greater influence on crop developmental rates than Ta during this period. To test this hypothesis, we monitored Tw, Ta, and crop phenology in three commercial irrigated rice fields in California, USA. Sampling locations were set up on along a transect from the water inlet into the field. (Water warms up as it moves into the field.) Ta averaged 22.7 °C across sampling locations within each field, but average seasonal Tw increased from 22 °C near the inlet to 23.4 °C furthest away from the inlet. Relative to Tw furthest from the inlet, low Tw near the inlet delayed time to panicle initiation (PI 5 days) and heading (HD 8 days) and the appearance of one yellow hull on the main stem panicle (R7 9 days). Using Tw instead of Ta when the active growing point is under water until booting (midway between PI and HD) in a thermal time model improved accuracy (root-mean-square error, RMSE) for predicting time to PI by 2.5 days and HD by 1.6 days and R7 by 1.8 days. This model was further validated under more typical field conditions (i.e., not close to cold water inlets) in six locations in California. Under these conditions, average Tw was 2.6 °C higher than Ta between planting and booting, primarily due to higher daily maximum Tw values. Using Tw in the model until booting improved RMSE by 1.2 days in predicting time to HD. Using Tw instead of Ta during this period could improve the accuracy of rice phenology models.

Crop phenology models that use constant temperature parameters across developmental stages may be less accurate and have temperature‐dependent systematic prediction error (bias). Using the DD10 model, we evaluated default and optimized (DD_Opt) temperature parameters using data from seven California rice (Oryza sativa L.) cultivars grown in six locations over 3 yr (2012–2014). Furthermore, we evaluated the effect of using stage‐dependent temperature parameters on model performance using two‐ and three‐stage optimization approaches. Optimized temperature parameters, or DD_Opt (RMSE: 2.3–5.4 d), performed better than DD10 (RMSE: 2.9–7.3 d). A temperature sensitivity analysis indicated that the time from planting to panicle initiation was most sensitive to temperature (every 1°C increase decreased days to panicle initiation by 1.8 d) while time from heading to R7 (marked by the appearance of one yellow hull on the main stem panicle) was not affected by temperature. Optimized temperature parameters varied between stages, with base temperature decreasing and optimum temperature increasing with plant development. Compared to the DD_Opt, two‐stage optimization (planting–heading and heading–R7) reduced the RMSE by 0.8 d and the systematic error by 0.6 d °C−1. Three‐stage optimization (planting–panicle initiation, panicle initiation–heading, and heading–R7) further reduced RMSE by 1.1 d and systematic error by 1.4 d °C−1 for preheading. These results demonstrate the importance of using stage‐dependent parameters to improve accuracy of phenological models, which may be important when models are used to study the crop response to climate change, field management options, ecosystem productivity, breeding, and yield gap analysis.

Rice (Oryza sativaL.) establishment systems based on resource‐conserving production practices are gaining popularity globally. To investigate the potential for improved N management and mitigation of methane (CH4) emissions, field experiments were conducted in California on three crop establishment systems: water‐seeded (WS) conventional, WS stale seedbed, and drill‐seeded (DS) stale seedbed. Fertilizer nitrogen recovery efficiency (NRE) and rice yield as affected by N rate, source, and application timing were evaluated for 2 yr in each system. Methane emissions were monitored over a full annual rice production cycle (growing season plus fallow period). Results indicated that neither split N applications nor ammonium sulfate increased yields or NRE compared with a single application of urea, regardless of system. However, the economic optimum N rate increased by approximately 30 kg N ha−1in WS stale seedbed compared with the conventional system. Since NRE generally remained similar across N treatments that maximized yields, applying the appropriate N rate as a single dose before the permanent flood would satisfy both agronomic and environmental goals of N management within each system. Both WS systems resulted in similar growing season CH4emissions. However, the DS system reduced CH4emissions by 47% compared with the conventional WS system, possibly due to a decreased period of anaerobic soil conditions. This study highlights the importance of assessing benefits as well as tradeoffs when evaluating opportunities for increasing the sustainability of direct‐seeded establishment systems with respect to N management and CH4emissions.

The efficacy of crop management is highly sensitive to the timing of operations. This study tested the hypothesis that using site-specific, real-time temperatures to predict weed emergence at the regional scale can improve the timing of weed management in stale-seedbed and drill-seeded rice (Oryza sativa) relative to the use of regional emergence averages that incorporate the spatiotemporal variability. First, thermal models of emergence for smallflower umbrella sedge (Cyperus difformis) and watergrass (Echinochloa ssp.), two of the most problematic weeds in California's direct-seeded rice system, were developed from field-scale observations made across 3 sites and 2 years. The models predicted smallflower umbrella sedge and watergrass emergence in an independently collected dataset with accuracy [root mean square error (RMSE) = 21% emergence and 1.3 d; model efficiency index (EF) = 0.80; and RMSE = 14% emergence and 2.2 d; EF = 0.88, respectively]. Subsequently, in order to quantify the degree to which spatially and temporally precise temperatures affect predicted emergence at the regional scale, the models were applied to a daily regional temperature dataset precise to 2 km x 2 km. For each species, the number of days to emergence was simulated for 48 dates (April 15-June 1), 9 years (2003-2011), and 193 locations in the Sacramento Valley rice growing region (83,376 total emergence predictions per species). The variability of the resulting emergence predictions due to the intra-annual, inter-annual and spatial heterogeneity of temperatures was measured with a linear model. Each of the spatiotemporal effects affected the emergence predictions (P < 0.001), with the temporal effects (intra- and inter-annual variability) having the greatest impact on predicted emergence. In management terms, using site-specific, real-time temperatures to predict weed emergence would have improved the timing of weed management by as much as 14 days for smallflower umbrella sedge and 12 days for watergrass when compared to using regionally-specific averages that ignored spatiotemporal variability for the simulated period. These results argue for further efforts to merge phenological models with spatiotemporally-specific environmental data in order to improve their accuracy when applied to real-time management decisions. (C) 2013 Elsevier Ltd. All rights reserved.

To meet growing global food demand with limited land and reduced environmental impact, agricultural greenhouse gas (GHG) emissions are increasingly evaluated with respect to crop productivity, i.e., on a yield-scaled as opposed to area basis. Here, we compiled available field data on CH4 and N2 O emissions from rice production systems to test the hypothesis that in response to fertilizer nitrogen (N) addition, yield-scaled global warming potential (GWP) will be minimized at N rates that maximize yields. Within each study, yield N surplus was calculated to estimate deficit or excess N application rates with respect to the optimal N rate (defined as the N rate at which maximum yield was achieved). Relationships between yield N surplus and GHG emissions were assessed using linear and nonlinear mixed-effects models. Results indicate that yields increased in response to increasing N surplus when moving from deficit to optimal N rates. At N rates contributing to a yield N surplus, N2 O and yield-scaled N2 O emissions increased exponentially. In contrast, CH4 emissions were not impacted by N inputs. Accordingly, yield-scaled CH4 emissions decreased with N addition. Overall, yield-scaled GWP was minimized at optimal N rates, decreasing by 21% compared to treatments without N addition. These results are unique compared to aerobic cropping systems in which N2 O emissions are the primary contributor to GWP, meaning yield-scaled GWP may not necessarily decrease for aerobic crops when yields are optimized by N fertilizer addition. Balancing gains in agricultural productivity with climate change concerns, this work supports the concept that high rice yields can be achieved with minimal yield-scaled GWP through optimal N application rates. Moreover, additional improvements in N use efficiency may further reduce yield-scaled GWP, thereby strengthening the economic and environmental sustainability of rice systems.

Seedling vigor is the basic component of the transplanted rice, which depends on its growing environment and proper age. Thus the influence of nursery management and seedling age was evaluated after transplanting 10, 20, 30, and 40-days older seedlings raised by using seed rate (high and low) and nitrogen (N) (with and without) in rice growing season 2008 and 2009. Study revealed that 10-days older nursery seedlings, irrespective to nursery-bed treatments, showed accelerated growth start after transplanting in main field while at later stages significant interaction was observed. Transplanting of 20-days older fertilized seedlings grown at low seeding density at nursery-bed resulted in similar growth performance as in younger. Maximum net-benefit (45111, 44402 Rs. ha(-1)) was recorded with transplanting 10-days older nursery seedlings while the fertilized seedlings grown at lower seeding density recorded highest net-benefit (37908, 36780 Rs. ha(-1)) in both the years.

Drill seeded rice ( L.) is the dominant rice cultivation practice in the United States. Although drill seeded systems can lead to significant CH and NO emissions due to anaerobic and aerobic soil conditions, the relationship between high-yielding management practices, particularly fertilizer N management, and total global warming potential (GWP) remains unclear. We conducted three field experiments in California and Arkansas to test the hypothesis that by optimizing grain yield through N management, the lowest yield-scaled global warming potential (GWP = GWP Mg grain) is achieved. Each growing season, urea was applied at rates ranging from 0 to 224 kg N ha before the permanent flood. Emissions of CH and NO were measured daily to weekly during growing seasons and fallow periods. Annual CH emissions ranged from 9.3 to 193 kg CH-C ha yr across sites, and annual NO emissions averaged 1.3 kg NO-N ha yr. Relative to NO emissions, CH dominated growing season (82%) and annual (68%) GWP. The impacts of fertilizer N rates on GHG fluxes were confined to the growing season, with increasing N rate having little effect on CH emissions but contributing to greater NO emissions during nonflooded periods. The fallow period contributed between 7 and 39% of annual GWP across sites years. This finding illustrates the need to include fallow period measurements in annual emissions estimates. Growing season GWP ranged from 130 to 686 kg CO eq Mg season across sites and years. Fertilizer N rate had no significant effect on GWP; therefore, achieving the highest productivity is not at the cost of higher GWP.

Fertilizer nitrogen (N) has been shown to impact both N2O and CH4 emissions from flooded rice systems, yet there is limited research on the effects of N rate when assessing global warming potential (GWP = N2O + CH4) per unit area and per unit grain yield (yield-scaled) on a seasonal and annual basis. A two-year on-farm experiment was conducted from 2010-2012 to test the hypothesis that optimal N rates result in maximum agronomic productivity and minimal yield-scaled GWP in water-seeded rice systems experiencing continuously flooded conditions during the growing season and fallow period. Five fertilizer N rates (0, 80, 140,200 and 260 kg N ha(-1) yr(-1)) were applied as aqua ammonia and annual N2O and CH4 emissions were quantified using the vented, closed chamber method. Results indicate that low N2O emissions occurred regardless of N rate when a permanent flood was maintained, but that large N2O fluxes occurred during discrete field drainage periods prior to harvest, particularly at high N rates. Hence, cumulative N2O emissions increased with N rate in a nonlinear manner during the growing season. Over the entire cropping cycle, the highest CH4 fluxes occurred during the middle of the growing season and following field drainage periods prior to harvest and at the conclusion of the fallow period. Mean seasonal and annual CH4 emissions tended to increase with N addition compared to the control, but significant differences were not observed between N rates. While CH4 and N2O emissions were generally not affected by N rate during the fallow period, the fallow period contributed significantly to annual emissions (e.g. 56% of annual N2O emissions across N rates). Across years, CH4 represented 94% of total GWP and as a result, mean annual GWP increased with N rate up to 140 kg N ha(-1). Maximum yields occurred between 140 and 200 kg N ha(-1), thus by employing the yield-scaled metric to begin to integrate climate change and global food demand concerns, mean annual yield-scaled GWP significantly decreased by 49% at these N rates. These findings suggest that optimal yields can be achieved with simultaneous reductions in yield-scaled GWP through efficient fertilizer N management in water-seeded rice systems experiencing continuously flooded conditions during the growing season and fallow period. (c) 2013 Elsevier B.V. All rights reserved.

Increasing food production and water saving are the major challenges for rice growers at micronutrient deficient soil in Pakistan. Thus the field experiment was conducted focusing on two emerging deficient micronutrients (B and Zn) through soil application in different rice cultures. Application of boron and zinc was done alone and in combination with each other having control treatment in various rice cultures: Aerobic rice, flooded rice and intermittent flooding and drying. Experiment was conducted by using randomized complete block design with split plot arrangements keeping rice cultures in main plot while micronutrients in subplots. Study revealed that the yield attributes (Plant height, productive tillers, panicle length and 1000 kernel weight) of rice crop were significantly hassled when it was subjected to grow in aerobic condition without micronutrients application. However, crop was not affected significantly when it was grown in modified rice culture and performed as good as the normal flooded rice. Rice crop matured in intermittent flooded and drying condition enhanced paddy yield with combined micronutrients application as compared to aerobic culture and resulted in similar performance with flooded rice. Maximum water productivity (0.26, 0.21g/liter) and benefit cost ratio (1.89, 1.72) was recorded in modified rice culture.

In water-seeded rice systems, cyanobacteria (Nostoc spongiaeforme) hinder early-season crop growth by dislodging and reducing light to seedlings. Since algae are often phosphorus (P) limited, we investigated whether changing the timing of P fertilizer application could reduce algal growth without reducing crop yields or increasing mid-season water P concentrations to levels of concern for water quality. Water P and algae were monitored in 10 and 12 (respectively) side-by-side fields (16-60 ha in size) where P fertilizer was applied pre-plant or where P application was delayed until after rice plants had emerged above the surface of the floodwater (2-5 weeks after seeding). Early-season water P concentration and algal occurrence were higher (P<0.001 and P=0.018, respectively) when P fertilizer was applied pre-plant as opposed to delayed. In fields receiving a delayed P application, water P increased to as high as 1.68 mg L-1 immediately following application and subsequently declined by 0.054 mg L-1 day(-1) (P=0.029). A separate study evaluated the effect of P fertilizer timing on crop productivity and P uptake. Triple-super-phosphate was either not applied or was applied to the soil surface in the fall prior to the cropping season, immediately prior to planting, 35 clays after seeding (DAS) and 49 DAS at a rate of 25 kg ha(-1) P. P uptake and agronomic P use efficiency (APUE) were similar when P was applied at seeding or 35 DAS. However, relative to P application at seeding, yields were reduced by 6% and there was lower APUE when P was applied after harvesting the previous crop or at 49 DAS (P<0.05). These results indicate that correctly timed, delayed fertilizer P applications can maximize rice yield while reducing early-season interference from algae. However, because delayed applications of P fertilizer also increased water P concentrations, drainage water must be managed carefully following application. (C) 2012 Elsevier B.V. All rights reserved.

Weed control is a primary concern in direct-seeded rice, particularly for herbicide-resistant weed species. which stand to threaten the long-term sustainability of California rice systems. In a four-year field study we evaluated the potential for improved weed control using no-till stale seedbed practices in water-seeded (WS) and drill-seeded (DS) rice establishment systems. In addition, as the agronomic performance of alternative tillage and crop establishment methods is not well understood, we assessed the productivity of these systems and estimated economic optimum nitrogen (EON) rates based on yield response to nitrogen (N) trials. Establishment system treatments included: water-seeded conventional tillage (WS conventional), water-seeded conventional tillage stale seedbed (WS stale), water-seeded no-till stale seedbed (WS no-till stale), drill-seeded conventional tillage (DS conventional), and drill-seeded no-till stale seedbed (DS no-till stale). Compared to the WS conventional system, WS stale and WS no-till stale treatments significantly reduced sedge weed biomass by 59 and 95%, respectively. Although redstem (Ammannia spp.) was not controlled, alternative WS systems reduced grass weed biomass by more than 99% when present. Within DS systems, no-till stale seedbed practices significantly reduced watergrass (Echinochloa spp.) biomass by 75% in the first two years but did not improve watergrass control during the second half of the study. Grain yields were not different for conventional and alternative rice establishment systems each year when N was applied at 168 kg N ha(-1) and weeds were fully controlled. However, yields were significantly lower for alternative establishment systems compared to the WS conventional system when no N fertilizer was applied, likely as a result of greater soil N losses. The response of grain yield to N rate was significantly different among systems and estimated EON rates indicated that WS stale and WS no-till stale systems required an increase of 30-35 kg N ha(-1) to maximize yields and returns to N compared to the WS conventional system. Results from this experiment demonstrate that alternative tillage and crop establishment systems can lead to improved weed control while remaining viable from an agronomic and economic standpoint in California. Provided N rates are close to optimal and WS and DS establishment methods are selected to target weed species of concern, these findings suggest that no-till stale seedbed practices should be considered as a component of integrated weed management strategies in direct-seeded rice moving forward. (C) 2012 Elsevier By. All rights reserved.

Phosphorus, an essential element for plant and animal growth, can also impair water quality. Understanding management effects on P dynamics can aid in the management of these systems to reduce nonpoint source pollution and improve fertilizer use efficiency. A sequential P fractionation procedure was used to evaluate labile to recalcitrant inorganic (Pi) and organic (Po) fractions in wetland rice soils. In this study we evaluated 71 wetland soils in the Sacramento Valley, California, consisting of different soil orders (Alfisols, Entisols, Mollisols and Vertisols) and different management systems (conventional rice, organic rice and natural wetlands). Total soil P ranged from 165 to 784 μg g−1 and averaged 415 μg g−1. Mollisols contained significantly more Pi and Po than all other soils; especially more HCl-Pi and Po. Although most studies ignore Po in the HCl fraction, 9% of total P was recovered in this fraction, suggesting that this fraction should not be ignored in studies aimed at quantifying and understanding organic P. The HCl-Po fraction was closely correlated with HCl-Pi, suggesting that it may be Ca bound P. Soils managed under organic rice production had higher NaHCO3-Pi and NaOH-Pi levels than conventional rice systems; while the natural wetland systems showed intermediate amounts. Organic or synthetic P fertilizers applied in excess of the amount of P that is removed during harvest was recovered as NaHCO3-Pi and NaOH-Pi.

Water quality concerns have arisen related to rice (Oryza sativa L.) field drain water, which has the potential to contribute large amounts of dissolved organic carbon (DOC) and total dissolved solids (TDS) to the Sacramento River. Field-scale losses of DOC or TDS have yet to be quantified. The objectives of this study were to evaluate the seasonal concentrations of DOC and TDS in rice field drain water and irrigation canals, quantify seasonal fluxes and flow-weighted (FW) concentrations of DOC and TDS, and determine the main drivers of DOC and TDS fluxes. Two rice fields with different straw management practices (incorporation vs. burning) were monitored at each of four locations in the Sacramento Valley. Fluxes of DOC ranged from 3.7 to 34.6 kg ha(-1) during the growing season (GS) and from 0 to 202 kg ha(-1) during the winter season (WS). Straw management had a significant interaction effect with season, as the greatest DOC concentrations were observed during winter flooding of straw incorporated fields. Fluxes and concentrations of TDS were not significantly affected by either straw management or season. Total seasonal water flux accounted for 90 and 88% of the variability in DOC flux during the GS and WS, respectively. Peak DOC concentrations occurred at the onset of drainflow; therefore, changes in irrigation management may reduce peak DOC concentrations and thereby DOC losses. However, the timing of peak DOC concentrations from rice fields suggest that rice field drainage water is not the cause of peak DOC concentrations in the Sacramento River.

Weeds are the major biotic constraint to rice production. Field observations have suggested that certain fertilizer regimes could enhance infestations of particular weed species emerging with rice. The study objective was to determine the effect of surface-applied calcium phosphate on weed growth in flooded California rice systems. In field and pot studies, triple superphosphate (TSP) applied to the soil surface increased weed emergence. Surface-applied TSP increased the number of sedge and broadleaf weeds, including smallflower umbrella sedge, blue-flowered ducksalad, redstem, ricefield bulrush, waterhyssop, and California arrowhead. A laboratory study measured germination of smallflower umbrella sedge and ricefield bulrush in response to the application of phosphorus (P) and calcium (Ca), which comprise 20 and 15% of TSP, respectively. Calcium stimulated smallflower umbrella sedge germination and had no effect on ricefield bulrush germination. Phosphorus did not stimulate either smallflower umbrella sedge or ricefield bulrush germination. Results indicate that surface applications of calcium phosphate increase the growth of certain weed species and that Ca may stimulate germination of smallflower umbrella sedge. By incorporating preplant applications of calcium phosphate into the soil profile, growers can reduce weed pressure from certain species. Alternatively, surface applications of calcium phosphate may be useful to stimulate weed emergence in stale-seedbed management.

California rice (Oryza sativa L.) growers typically use two forms of preplant N fertilizer: aqua NH 3 applied 7 to 10 cm below the soil surface (subsurface N) and surface-applied N. The rational for applying about 25% of the total N rate to the surface is to provide a readily available N source for young rice seedlings; however no research has been done to verify this. On-farm field studies were conducted over a 3-yr period (12 site-years) with the specific objectives of determining when rice begins to use subsurface N and to compare the efficiency of surface and subsurface applied N. Rice seedlings began accumulating subsurface N within 2 wk after sowing at some sites. When a portion of the N rate was applied to the surface, early season plant biomass and N uptake was higher than when all of the fertilizer-N was applied subsurface. In contrast, grain yields were higher when all of the N fertilizer was applied subsurface. Averaged across all sites, the fertilizer-N recovery efficiency of surface-applied N was 38% compared to 53% when only subsurface N was applied. As aqua NH 3 is less expensive than NH 4 + based fertilizers and the application of surface N requires an additional field operation, there is no justification to recommend the practice of applying surface N fertilizer in these rice systems. Instead, all of the preplant N should be applied subsurface as aqua NH 3 .

Field research and grower interviews were used to evaluate the potential of minimum tillage for California rice systems. We found that by tilling only in the fall (instead of both the fall and spring), rice farmers can control herbicide-resistant weeds when combined with a stale rice seedbed, which entails spring flooding to germinate weeds followed by a gly-phosate application to kill them. Our results indicated that yield potentials are comparable between water-seeded minimum- and conventional-till systems. We also found that rice growers can reduce fuel costs and plant early. However, minimum tillage may require more nitrogen fertilizer to achieve these yields.

Pests were monitored for three seasons in 120 farms across South and Southeast Asia to generate pest profiles under practice (TFP) and site-specific nutrient management (SSNM) schemes, and to determine the nutrient pest relationship at the production level. Totals of N and K (K2O) fertilizers applied and frequencies of application differed between FFP and SSNM plots. FFP plots received more N (approximate to 11 and 9 kg/ha for seasons 1 and 2, respectively) but lesser K2O (approximate to 11 and 21 kg/ha for seasons 1 and 2, respectively) than SSNM plots. The frequencies of N and K applications were higher in SSNM than FFP plots. A total of 16 pest incidences (8 diseases, 7 insect pests and I rat damage) were documented Sheath blight and grain discoloration were dominant in the monitoring sites. Sheath blight was observed in all sites for the first two seasons. Grain discoloration was dominant in four sites (Omon, Munoz, Sukamandi and Thanjavur). Red stripe was observed in Onion and Sukamandi for both dry and wet seasons. Incidences of sheath blight and grain discoloration, infestations of stemborer, leaffolder and whorl maggot, and rat damage were more frequent in FFP than SSNM plots. Incidences of brown spot, red stripe, and whorl maggot infestation did not significantly differ between FFP and SSNM plots. Bacterial blight, narrow brown spot, stem rot and brown plant hopper damages (hopperburn) were positively correlated with N concentration in the plant. Sheath blight and grain discoloration were negatively correlated with plant N concentration, while sheath blight was positively correlated with K concentration. Correspondence analysis by STAT-ITCF of two-season data from Omon, Vietnam showed a close association of pest injury with nutrient levels, nutrient levels with yield and yield with season.