Rice Density Research Articles

Inhibition of Pre-Emergent Herbicide on Weedy Rice under Flooded and Saturated Soil Conditions in Rice

Weedy rice poses a formidable challenge in rice cultivation due to its genetic similarity to cultivated rice, making selective herbicides less effective in controlling it without causing harm to the cultivated rice. The potential use of pre-emergent herbicides before rice sowing to inhibit emergence and growth of weedy rice seedlings in the soil remain unknown. Thus, this study aimed to evaluate the responses of weedy rice towards selected pre-emergent herbicides under different soil water conditions and to identify the optimal timing for rice seed sowing following the application of pre-emergent herbicide to the soil under glasshouse conditions. The results showed that oxadiazon and pretilachlor at 500 g ai ha-1 exhibited higher reductions in weedy rice density and dry weight compared to those given by pendimethalin. Under saturated soil conditions, oxadiazon demonstrated 79 and 80% reductions whereas pretilachlor gave 50 and 59% reductions in weedy rice density and dry weight, respectively. By contrast, under flooded soil conditions both oxadiazon and pretilachlor resulted in complete inhibition of weedy rice. The study showed that delaying rice seed sowing for at least six days after application of pretilachlor or oxadiazon effectively minimized phytotoxic effects on rice. These findings provide valuable insights on the impact of soil water conditions when applying pretilachlor and oxadiazon for effective weedy rice control in direct-seeded rice systems.

Modelling rice competition with Leptochloa fusca, Bidens frondosa and Ammannia coccinea in transplanted rice cultivation

AbstractA 2‐year field experiment was conducted in Daejeon, South Korea, to investigate changes in the growth parameters of rice plants during the growth and model the rice yield loss as affected by the competition with Leptochloa fusca, Bidens frondosa and Ammannia coccinea under transplanted rice cultivation. Compared with B. frondosa and A. coccinea, L. fusca significantly reduced the number of tillers at the early growth stage, resulting in more reduction in the number of panicles and other yield components such as the number of spikelets, ripened grain ratio and the ultimate rice yield at the later growth stage. Among the components, the panicle number was the most negatively and positively correlated with weed density and rice yield, respectively. Cousens' rectangular hyperbola model predicted that weed competitiveness value represented by parameter β was 0.0337 and 0.0572 for L. fusca, 0.0124 and 0.0165 for B. frondosa and 0.0059 to 0.0061 for A. coccinea in 20.8 and 15.2 hill m−2 rice densities, respectively, indicating the competition effect of L. fusca on rice yield was greater than that of B. frondosa and A. coccinea. Weed densities corresponding to a 50% reduction in the rice yield were 27 and 18 for L. fusca, 81 and 61 for B. frondosa and 169–164 for A. coccinea in 20.8 and 15.2 hill m−2 rice densities, respectively. Weeds are more competitive in low‐density rice plots, leading to greater yield loss. In addition, the economic thresholds (ET) are positively correlated with rice densities and negatively correlated with weed competitiveness. Although low‐density rice transplanting may offer economic benefits during cultivation, optimal rice density and weed control strategies are important for securing a high yield. Therefore, the model and findings could be applied to decision‐making systems in rice farming where weed interference is present.

Variance of the number of zeros of dependent Gaussian trigonometric polynomials

We compute the variance asymptotics for the number of real zeros of trigonometric polynomials with random dependent Gaussian coefficients and show that under mild conditions, the asymptotic behavior is the same as in the independent framework. In fact our proof goes beyond this framework and makes explicit the variance asymptotics of various models of random Gaussian processes. Our proof relies on intrinsic properties of the Kac–Rice density in order to give a short and concise proof of variance asymptotics.

Deep Convolutional Neural Network for Rice Density Prescription Map at Ripening Stage Using Unmanned Aerial Vehicle-Based Remotely Sensed Images

In this paper, UAV (unmanned aerial vehicle, DJI Phantom4RTK) and YOLOv4 (You Only Look Once) target detection deep neural network methods were employed to collected mature rice images and detect rice ears to produce a rice density prescription map. The YOLOv4 model was used for rice ear quick detection of rice images captured by a UAV. The Kriging interpolation algorithm was used in ArcGIS to make rice density prescription maps. Mature rice images collected by a UAV were marked manually and used to build the training and testing datasets. The resolution of the images was 300 × 300 pixels. The batch size was 2, and the initial learning rate was 0.01, and the mean average precision (mAP) of the best trained model was 98.84%. Exceptionally, the network ability to detect rice in different health states was also studied with a mAP of 95.42% in the no infection rice images set, 98.84% in the mild infection rice images set, 94.35% in the moderate infection rice images set, and 93.36% in the severe infection rice images set. According to the severity of rice sheath blight, which can cause rice leaves to wither and turn yellow, the blighted grain percentage increased and the thousand-grain weight decreased, the rice images were divided into these four infection levels. The ability of the network model (R2 = 0.844) was compared with traditional image processing segmentation methods (R2 = 0.396) based on color and morphology features and machine learning image segmentation method (Support Vector Machine, SVM R2 = 0.0817, and K-means R2 = 0.1949) for rice ear counting. The results highlight that the CNN has excellent robustness, and can generate a wide range of rice density prescription maps.

Morphological variation, distribution and relationship of weedy rice (Oryza sativa L.) in Peninsular Malaysia

AbstractThe dynamic changes in agricultural practices have negatively impacted rice cultivation resulting in serious weedy rice (Oryza sativa L.) infestations. In this study, field surveys were conducted at six granaries to evaluate the abundance, spatial‐distribution patterns, and morphological characteristics of weedy rice in Peninsular Malaysia. An average of 5.28 plants/m2 weedy rice density was recorded, with coverage area that ranged from 29% to 66% and occurrence at >67% frequency. Seed morphological characteristics identified the existence of at least 18 biotypes. A total of 134 weedy rice samples were characterized morphologically, and Principle Component Analysis results showed that these samples are highly diverse and are dispersed across Peninsular Malaysia. These genetically diverse reservoir of weedy rice characteristics present opportunities of exploitation in the future, especially through rice breeding and crop improvement. Weedy rice from Kelantan and Selangor displayed unique clusters in the PCA plot which suggest higher tendencies for environmental adaptation although the influencing factors have yet to be determined. Dispersion studies based on Lloyd patchiness index revealed that more than 50% of surveyed field blocks in five states displayed uniform distribution as opposed to cluster distribution observed in the remaining field blocks. These variations in dispersion suggested that farmers in Peninsular Malaysia employed inconsistent weedy rice management practices in every granary and farm block. Observation of weedy rice infestation pattern via spatial distribution suggested that weed management requires proper coordination of control efforts among farmers to ensure the degree of infestation is maintained under a controlled level.

Optimization of the rice-fish coculture in Qingtian, China: 1. Effects of rice spacing on the growth of the paddy fish and the chemical composition of both rice and fish

The sustainability of Global Important Agriculture Heritage System (GIAHS) such as the Qingtian rice-fish coculture system relies heavily on the good economic return that ensures the willingness of farmers to adopt, which in turn relies on the effectiveness of the technology. The dynamic conservation concept makes it possible for each GIAHS project to be optimized in technology while overall systems are conserved. To reach this goal, we compared the effects of three rice planting densities (20 cm × 30 cm, 30 cm × 30 cm and 40 cm × 30 cm) on both the fish growth performance and the rice productivities in this study. In one hand, the survival rate (SR), specific growth rate (SGR), amino acid (AA) and fatty acid (FA) of the fish (Cyprinus carpio var. color) in paddy field under three rice planting densities were compared. In the other hand, the rice yield indicators and rice stem characters were also calculated and compared among the different rice planting densities. The results showed that the SGR of fish reached the highest (4.54 ± 0.03) % in the medium rice density group (i.e., the 30 cm × 30 cm group), which was extremely significantly higher than that in the high density (20 cm × 30 cm) (3.62 ± 0.04) % (P = .001). However, the survival rates of fish were not significantly different among treatments (P = .298). The content of essential amino acid (EAA) in fish muscle under the high, medium and low rice cultivation densities were (37.35 ± 1.07) %, (37.35 ± 0.27) %, (32.20 ± 1.29) %, respectively. The first limiting amino acid was Met; the the total essential amino acid (TEAA) and the essential amino acid index (EAAI) in the medium and high density were all slightly higher than those in the low density. There were no significant differences in rice yield indicators among the treatments (P = .687); the number of effective panicles in the low density was significantly higher than that in the high density (P = .000), and panicle setting rate in the low density was significantly higher than that in the medium and high density (P = .038). In summary, the fish can obtain a faster growth rate while ensuring the rice yield in the rice density of 30 cm × 30 cm.

High-Throughput Rice Density Estimation from Transplantation to Tillering Stages Using Deep Networks.

Rice density is closely related to yield estimation, growth diagnosis, cultivated area statistics, and management and damage evaluation. Currently, rice density estimation heavily relies on manual sampling and counting, which is inefficient and inaccurate. With the prevalence of digital imagery, computer vision (CV) technology emerges as a promising alternative to automate this task. However, challenges of an in-field environment, such as illumination, scale, and appearance variations, render gaps for deploying CV methods. To fill these gaps towards accurate rice density estimation, we propose a deep learning-based approach called the Scale-Fusion Counting Classification Network (SFC2Net) that integrates several state-of-the-art computer vision ideas. In particular, SFC2Net addresses appearance and illumination changes by employing a multicolumn pretrained network and multilayer feature fusion to enhance feature representation. To ameliorate sample imbalance engendered by scale, SFC2Net follows a recent blockwise classification idea. We validate SFC2Net on a new rice plant counting (RPC) dataset collected from two field sites in China from 2010 to 2013. Experimental results show that SFC2Net achieves highly accurate counting performance on the RPC dataset with a mean absolute error (MAE) of 25.51, a root mean square error (MSE) of 38.06, a relative MAE of 3.82%, and a R2 of 0.98, which exhibits a relative improvement of 48.2% w.r.t. MAE over the conventional counting approach CSRNet. Further, SFC2Net provides high-throughput processing capability, with 16.7 frames per second on 1024 × 1024 images. Our results suggest that manual rice counting can be safely replaced by SFC2Net at early growth stages. Code and models are available online at https://git.io/sfc2net.

Does direct-seeded rice decrease ecosystem-scale methane emissions?—A case study from a rice paddy in southeast China

Rice paddy fields are one of the world’s largest anthropogenic sources of methane (CH4). With the largest area of rice paddy fields, China is experiencing a rapid shift from conventional seedling-transplanted rice (TPR) to direct-seeded rice (DSR) due to efforts to introduce labor-saving practices. However, the potential effect of this change on agricultural ecosystem CH4 flux (FCH4) are less studied and remain poorly understood. Here, we analyze FCH4 measured with the eddy covariance technique over a rice paddy where TPR was applied in 2013 and DSR in 2016. Meteorological conditions (i.e., friction velocity, radiation and temperature) between the two growing seasons were similar. However, compared to the TPR system, cumulative CH4 emissions in the DSR system were 25% higher (610.5 ± 73.3 vs 488.8 ± 56.2 kg CH4 ha−1). The increase in CH4 emissions mainly occurred during the flooding periods (i.e. DOY 173–203 and 222–260). After eliminating the effect of differences in weather conditions and water management practices between the TPR and DSR systems, daily CH4 emissions in the DSR system remained significantly higher than in the TPR system. Gross ecosystem productivity (GEP) and rice density were higher in the DSR system than in the TPR system. Cross correlation and wavelet coherence analyses showed that FCH4 were significantly correlated to GEP. Thus, increased CH4 emissions in the DSR system are most likely due to greater GEP, which was associated with higher rice plant density. With the rapid development of DSR, a scientifically sound reduced seeding density could be a promising strategy to reduce CH4 emissions.

Weed management in rice established by different methods

The field experiment was conducted at Agronomy Department Farm, College of Agriculture, Dapoli, during Kharif (rainy) seasons 2016 and 2017 to study the effect of established by different methods of rice establishment on weeds. The field experiment was laid out in a strip plot design comprising of twenty five treatment combinations replicated thrice. The horizontal strips comprised five rice establishment methods, viz. sowing of dry seeds by drum seeder at onset of monsoon, sowing of sprouted seeds (Rahu) by drum seeder in puddled field, broadcasting of sprouted seeds (Rahu) in puddled field, system of rice intensification (SRI) method and conventional transplanting. The vertical strips consisted five weed management practices, viz. need based two hand weedings at 20/30 and 40/60 DAS/DAT, pre-emergence application of oxadiargyl 80 WP 100 g/ha + 1 hand weeding at 20/30 DAS/DAT, pre-emergence application of oxadiargyl 80 WP 100 g/ha + 1 hand weeding at 40/60 DAS/DAT, pre-emergence application of oxadiargyl 80 WP 100 g/ha + post-emergence application of Almix 20 WP 4 g/ha and unweeded control. SRI method with two hand weedings carried out at 20 and 40 DAT recorded highest grain and straw yield of rice and lowest weed density and dry weight of weeds as compared to remaining treatment combinations during both the seasons.

Density Dependent Competitive Interactions between Weedy Rice (<em>Oryza sativa F. Spontaneae</em>) and Cultivated Rice (Bg 379-2) in Sri Lanka

Weedy rice (Oryza sativa f. spontaneae) is a recently emerged major threat in many rice-growing areas in tropical Asia including Sri Lanka since 1990s. Though there are scientific investigations on origin, diversity, distribution and controlling technique of weedy rice in regional and global scales, studies on the interactions between weedy rice and cultivated rice varieties under local situations are limited. A green-house experiment was conducted at the Open University, Nawala, Sri Lanka (30oC, 40% humidity) in 2015 using an improved-cultivated rice variety (Bg379-2) with varying percentages of cultivated: a black-hulled weedy rice bio-type (0, 20, 40, 60 80 100 /pot). Influence of weedy rice on growth and yield of Bg379-2 was assessed by measuring plant height and number of tillers through 2, 4, 6 and 8 weekly. Dry shoot-and root- biomass of uprooted plants at 8 weeks after sowing (8WAS)were determined through oven-dried to a constant weight at70 o C for 72 hrs and weighed to the nearest 0.01 g. Yield components of Bg379-2 including mean weight of 100 grains/plant, number of panicles/plant, number of grains/panicle and yield weight/plant were calculated. Data were statistically analyzed using SAS Ver. 9.0. Significant interactions between weedy rice bio-type and Bg379-2 were apparent after 6 weeks after sowing (WAS). A yield loss of 30% was observed even at a percentage of 20 of weedy rice at 20 /pot. The competition of weedy rice resulted in 90% loss of yield of variety Bg379-2at 80% plants/pot. For the purpose of generalization, further research is recommended to determine the effect of weedy rice density on growth and yield components of different new improved rice varieties. KEYWORDS: Competition, Oryza sativa f. spontaneae, Yield loss, Sri Lanka, Weedy rice

Effect of weedy rice at different densities on photosynthetic characteristics and yield of cultivated rice

In order to evaluate effect of weedy rice on the photosynthesis and grain filling of cultivated rice, cultivated rice ‘Nanjing 44’ was planted in the field under different densities of weedy rice ‘JS-Y1’ for two years. The results showed that net photosynthetic rate (PN), net assimilation rate, grain filling rate, and the grain yield of cultivated rice all decreased with increasing weedy rice density. Furthermore, yield component analysis revealed that increasing weedy rice density had the most significant effect on the percentage of filled grains and the number of rice panicles. The correlation analyses indicated that the yield of cultivated rice was highly correlated with the net photosynthetic rate and the net assimilation rate. Our results illustrated that high density of weedy rice might cause yield losses in cultivated rice by inhibition of photosynthesis and grain filling.

Seed density and waterfowl use of rice fields in Camargue, France

ABSTRACTEuropean rice fields provide potential nocturnal foraging habitat to waterfowl during winter. Agricultural practices may influence seed density and accessibility, thereby influencing the extent to which ducks use this managed habitat. To assess the potential influence of post‐harvest stubble‐management practices (i.e., flooding, burning, chopping, disking, plowing, and their combinations) on waste rice and weeds, and to evaluate the link of the latter 2 with nocturnal duck abundance, we conducted an experiment in rice fields of the Camargue, France. We assessed the density of available seeds 3 times across the winter, from harvest in late October to departure of the wintering ducks in early February, over 2 winters (2012–2013, 2013–2014). We monitored nocturnal duck use of the fields during the second year. Initial post‐harvest rice and weed seed densities were positively correlated and highly variable. Mean seed density was not dependent on straw management procedure (i.e., burning, chopping, disking). Average rice density decreased by 89% and weed seed density by 69% between harvest and late winter. Seed depletion rate was dependent on initial seed density, and was greatest in plowed fields for rice. Winter flooding did not significantly affect seed depletion rate but was the main determinant of nocturnal duck use of the fields. Duck numbers were lower in burned fields than in chopped or disked fields. One hectare of rice field could support an average of 43 mallard (Anas platyrhynchos)‐sized ducks/day for the entire wintering season (i.e., 5,202 duck‐energy days/ha) given initial post‐harvest seed densities. Harvested rice fields have a great potential as nocturnal foraging habitat for wintering ducks in the Camargue. Plowing should be avoided and flooding promoted to better allow ducks to use these fields. © 2016 The Wildlife Society.

Management of Cleome rutidosperma DC. using high crop density in dry-seeded rice

Cleome rutidosperma is one of the invasive C3 weed species in dry-seeded rice systems in South and Southeast Asia. Cleome species have shown a cytological developmental progression from C3 to C4 photosynthesis. Hence, a better understanding of growth responses of the weed in rice is critical in developing sustainable integrated management approaches for the suppression of this weed. Physiological and morphological changes of C. rutidosperma were studied in a pot experiment by growing the weed alone (1 plant per pot; 0.045 m2) and in competition with 4, 8, 12, and 16 plants per pot of two rice cultivars namely, IR64 and SACG4, in separate pots. The plant height of C. rutidosperma was not affected by the rice density or cultivar, and the height increased linearly with increasing time. The leaf area production of the lower half of the weed was reduced by 80–88% when grown with 12 and 16 rice plants compared to that grown without rice interference. Rice interference at the density of 16 plants per pot reduced leaf biomass of the weed by 92%, inflorescence biomass by 93%, stems biomass by 90%, and root biomass by 92% compared to the weed plants grown alone. The leaf and stem biomass allocation was higher to the lower half of the weed than that to the upper half, when grown in competition with rice. The specific stem length of C. rutidosperma increased by 84% in the presence of 12 rice plants (sowing density 60 kg/ha) compared to 4 plants (sowing density 20 kg/ha), indicating the change in resource allocation to stem elongation to enable the weed to compete effectively under shade to acquire light. However, the relative growth rate of the weed declined by 28% and 54% in the presence of 16 rice plants (sowing density of 80 kg/ha) during 3–5 and 5–7 weeks after sowing, respectively, compared to when the weed was grown without rice interference, reflecting less competitiveness of the weed for the resource acquisition under crop competition. The results of this study showed that the rice interference alone can reduce the weed's growth to a great extent. Therefore, the need of integration of other management practices in dry-seeded rice along with crop interference is highlighted to achieve complete control of C. rutidosperma and similar weeds.

Impact of volunteer rice infestation on yield and grain quality of rice.

Volunteer rice (Oryza sativa L.) grains may differ in physicochemical traits from cultivated rice, which may reduce the quality of harvested rice grain. To evaluate the effect of volunteer rice on cultivated rice, fields were surveyed in Arkansas in 2012. Cropping history that included hybrid cultivars in the previous two years (2010 and 2011) had higher volunteer rice infestation (20%) compared with fields planted previously with inbred rice (5.5%). The total grain yield of rice was reduced by 0.4% for every 1% increase in volunteer rice density. The grain quality did not change in fields planted with the same cultivar for three years. Volunteer rice density of at least 7.6% negatively impacted the head rice and when infestation reached 17.7%, it also reduced the rice grain yield. The protein and amylose contents of rice were not affected until volunteer rice infestation exceeded 30%. Crop rotation systems that include hybrid rice are expected to have higher volunteer rice infestation than systems without hybrid rice. It is predicted that, at 8% infestation, volunteer rice will start to impact head rice yield and will reduce total yield at 18% infestation. It could alter the chemical quality of rice grain at >30% infestation. © 2016 Society of Chemical Industry.

Competitive ability of cultivated rice against weedy rice biotypes: A review

Weedy rice has been identified as a threat to rice production worldwide. Its phenotypic and genotypic diversity and its potential to compete against rice in all development stages from germination to maturity have resulted in a loss of rice yield and grain quality, which is remarkably high in direct- seeded rice cultivation. Weedy rice dormancy varies, it has a higher germination rate, and tolerates deeper germination depth compared to rice cultivars. Interactions of weedy rice with cultivars often reflect early vigor, more tillering, nutrient utilization ability for shoot development with respect to rice cultivars even though the latter also show an improvement in shoot development under competition. An exponential relationship has been reported between cultivated rice loss and weedy rice density: this is true for all rice cultivars. The degree of loss is dependent on the competitive ability of the rice cultivar being studied, and each weedy rice biotype also interacts differently. Hence, the need for a comprehensive study of the biology of various weedy rice variants. Difficulties arise in the management of weedy rice due to its physiological, anatomical, and morphological similarities to cultivated rice. The manipulation of the environment to improve cultivated rice production and suppress the emergence of weedy rice variants is important in the management of weedy rice, as well as other cultural practices and use of pesticides. The development of herbicide-resistant rice cultivars is necessary to totally eliminate the weedy rice variants. This review provides information on the competitive ability of weedy rice against rice cultivars; this information is essential to create management options to control weedy rice.

On the zeros of random harmonic polynomials: The truncated model

Motivated by Wilmshurst's conjecture and more recent work of W. Li and A. Wei [17], we determine asymptotics for the number of zeros of random harmonic polynomials sampled from the truncated model, recently proposed by J. Hauenstein, D. Mehta, and the authors [10]. Our results confirm (and sharpen) their (3/2)-powerlaw conjecture [10] that had been formulated on the basis of computer experiments; this outcome is in contrast with that of the model studied in [17]. For the truncated model we also observe a phase-transition in the complex plane for the Kac–Rice density.

Effect of Nitrogen Application, Rice Planting Density, and Water Regime on the Morphological Plasticity and Biomass Partitioning of Chinese Sprangletop (Leptochloa chinensis)

Due to the looming water and labor crisis, farmers are adopting dry-seeded rice establishment, in which Chinese sprangletop is becoming a major weed. Concerns about the excessive use of herbicides in controlling Chinese sprangletop highlight the need for cultural weed management strategies. Such strategies require an adequate understanding of Chinese sprangletop response to rice plant density, nutrition, and water regime. Therefore, a greenhouse study was conducted to determine the effect of water regime (aerobic and saturated), nitrogen (N) fertilization (50 and 150 kg ha−1), and rice density (0, 160, and 640 plants m−2) on the growth and reproduction of Chinese sprangletop. Chinese sprangletop plants were taller than rice in aerobic conditions than in saturated conditions. All growth parameters (shoot, root, and inflorescence biomass and leaf area, leaf weight, and inflorescence weight ratio) of Chinese sprangletop were higher in aerobic conditions than in saturated conditions when grown without rice. However, no difference was observed for these parameters between water regimes when Chinese sprangletop was grown with rice. Chinese sprangletop growth and seed production was not affected, but rice growth was affected by N rates. Irrespective of N rate and water regime, Chinese sprangletop height (34 to 59%), tiller number (87 to 92%), leaf number (83 to 89%), shoot biomass (93 to 99%), and inflorescence biomass (95 to 99%) decreased as rice density increased from 0 to 640 plants m−2. The ability of Chinese sprangletop to grow taller and produce more plant biomass (107%) and inflorescence biomass (183%) under aerobic than saturated conditions suggests the need for integrated weed management strategies for controlling weeds under water-limited environments. Such strategies might include the use of weed-competitive and drought-tolerant rice cultivars, high seeding rates, and optimum rate of fertilizer application.

Growth analysis and biomass partitioning of Cyperus iria in response to rice planting density and nitrogen rate

Cyperus iria is a weed of rice with widespread occurrence throughout the world. Because of concerns about excessive and injudicious use of herbicides, cultural weed management approaches that are safe and economical are needed. Developing such approaches will require a better understanding of weed biology and ecology, as well as of weed response to increases in crop density and nutrition. Knowledge of the effects of nitrogen (N) fertilizer on crop-weed competitive interactions could also help in the development of integrated weed management strategies. The present study was conducted in a screenhouse to determine the effects of rice planting density (0, 5, 10, and 20 plants pot−1) and N rate (0, 50, 100, and 150 kg ha−1) on the growth of C. iria. Tiller number per plant decreased by 73–88%, leaf number by 85–94%, leaf area by 85–98%, leaf biomass by 92–99%, and inflorescence biomass by 96–99% when weed plants were grown at 20 rice plants pot−1 (i.e., 400 plants m−2) compared with weed plants grown alone. All of these parameters increased when N rates were increased. On average, weed biomass increased by 118–389% and rice biomass by 121–275% with application of 50–150 kg N ha−1, compared to control. Addition of N favored weed biomass production relative to rice biomass. Increased N rates reduced the root-to-shoot weight ratio of C. iria. Rice interference reduced weed growth and biomass and completely suppressed C. iria when no N was applied at high planting densities (i.e., 20 plants pot−1). The weed showed phenotypic plasticity in response to N application, and the addition of N increased the competitive ability of the weed over rice at densities of 5 and 10 rice plants pot−1 compared with 20 plants pot−1. The results of the present study suggest that high rice density (i.e., 400 plants m−2) can help suppress C. iria growth even at high N rates (150 kg ha−1).

Ecological significance of rice (Oryza sativa) planting density and nitrogen rates in managing the growth and competitive ability of itchgrass (Rottboellia cochinchinensis) in direct-seeded rice systems

Current understanding is that high planting density has the potential to suppress weeds and crop-weed interactions can be exploited by adjusting fertilizer rates. We hypothesized that (a) high planting density can be used to suppress Rottboellia cochinchinensis growth and (b) rice competitiveness against this weed can be enhanced by increasing nitrogen (N) rates. We tested these hypotheses by growing R. cochinchinensis alone and in competition with four rice planting densities (0, 100, 200, and 400 plants m-2) at four N rates (0, 50, 100, and 150 kg ha-1). At 56 days after sowing (DAS), R. cochinchinensis plant height decreased by 27-50 %, tiller number by 55-76 %, leaf number by 68-84 %, leaf area by 70-83 %, leaf biomass by 26-90 %, and inflorescence biomass by 60-84 %, with rice densities ranging from 100 to 400 plants m-2. All these parameters increased with an increase in N rate. Without the addition of N, R. cochinchinensis plants were 174 % taller than rice; whereas, with added N, they were 233 % taller. Added N favored more weed biomass production relative to rice. R. cochinchinensis grew taller than rice (at all N rates) to avoid shade, which suggests that it is a "shade-avoiding" plant. R. cochinchinensis showed this ability to reduce the effect of rice interference through increased leaf weight ratio, specific stem length, and decreased root-shoot weight ratio. This weed is more responsive to N fertilizer than rice. Therefore, farmers should give special consideration to the application timing of N fertilizer when more N-responsive weeds are present in their field. Results suggest that the growth and seed production of R. cochinchinensis can be decreased considerably by increasing rice density to 400 plants m-2. There is a need to integrate different weed control measures to achieve complete control of this noxious weed.

Integrated damage management reduces grazing of wild rice by resident Canada geese in New Jersey

ABSTRACTTidal freshwater marshes of the Maurice River, New Jersey, USA, have been long renowned for robust stands of wild rice (Zizania aquatica). During the 1990s, these marshes experienced an apparent decline in wild rice. During 2000–2002, I used paired fenced exclosures and open control plots to measure herbivory by the Atlantic Flyway Resident Population of Canada geese (Branta canadensis) on wild rice and response of rice to an integrated damage management program (IDMP). The IDMP consisted of rendering goose nests unhatchable, shooting, and culling molting geese. The IDMP reduced the number of goslings by 60% during the first year and essentially eliminated recruitment during the second year. Prior to the IDMP, grazing by geese reduced the density of rice by 78% and the height of plants surviving grazing by 17%. With implementation of an IDMP, rice density between exclosures and control plots did not differ. Wetland managers should consider the grazing impacts that resident population Canada geese can incur on native plant communities and develop a plan for mitigating that damage. © 2014 The Wildlife Society.