Facilitating Integrated Weed Management in California Rice: Predicting E. spp. and C. difformis emergence across heterogeneous growing environments

Project Overview

Project Type: Graduate Student
Funds awarded in 2011: $17,120.00
Projected End Date: 12/31/2012
Grant Recipient: University of California, Davis
Region: Western
State: California
Graduate Student:
Major Professor:
Dr. Chris van Kessel
University of California, Davis

Annual Reports

Information Products


  • Agronomic: rice


  • Crop Production: conservation tillage
  • Education and Training: decision support system, extension, on-farm/ranch research, workshop
  • Farm Business Management: risk management
  • Pest Management: cultural control, genetic resistance, precision herbicide use, weather monitoring, weed ecology
  • Production Systems: agroecosystems


    Herbicide resistant weeds are an ever-worsening problem in California rice systems. By integrating cultural and chemical control of weeds, alternative stand establishment approaches have been shown to reduce herbicide resistant weed populations while simultaneously reducing herbicide inputs in direct seeded rice. However, management decisions are more complicated and time-sensitive when growers use alternative establishment methods for weed control.

    The overarching goal of the proposed research was to develop a geographically and temporally sensitive decision support tool that predicts the minimum time required to achieve control of watergrass (Echinochloa spp.) and smallflower umbrella sedge (Cyperus difformis) populations using alternative establishment management methods in direct-seeded rice. The project was successful in developing and validating regional-scale emergence models for both weed species, quantifying the spatial and temporal variability in emergence, and making site-specific, real-time and historical emergence data publicly available via a web-based interface.

    Coupled with effective outreach, this tool will facilitate the planning and execution of weed control via cultural methods and enable wider adoption of alternative establishment approaches. It will also serve as a platform for further efforts to translate management-related modeling outputs to rice growers in the California region.


    California rice has among the highest number of herbicide resistant weeds in the United States (Heap, 2012) due, in part, to a reliance on herbicides for weed control and soils that offer limited options in terms of crop rotation (Hill et al., 1994; Pittlekow et al., 2012). As a response to the growing problem of herbicide resistant weeds (Fischer et al., 2000; Osuna et al., 2002), the use of stale seedbeds and drill-seeded systems with intermittent early-season flooding has been investigated in recent years (Pittlekow et al., 2012). These establishment systems attempt to diversify the weed recruitment environment and improve the timing and efficacy of herbicide applications, thereby reducing the overall usage of herbicides.

    In both systems, ground is broken in the early spring, as soon as the fields are sufficiently dry from the rainy season to permit tillage. Tillage generally includes passes with a chisel plow, disc, tri-plane and roller. In the stale seedbed system, the tilled, rolled field then experiences two to four “flushes” of irrigation over the course of several weeks. The flushes completely saturate the seedbed and last for one to two days each, depending on the size of the field, the soil type and the design of the irrigation delivery system. Water is managed similarly in drill seeded fields; however, rice will have already been planted prior to the flushes. In either case, the management objective is a moist, primarily aerobic seedbed that rapidly recruits problematic weeds such as watergrass (Echinochloa phyllopogon and E. oryzoides) and smallflower umbrella sedge (Cyperus difformis), which are resistant to the greatest variety of herbicide formulations and cause the most economic damage in the California rice system (Fischer et al., 2000; Osuna et al., 2002).

    In the stale seedbed system, once the weed population has reached a critical growth threshold, a broad-spectrum herbicide (such as glyphosate) is then applied to the field. The field is subsequently flooded, and rice is planted aerially without further seedbed disturbance. The drill seeded system primarily recruits watergrass, which is controlled via propanil and/or pendimethalin applied to the unflooded seedbed after a critical growth threshold is reached and prior to the permanent flood (CRPW, 2011). Both systems have shown promising results as alternatives to the conventional establishment systems by controlling weeds while reducing herbicide use and maintaining yields (Pittlekow et al., 2012).

    Despite the promise of these systems, their efficacy is sensitive to the timing of herbicide applications. Also, in the case of stale seedbeds, planting of rice is delayed until at least as long as is required for the weeds to reach their critical growth threshold and be eliminated. This affects variety choice and introduces late season risk of blanking (Board et al., 1980). As a result, the timing of weed emergence and early growth in alternative stand establishment systems is relevant and useful information for farmers to plan for and implement effective weed management.

    We undertook a two-year study to develop and validate regional temperature-based emergence models for smallflower umbrella sedge and watergrass in stale seedbeds and drill-seeded rice systems. We then merged these models with a regional temperature dataset available at fine-scale temporal (daily) and spatial (4km2) resolutions to quantify the predicted emergence and its variability across the period of interest (April 15 to June 1). To communicate the resulting information to land managers in the region, we constructed a web-based interface that reports real-time and historical weed emergence predictions and also presents the models that underpin this decision support tool. We expect this website to be a platform from which these and other temperature-based plant growth models can be translated to site-specific management information, thereby improving the precision and efficacy of rice management in the region.

    Project objectives:

    Objective 1: Produce an empirical model that predicts the emergence of watergrass (E. spp.) and smallflower umbrella sedge (C. difformis) as a function of air temperature in rice fields with early-season irrigation flushes.

    Objective 2: Determine the spatial and temporal resolution at which air temperature can be predicted across the Sacramento Valley between April 15th and June 1st based on available historical data.

    Objective 3: Validate the accuracy of the model's emergence predictions across heterogeneous spatial and temporal environments.

    Objective 4: Implement a web-based tool that communicates spatially- and temporally-sensitive emergence predictions to growers.

    Objective 5: Communicate management-related results of research to growers via UC extension networks, field days and UC websites and publications.

    Objective 6: Publish a peer-reviewed scientific paper that reports on the accuracy and efficacy of a spatio-temporally sensitive weed emergence model in alternative establishment rice systems.

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.