Competitiveness of Rice Cultivars as a Tool for Crop-Based Weed Management

Abstract

Rice (Oryza sativa L.) is one of the most important crops in the world both in terms of acreage planted and human consumption (Evans, 1998). Millions of people derive a substantial portion of their caloric intake from rice and rice is grown on every continent except Antarctica (IRRI, 1993). Rice is unique among crops in the variety of environments in which it can be grown which range from drought-prone sites to tropical rainforest and from sea level to the slopes of the Himalayas (IRRI, 1993). Cropping systems range from highly mechanized, high yielding, irrigated systems in which fields are continually flooded to lower-yielding rainfed upland rice where prolonged periods of moisture stress may occur (IRRI, 1993). Weed control is essential across rice cropping systems and the economic costs of weed management practices and yield losses can be substantial (Moody, 1996). Weed management practices are also diverse and range from hand-weeding to the near-exclusive reliance on herbicides for control. Despite the diversity of environments and weed management practices, these cropping systems all rely to some extent on vigorous crop growth to interfere with weed growth. Enhanced crop competitive ability has been proposed for reducing the costs of weed control and as an environmentally-compatible tool for rice farmers (Moody, 1996). The rationale for adopting more competitive cultivars varies with cropping systems. In the Ivory Coast, approximately half of the rice yield decline was attributed to increased weed pressure (Becker and Johnson, 1995). Johnson et al. (1998) suggested that more competitive upland rice cultivars might reduce the need for hand-weeding in West Africa where only a small proportion of farmers rely on herbicides for weed control. In contrast, rice production in California is highly dependent on herbicide use and work on competitive cultivars (Gibson et al., 2001 a, b) was driven by concerns related to reducing pesticide loads in the environment and to the evolution of cross-resistant watergrass (Echinochloa spp.) populations (Fischer et al., 2000).