Purposeful application of the agents of such diseases to weed pests among rice crops could constitute another approach to weed control. Researchers from IRRI, Maejo University and Chiang Mai University launched a study in 1993 of the interactions between weeds, crop environmental conditions, and farmers' practices in upper northern Thailand. The goals are to understand the diversity of farmers' practices and decisionmaking processes and to grade the factors that limit rice crop yields. IRRI scientists are also studying how fertilizer and cultural practices influence weed communities. In one project on phosphorus management, they are investigating how weed communities change as soil fertility is improved over time in the Philippines, Indonesia, and Thailand.
Riceplant cultivars differ in their ability to compete with weeds in the field.
Scientists in the Philippines tested the competitiveness of a dozen cultivars against weeds to help farmers choose the most highly competitive one. By planting this cultivar and enhancing its competitive ability through good management practices, farmers should be able to reduce the number of handweedings necessary while achieving maximum yields. Their suspicions arose from the fact that many highly weathered upland soils are inherently low in phosphorous and are acidic. But the scientists found that the lack of phosphorus will limit production even if calcium is added to the soil to overcome the acidity, or if acid-tolerant cultivars are planted. Rotations of rice and legumes could lead to stable, higher value production, they concluded.