Prediction of Heteranthera reniformis competition with flooded rice using day–degrees

Summary In field studies carried out in 1992 and 1993, several durations of growth of Heteranthera reniformis Ruitz et Pavon were established in flooded rice ( Oryza sativa L.) to evaluate weed influence on crop yield using day‐degree predictive models for weed growth. H. reniformis was allowed to emerge at 7‐day intervals from rice emergence until 49 days later. Weeds that emerged with the rice accumulated 403 day‐degrees during the first 49 days. Weeds allowed to utilize 308 day‐degrees had a dry weight reduction of 20% compared with plants that emerged with the rice. With the loss of all 403 day‐degrees dry weight was reduced by about 95%. Maximum leaf area index (LAI) was 2.8, reached at 308 day‐degrees. Weed density ranged from 48 to 5 plants m ‐2 when all 403 day‐degrees were accumulated or lost by the plant respectively. When the weed lost only 95 day‐degrees out of the possible 403 (1 week's delay in emergence) rice yield was 34% and 39% lower in 1992 and 1993 respectively. When the weed was allowed to accumulate ail 403 day‐degrees yield reduction reached 62% in 1992 and 68% in 1993, very similar to the control plot, which were maintained weedy throughout (64% and 70.5%). After an accumulation of 403 day‐degrees the weed reduced the number of rice panicles by 45% in 1992 and 38% in 1993.