INCIDENT PHOTOSYNTHETICALLY ACTIVE RADIATION AS A BASIS FOR INTEGRATED MANAGEMENT OF PURPLE NUTSEDGE (CYPERUS-ROTUNDUS)

Artificial shading studies indicated that competition for photosynthet ically active radiation (PAR) will limit tuber production in purple nu tsedge. There were no data available to test whether there is a relati onship between incident PAR underneath crop canopies and tuber product ion of this weed. In this study, the effect of crop competition on net reproductive rates of purple nutsedge tubers was measured under field conditions. Purple nutsedge plants were grown in association with bus h beans, maize, maize and beans intercropped, sweet potato, pole beans , and bell pepper in a 2-yr field study in Costa Rica. Measurements we re taken on the number of tubers produced during the growing season of each crop, and PAR transmittance was monitored weekly for the duratio n of the respective cropping cycles. Data on transmittance and inciden t solar radiation were used to calculate the daily average amount of P AR, available 15 cm above the soil surface. Regressions indicated that average incident PAR accounted for 95% of the variation in net reprod uctive rates. Average incident PAR also allowed a more precise competi tive ranking of crops than either average or minimum transmittance. Bu sh beans had consistently the lowest average incident PAR values and t herefore ranked as the most competitive crop in both years. Our data s uggest that no net increase in tuber populations occurs if average inc ident PAR is below 2.7 MJ m(-2) d(-1). Differences in the duration of the cropping cycle accounted only for a small proportion of the overal l variation in net reproductive rates of purple nutsedge tubers. Infor mation on the competitive ranking of different crops can be used to de sign crop rotations that could reduce reliance on herbicides on small farms in Costa Rica.