Delayed or early sowing : Timing as parasitic weed control strategy in rice is species and ecosystem dependent

Parasitic weeds are a severe problem in rain-fed rice production ecosystems in sub-Saharan Africa. In this study, effects of sowing time of rice on parasitic weed infection and crop yields were investigated. Field experiments were conducted in Striga asiatica-infested upland and Rhamphicarpa fistulosa-infested lowland systems from 2012 to 2014 in Kyela district, Tanzania. In each system, three rice varieties were planted at five sowing times, the first coinciding with the start of the rainy season and the four other ones followed each at two weeks’ intervals. The chosen rice varieties were the late maturing local varieties Supa India (upland and lowland) and Mwangulu (upland) and earlier-maturing NERICA-14 (upland), NERICA-L-20 (lowland) and IR64 (lowland) as alternatives. A greenhouse pot-experiment was conducted in Wageningen, the Netherlands, combining staggered planting of rice at two weeks’ intervals with introduction of either S. asiatica or R. fistulosa at one moment in time. In both field and pot experiments sowing time influenced parasite growth and rice grain yield, but the direction of these effects differed considerably between weed species and associated agro-ecosystems. In upland, S. asiatica number and biomass decreased with a delay in sowing time. It was postulated that with these delays an increasing share of the S. asiatica seed population would return to a state of dormancy from where they are unable to germinate. Under conditions of heavy infection (2013) rice yields were highest at later sowing dates. Under moderate S. asiatica infection levels (2012 and 2014) the positive effects of late sowing on rice yield were annihilated, due to increased chances of drought stress during kernel filling. This risk was mitigated by the use of an improved early-maturing variety (NERICA-14). In lowland, there was a significant increase in R. fistulosa biomass with delayed sowing times. Planting rice before optimum soil moisture conditions (i.e. saturation) for R. fistulosa seed germination are met, would result in partial escape from infection by this facultative parasite, and consequently higher rice grain yields. Manipulating rice-sowing time is a feasible control strategy to minimize parasitic weed infection, but the proper application and associated risk of this practice are strongly species and ecosystem dependent