Fertilisers differentially affect facultative and obligate parasitic weeds of rice and only occasionally improve yields in infested fields

Abstract

Different fertilisers were field tested to investigate whether they (1) suppress the obligate parasitic weed Striga asiatica and the facultative parasitic weed Rhamphicarpa fistulosa, and (2) favour rainfed rice yields under parasitic weed infestation. Four years of experiments were conducted in southwest Tanzania, in a S. asiatica-infested rainfed upland and a R. fistulosa-infested rainfed lowland field. Treatments included sole mineral (NPK or Di-Ammonium Phosphate —DAP— plus urea), organic (cattle manure or rice husk), combinations of mineral and organic fertilisers and a no-fertiliser control. Fertilisers moderately suppressed the obligate parasite S. asiatica, but no correlations between infestation level and soil fertility status were observed. In the lowland field, fertilisers promoted the facultative R. fistulosa, in particular the ones that included organic components. Plant-available phosphorus, exchangeable potassium and soil organic matter content correlated with R. fistulosa infestation levels. Positive fertiliser effects on yields were found in both parasitic weed infested fields, except in years with high S. asiatica infestation levels. Rice husks alone and rice husks or manure combined with DAP and urea increased yields and soil fertility most. We conclude that fertilisation differentially affects the obligate and facultative parasitic weeds of rice systems; obligate parasites may be slightly suppressed, but facultative parasites may even be stimulated. Meanwhile, consistent rice yield increases under parasitic weed infested conditions cannot be obtained with fertiliser application when parasitic weed infestation levels are too high.