Parthenium is a competitive weed spreading in tropical countries. Field experiments were conducted to compare the effect of hand hoeing, growing a smother crop, and applying a herbicide (2,4-D) on parthenium growth and on yield of grain sorghum in smallholder farming systems in eastern Ethiopia. Hand hoeing twice and a smother crop (cowpea) in combination with hand hoeing once consistently suppressed parthenium at the experimental sites. Application of 2,4-D provided inconsistent control of parthenium, possibly because of reemergence from the soil seed bank after control. Growing cowpea as the smother crop suppressed parthenium, but sorghum grain and stalk yields were reduced even when the smother crop was combined with hoeing. Hand hoeing twice, 4 and 8 wk after emergence of sorghum, consistently resulted in better yields than application of 2,4- D and growing smother crop. It is possible that hoeing, apart from controlling the weed, also created better soil conditions for rain water infiltration.

Biological control of parthenium, a major weed in grazing areas in Australia, was initiated in the mid 1970s. Since then, nine species of insects and two rust fungi have been introduced. Evaluation using pesticide exclusion at two sites (Mt. Panorama and Plain Creek) in Queensland, Australia, revealed that classical biological control had a significant negative effect on the target weed, but the impact varied between years. In this study, I quantified the effects of biological control of parthenium on grass production. Grass production declined with the increase in parthenium biomass. Significant increase in grass production due to biological control was observed, but only in 1 of 4 yr at Mt. Panorama and 2 of 4 yr at Plain Creek. At Mt. Panorama, there was a 40% increase in grass biomass in 1997 because of defoliation by Zygogramma bicolorata and galling by Epiblema strenuana. At Plain Creek, grass biomass increased by 52% in 1998 because of E. strenuana and by 45% in 2000 because of combined effects of E. strenuana and the summer rust Puccinia melampodii. This study provides evidence on the beneficial effects of biological control of parthenium in areas under limited grazing.