Effects of fodder legume species on germination, infestation and parasitism of striga hermonthica (del.) benth. on maize (zea mays L.)
Striga hermonthica (del.) Benth, a parasitic weed reduces maize yields in western Kenya Striga infestation in maize can be reduced by use of legumes (i.e. Mucuna pruriens, Stylosanthes guyanensis and Desmodium intortum). Controlled experiments were conducted between September 1999 and March 2000 at Kibos in Kisumu to determine the effects of these fodder legumes on germination and attachment of Striga on maize roots. A field experiment was also conducted to study the effect of the legumes on Striga emergence and maize growth and yield. Laboratory petri-dish experiment showed that root exudates of fodder legumes stimulated more Striga seeds to germinate than exudates of maize or cowpea (Vigna unguiculata), suggesting that the amount of stimulant compound exuded by the roots of test plants followed the order: Stylosanthes > Mucuna ~ Desmodium > maize> cowpea. In the root chamber experiment growth of Desmodium and Stylosanthes was a major problem and only maize and Mucuna stimulated Striga seeds to germinate. Greenhouse pot experiments showed significant effect of fodder legumes on Striga attachment on maize roots. There was significantly higher number of Striga attached on maize roots when maize was intercropped with Mucuna (158 Striga germlings/plant) or Stylosanthes (153) compared to maize with cowpea (105), or Desmodium (81) or maize grown alone (61). There was a positive correlation (r=0.83) between Striga attachment on roots of maize and legume root biomass. However there were significantly fewer Striga attachmentson maize where pots were previously grown with cowpea (25/plant), Mucuna (24), Stylosanthes (7) or Desmodium (3) compared to the control (41). There was a negative correlation (~=-0.71) between legume root biomass and number of attached Striga on maize roots. Striga seed bank in the soil decreased in both the maize legume intercrop and undersowing systems compared to bare soil. In the field experiment, fodder legumes did not reduce Striga emergence significantly in season 1 (16th November to march 2000). However in seasons 2 and 3, (April-July 2000 and August -November 2000, respectively) they reduced Striga emergence significantly. Stylosanthes and Desmodium intercropped or undersown with maize suppressed Striga emergence by 92-100% in the second season. In the third season fodder legumes in both cropping systems gave 37 to 96% suppression. There was no significant difference in maize yield between all treatments in both systems in the first season. However, in the second season maize undersown or intercropped with Mucuna had higher grain yield (3060 and 2530 kg ha-I respectively) than the control (2060 kg ha-I ). In the second season there was no significant difference in maize grain yield between the control and maize associated with Stylosanthes or Desmodium or cowpea in the undersowing experiment. However maize grain yield was lower where legumes were intercropped with maize. In the third season, lower maize grain yields were attributable to drought and termite damage and all treatments were not significantly different. Legume herbage yields, was 2.6 tons ha" for Mucuna and 2 tons ha-I for Desmodium and Stylosanthes. Fodder legumes stimulated Striga seeds to germinate and reduced Striga seed bank.