INTEGRATION OF CULTURAL PRACTICES FOR WITCHWEED [Striga hermonthica Del. (Benth)] MANAGEMENT IN SORGHUM

Abstract

Field trials and a greenhouse experiment were conducted, during the rainy seasons in the period 2006 – 2009, at the experimental farm of the Faculty of Agriculture, University of Sinnar, Abu Naama. The objective of the experiments was to study the effects of various cultural practices, including nitrogen, intercropping with legumes, crop sequence, tied ridges and the herbicide 2,4-D on Striga incidence and sorghum growth and yield. Nitrogen, as urea, reduced Striga emergence, but the reduction was only significant in one out of the two seasons. 2,4-D applied, as a basal treatment 3 WAS, to plots previously treated with nitrogen, reduced Striga emergence significantly early in the season, however, the reduction, was not significant late in the season. The herbicide applied three and six weeks subsequent to the basal 2,4-D treatment effected significant and persistent reductions in Striga emergence. 2,4-D applied nine weeks after the initial 2,4-D treatment was less effective. Nitrogen and 2,4-D reduced capsules and seeds production by the parasite. However, the reduction was only significant in the first season. None of the treatments had a significant adverse effect on sorghum stand early in the season. At harvest crop stand tended to decrease irrespective of treatment. 2,4- D treatments made subsequent to nitrogen resulted in crop stand better than 2,4-D and nitrogen each applied alone. Nitrogen and 2,4-D, invariably, increased sorghum grain yield. 2,4-D applied three WAS to plots previously treated with nitrogen out-yielded most of the treatments significantly. Early in the season, the combination nitrogen and tied ridges increased Striga emergence albeit not significantly. 2,4-D and the combination 2,4-D and nitrogen, 2,4-D and tied ridges, and 2,4-D, nitrogen and tied ridges reduced Striga emergence considerably. Late in the season, nitrogen, 2,4-D, and the combinations tied ridges with 2,4-D, nitrogen with 2,4-D, and tied ridges with nitrogen and 2,4-D reduced Striga emergence significantly. All treatments invariably, reduced capsules production and seeds/capsule. None of the treatments had adverse effects on crop stand early in the season. At harvest all treatments increased crop stand over the respective control. However, only the combinations nitrogen and 2,4-D, 2,4-D with nitrogen and tied ridges, showed significant increments in crop stand over the control. The combinations 2,4-D and TR, and 2,4-D, N, and TR increased, significantly, grain and straw yields over the untreated Striga infested control in both seasons. Trap crops, invariably, reduced Striga emergence, significantly, on sorghum planted in the third season. Sunflower (SF) and sesame (SE) grown for two consecutive seasons or a fallow (FA) followed by millet (MI) or sunflower showed the highest reductions. Crop sequence, invariably, reduced Striga capsules production. All treatments reduced Striga seed bank 17 and 29 months after initiation of the experiment. Among the treatments SF/SF sequence, invariably, effected the highest reductions. No significant effects were observed for crop sequence on sorghum stand early in the season. However, at harvest significant reductions in crop stand were observed. Sorghum monoculture (SO/SO/SO) for three consecutive seasons resulted in the lowest stand, whereas sunflower grown for two consecutive seasons followed by sorghum (SF/SF/SO) displayed the highest stand. Sorghum planted subsequent to two consecutive SF or MI crops consistently displayed significant increase in both grain and straw yields. Sorghum following two consecutive sorghum crops, displayed the lowest grain and straw yields. Intercropping sorghum with cowpea, green gram and phillipisara, employing four methods of planting namely between sorghum holes (BH), within holes (WH), on one side of the ridge (OSR) and on two sides of the ridge (TSR), 2 reduced Striga emergence, often, significantly. The intercrops were more suppressive to the parasite late in the season. Planting BH and WH were often the most effective. Intercropping significantly suppressed capsules production and reduced Striga shoot dry weight. Intercropping had no significant effects on sorghum stand early in the season. However, at harvest intercropping improved crop stand albeit not significantly. The intercrops displayed no significant effects on sorghum grain yield. In among the legume intercrops, cowpea, invariably displayed the highest grain yield, followed in descending order by green gram and phillipisara. Intercropping sorghum with cowpea in a Striga-free field had no significant effects on crop stand early in the season. At harvest, considerable reductions were displayed. Unrestricted cowpea growth for six weeks or more, significantly (p = 0.05), reduced sorghum grain and straw yields in the first season. In the second season, significant stand reductions were effected, only, when cowpea was allowed to grow with the crop for 10 weeks or more. No significant effects were observed on grain and straw yields. Under greenhouse conditions, Striga emergence progressively increased with seed bank size and was positively correlated with it (r = 0.85). Capsule production consistently increased with seed bank size in the first season, but it was not affected in the second season. Sorghum stand at 30 DAS and at harvest, sorghum height, head length, and grain yield consistently declined with increasing Striga seed bank and were negatively correlated with it (r = - 0.77 to -0.93). The results confirmed that Striga is a noxious weed that cannot be controlled by a single method. A combination of cultural practices, comprising of water harvest, nitrogen fertilization, and 2,4-D displayed the most consistent results. A combination of crop rotation, water harvest and fertilization should be a core treatment in integrated Striga management strategies.