African Crop Science Journal, Vol. 8. No. 3, pp. 295-300
ON-STATION VERIFICATION OF THE USE OF SOYBEAN TRAP-CROP FOR THE CONTROL OF Striga IN MAIZE
I. Kureh, U.F. Chiezey and B.D. Tarfa
(Received 13 July, 1999; accepted 10 July, 2000)
Code Number: CS00031
The major obstacles to maize (Zea mays L.) production in the West and Central African savanna are nitrogen deficiency and Striga hermo-nthica parasitism (Berner and Kling, 1995). Striga is endemic in West and Central African savannas and can cause serious devastation to maize especially on the fields of resource-poor farmers (Parker, 1991; Lagoke et al., 1994; Abayo et al., 1997). Even under good management condition, about 79% reduction in yield was observed in susceptible hybrid maize . The value of total annual crop loss due to Striga in Africa has been estimated at 7 billion US Dollars.
Traditional African cropping systems which have included prolonged fallow, rotation and inter-cropping were common management practices that were used in the past to improve soil fertility and keep the infestation of Striga spp at tolerable levels. However, increasing human population has resulted in intensive land use and a shift away from traditional cropping systems, which in turn has resulted in the depletion of soil fertility and decrease in Striga control (Berner et al., 1996). Inter-cropping of maize with non host crops has been reported to increase the efficiency of land use through improved soil productivity and reduction of Striga hermonthica soil seed bank as a result of the stimulation of suicidal germination of Striga seeds by non hosts (Berner et al., 1996; Lagoke et al., 1997; Oswald et al., 1997). Carson (1988), for example, found that Striga densities were reduced when sorghum was intercropped with groundnuts. Similarly, Odhiambo and Ranson (1993) and Carsky et al. (1994) tested maize/sorghum/cowpea combinations and achieved a significant reduction of Striga population. The spreading vegetation of non-host crops (trap-crops) also smother emerging Striga plants. There is considerable variability among non-host crops and within crop cultivars in their ability to fix nitrogen and stimulate Striga seed germination. Varieties of soybean, cowpea, groundnut, pigeon pea and cotton with very high Striga seed germination potential have been identified (Carson, 1988; Ariga et al., 1994; Carsky et al., 1994). These non-hosts, if identified for various production and cropping systems would reduce soil seed bank and incidence of Striga hermonthica in maize.
MATERIALS AND METHODS
Field trials were conducted in 1997 and 1998 wet seasons at Samaru (11°11N, 7°36E; 686 m above sea level) in the Northern Guinea savanna agroecology of Nigeria. Treatments included a mixture of Striga tolerant maize hybrid (Oba super 1) and two soybean varieties (TGX 1019-2E and TGX 1440-1E - identified as potential trap-crop varieties) with a sole maize crop as control. The treatments which were supposed to be evaluated on-farm were verified on-station in a complete randomised block design replicated three times.
The trials were established on Striga sick plots which were further inoculated with about 3,000 germinable Striga seeds hill-1 at about 50 cm apart. Maize seeds were planted on the spots one week later. Intercropped soybean was drilled, on the same day, along the edges of the 75 cm spaced ridges. Spot application of fertiliser was carried out at the rate of 100 kg N ha-1, 50 kg P2O5 ha-1 and 50 kg K2O ha-1 to maize using 20-10-10 compound fertiliser and urea. The intercropped soybean received a basal application of 20 kg N ha-1 as starter dose using urea and 50 kg P2O5 using single super phosphate (SSP - 18% P2O5). Hoe weeding was carried out at 3 and 5 weeks after sowing (WAS) followed by hand pulling of other weeds which was carried out at 7 WAS. The plot size was 22.5 m2. Data collected included Striga incidence (percentage of crop plants infested), infestation (Striga shoot count), crop syndrome reaction score (where 1 = healthy plants to 9 = dead plants), stand count, yield and yield components of maize and soybean. The data were subjected to analysis of variance as described by Cochran and Cox (1957). Significant differences among treatment means were compared using Fisher's protected least significant difference (LSD) test at P=0.05.
RESULTS AND DISCUSSION
Striga incidence and infestation as reflected in the Striga shoot count and number of maize plants infested by Striga at 9 and 12 WAS were significantly (P=0.05) affected by the type of cropping system (Table 1). Intercropping maize with soybeans supported lower incidence and infestation of Striga and exhibited significantly (P=0.05) lower crop syndrome reaction score than sole maize. Earlier reports indicated that intercropping maize with non-host crops increase the efficiency of land use through improved soil productivity and reduction of Striga hermonthica soil seed bank (Ariga et al., 1997). In our study, emerged Striga plants in the intercrop were etiolated in growth and died earlier than the Striga that emerged in the sole crop probably as result of the smothering effect of soybeans. This finding confirmed earlier report by Carson (1988) who found that the spreading vegetation of non-host crops (trap-crops) also smother emerging Striga plants.
The mixture of maize with soybean produced significantly (P=0.05) higher grain yield than sole maize crop under Striga infestation (Table 2). Likewise, the mixture of local sorghum variety with soybean or cowpea produced significantly (P = 0.05) higher head weight, 1000-seed weight and grain yield than sole sorghum. Traditional African cropping systems which included rotation and intercropping were common management practices in the past that improved soil fertility, kept Striga infestation at tolerable levels and increased seed yields of crops (Berner et al., 1996). Intercropping maize with the two soybean varieties (TGX 1019-2EB and TGX 1440-1E) did not significantly (P=0.05) affect Striga incidence, infestation, crop syndrome reaction score and grain yield. This suggested that the soybean varieties tested had similar potential for use as trap-crop varieties.
The soybean varieties tested differed only significantly (P=0.05) in lodging and 100-seed weight (Table 3). TGX 1019-2EB was moderately susceptible to lodging but produced significantly higher 100-seed weight than TGX 1440-1E. Being an early maturing variety, TGX 1019-2EB was probably able to translocate enough assimilate for seed filling before the end of the season. Thus, it produced heavier seeds than TGX 1440-1E which is medium maturing.
As shown in Table 4, intercropping maize with soybean for Striga suppression produced three times more crop value than sole maize crop. Kureh et al. (1997, unpubl.) similarly reported that the highest crop value was obtained when soybean was intercropped with either the farmers local sorghum or improved sorghum variety than sole sorghum.
In conclusion, intercropping is a good agronomic practice for Striga management in maize due to reduced striga infestation and high total crop value obtained. Soybean varieties TGX 1019-2EB and TGX 1440-1E exhibited similar potential for use as trap-crops.