Maize-Soybean Intercropping System: Effects on Striga Control, Grain Yields and Economic Productivity at Tarka, Benue State, Nigeria

On-Farm trials were conducted from July to November during the 2009 and 2010 cropping seasons, at Tarka, Benue State, Nigeria to evaluate the effects of intercropping maize and soybean on striga control, grain yields and economic productivity. The treatments consisted of sole maize, sole soybean and the intercrop of maize and soybean, replicated three times in a randomized complete block design. The results obtained showed that intercropping maize and soybean significantly (P ≤ 0.05) reduced striga shoot count by 55.9 % and 56.1 % respectively, in 2009 and 2010 compared to that produced on pure maize plots. Number of affected maize plants, lodging score of maize and infestation rate were lower for intercropping than for sole maize plots. The severity level was also recorded lower for intercropping compared to that recorded for pure maize stands, where severity level is in the range of high to very high. Though, soybean yield was reduced by intercropping, however, soybean and maize intercropping system increased maize grain yield, total intercrop yield, land equivalent coefficient greater than 0.25, land equivalent ratio values greater than one (LER > 1), higher total intercrop values and monetary equivalent ratio greater than 1.00, indicating yield and economic advantages. The implication of study showed that intercropping maize and soybean can be adopted by farmers as an efficient cropping system strategy to reduce striga infestation, increase maize yield and give greater economic productivity.


INTRODUCTION
Striga hermonthica (Del.) Benth, commonly known as purple witch-weed is a parasitic plant belonging to the family Scrophulariales (Abbasher et al., 1998). The origin of striga hermonthica is unclear. It may have originated in north-east Asia (Scholes and Press, 2008). It is the largest and most destructive of the striga species and considered as one of the most serious weeds in Africa (Oswald, 2005).
In the late 1990s, 21 million hectares of cereals in Africa were estimated to be infested by S. hermonthica, leading to an estimated annual grain loss of 41 million tons (Gressel et al., 2004). Incidence and severity of S. hermonthica are exceptionally high on sorghum, pearl millet and maize, the main staple foods for over 300 million people in sub-saharan Africa (Scholes and Press, 2008). Striga infestation is extending in Africa because of the high pressure on land due to population crowding (Berner et al., 1995). However, the impact of  LTD, 2014 striga damages depends on ecological conditions, cropping systems, local cultural practices and farmers' skills on the ecology (IITA, 2002) In Tarka, a location in the southern guinea savanna agro-ecological zone of Nigeria,  striga was rated as the farmers most serious weed problem as it leads to total crop failure  (IITA, 2002). Methods commonly used in the locality in controlling striga include handpulling, root digging, early planting and crop seed dressing with salt before planting (IITA, 2002). Unfortunately, these cultural practices do not lead to any significant reduction in the density of S. hermonthica in affected fields (IITA, 2002). Parker (1991) observed that intercropping sorghum with cowpea invariably reduced striga infestation. Lagoke et al., (1994) also explained that intercropping is one of the striga control practices that require only adjustments in the farming systems without any additional inputs.
Though few farmers in the locality have made attempts at intercropping maize and soybean, however, there is paucity of information on its efficacy in reducing striga infestation, as well as increasing yield and economic productivity. The aim of this study, was therefore to evaluate the effects of intercropping maize and soybean on striga control, grain yields and economic productivity with the objective of determining the efficacy of the intercropping system.

1. Location of study and farmer selection
On-farm experiments were conducted in farmer's field at Tarka district, Benue State, Nigeria, from July to November, during the planting seasons of 2009 and 2010 to evaluate the effects of intercropping maize and soybean on striga control, grain yields and economic productivity.
The farmer was selected based on availability of striga-infested plot for intercropping, willingness to grow the crop combinations, availability of labour to carry out treatment operations in time and as required, as well as allowing access of experimental farm to other interested farmers.

Experimental area, design, treatments, variety of crops and planting
The field (75.0 m 2 ) was ploughed, harrowed, ridged and divided into twelve treatment plots, each measuring 4.5 m 2 . Each plot consisted of 3 ridges, spaced 1m apart. The cropping systems employed include sole maize, sole soybean and the intercrop of maize and soybean on striga infested plots. The three treatments were arranged in a randomized complete block design (RCBD) with three replications. The variety of maize used was 'Suwan-1-SR' (an open pollinated striga tolerant variety), while that of soybean was 'TGX 1448-2E' (medium maturing variety, identified as a potential trap crop).
The trials were established on striga infested plots. Five maize stands per ridge were sown at a spacing of 1 m x 30 cm, giving a total plant population of 15 maize plants per plot (33,333 maize plants per hectare equivalent). Soybean was spaced at an intra-row spacing of 5 cm to give a plant population of 90 plants per plot (200,000 plants per hectare equivalent). In soles and intercrop, maize and soybean were sown at the depth of 2-3 cm.

3. Cultural practices
Mixed fertilizer NPK (15-15-15) was applied to sole maize at the rate of 200 kg ha -1 , while 100 kg ha -1 of single superphosphate was applied to sole soybean and for soybeanmaize mixture, 100 kg N ha -1 , 100 kg P ha -1 and 100 kg K ha -1 was applied (Enwezor et al., 1989). One hoe weeding was undertaken 3 weeks after planting (WAP), followed by handpulling of other weeds which was carried out at 7 WAP. Soybean was harvested when the pods have turned brown (Dugje et al., 2009). Maize was harvested at 12 WAP, when the leaves turned yellowish and fallen off which were signs of leaf senescence and cob maturity (Ijoyah and Jimba, 2012).

4. Data Collection
Data collected include striga shoot count, number of affected maize plants, lodging score of maize using a scale of 1-5, where 1 indicate all maize plants erect and 5 indicating all maize plants lodged (Berner et al., 1995), infestation rate of maize calculated as the ratio of plants affected to total number of plants sown (Carson, 1988), severity level of infestation using a scale of 0-4 where 0 indicate no infestation and 4 indicating a very high severity level (Carson, 1988), and grain yields of maize and soybean. Other data calculated include total intercrop yield, striga weed yield, land equivalent ratio (LER) as described by Willey (1985), land equivalent coefficient (LEC) as described by Adetiloye et al., (1983), maize and soybean crop values as soles and in intercrop, total intercrop values of the component crops and monetary equivalent ratio (MER) as described by Adetiloye (1988).

5. Statistical analysis
The data were statistically treated using the Analysis of variance (ANOVA) for randomized complete block design and the Least Significant Difference (LSD) was used for mean separation (P ≤ 0.05) following the procedure of Steel and Torrie (1980).

1. Effect of intercropping on striga control
Intercropping maize and soybean significantly (P ≤ 0.05) reduced striga shoot count at 12 WAP. Intercropping significantly (P ≤ 0.05) reduced striga shoot count by 55.9 % and 56.1 % respectively, in 2009 and 2010, compared to that produced from sole maize plots (Table 1). This result confirmed that of Dembele and Kayentao (2002) who reported that intercropping sorghum-cowpea reduced by 83 % emerged striga. Mashark et al., (2006) also reported that the maize varieties grown in Ghana under intercropping supported fewer striga plants compared to those grown in sole cropping.
The lower number of affected maize plants produced under intercropping with soybean (Table 1) could be due to the smothering effect of the soybean plants, which might have created a microclimate that could have affected the emergence and growth of striga plants. The created micro-climate could also have been conducive to the growth of micro-organism such as Fusaria, a bio-control agent against striga. Carson (1988) also reported that the spreading vegetation of non-host crops (trap crops) smothers emerging striga plants.
Intercropping maize and soybean significantly (P ≤ 0.05) reduced number of affected maize plants by 43.3 % and 50.5 % respectively, in 2009 and 2010 compared to that obtained from pure maize plots. The lodging score recorded from pure maize stands was higher compared to that obtained from intercropping ( Table 1). The greater number of striga shoot count produced from pure maize plot and the greater number of affected maize plants could have been responsible. Intercropping maize and soybean significantly (P≤0.05) reduced infestation rate compared to that recorded for pure maize plot ( Table 1).

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The severity level of striga was lower under intercropping than in pure maize stands, where the severity level was in the range of high to very high (Table 1).

2. Effect of intercropping on grain yields and Striga weed yield
In both years, soybean planted as pure stands recorded greater yield than that produced from intercropped soybean ( Table 2). This could be attributed to the shading effect of maize over soybean, and the variety of soybean used as a trap crop.
Intercropping maize with soybean increased maize grain yield by 50.0 % and 51.7 % respectively, in 2009 and 2010 compared to that produced from pure maize stands. Dembele and Kayentao (2002) reported that sorghum grain yield had been improved from 37.0 % to 80.0 % in the intercropped plots of sorghum and cowpea.
Intercropping maize and soybean also increased total intercrop yield. The total intercrop yields produced in both years were greater than the component crop yields and sole crop yields ( Table 2).
The striga weed yield was higher in sole maize plots than in sole soybean plots. The reduction in weed yield in soybean plots could be due to the smothering effect of soybean on striga emergence. Intercropping maize and soybean reduced striga weed yield by 57.1 % and 75.0 % respectively, in 2009 and 2010, compared to that obtained from sole maize plots, and

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by 50.0 % and 60.0 % respectively, in 2009 and 2010, compared to that produced from sole soybean plots (Table 2). Land equivalent ratio values were greater than one (LER > 1), indicating that it was advantageous having the component crops in mixture. Land equivalent coefficient (LEC) values were also greater than 0.25, signifying yield advantage of the intercropping system (Table 2).   (Table 3). Focus group discussion (FGD) and pair-wise ranking were used in obtaining farmers evaluation on the effectiveness of maize-soybean intercropping in the control of striga (Table  4). Farmers expressed preference for soybean (TGX 1448-2E) used in intercrop with maize (SUWAN-1-SR) as a cropping system strategy in the control of striga hermonthica, as it promotes higher maize yield, greater soil improvement and requires less labour. However, they indicated more access to the maize and soybean varieties for distribution to other interested farmers wishing to adopt the technology.

CONCLUSION
From the results obtained, it can be concluded that it is effective intercropping maize and soybean in the control of striga, as well as improving maize grain yield and economic productivity. This is associated with a significant reduction of striga shoot count, reduced number of affected maize plants, higher maize grain yield, higher total intercrop yield, higher total intercrop value, land equivalent ratio values greater than one (LER > 1), land equivalent coefficient values greater than 0.25 and monetary equivalent ratio (MER) value greater than 1.00.
It is however, recommended that further investigation be evaluated across a wider combination of maize and soybean varieties and across different locations within the southern guinea savannah agro-ecological zone of Nigeria.