“Global Change” refers to the complex of interlinked changes that are altering the contemporary earth at an unprecedented and accelerating rate. It includes the growing demands of the human population for food, fibre and water that have led to a dramatic transformation of the land surface, from quasi-natural cover to cultivated lands (Scholes et al., 2006). In addition to the population driven increase in demand for food, the demand for basic infrastructural facilities due to population growth and increasing pace of urbanization has increased the rateof agricultural land alienation especially in Sub Sahara Africa. These factors have to a large extent, been major drivers of increased pressure on agricultural land across these countries.

In addition to the problem of land alienation, the rapid population increase estimated to be at an annual average of 2 to 4% in the midst of lower food production growth rate of 1.9% in Sub Sahara Africa is viewed as being capable of exacerbating the precarious food situation in the region if unabated (FAO, 1996; Rosegrant et al., 2001; Scholes et al., 2006; USDA, 2006; Tappan and McGahuey, 2007). In view of this, a long-standing interest has been generated on greater and more effective utilization of arable land for increased productivity at the farm level.

In Nigeria, increased pressure on arable land is evident in reduction in land-man ratio, average size of farmland (Bamire and Manyong, 2003), increased land use intensity characterized by multiple cropping and intercropping of two or more crops under shortened fallow (Oluwatosin et al., 2008; Saka et al., 2011). While intercropping of two or more crops under shortened fallow are indicative of increased land-use intensification, proponents of agricultural intensification (Sivanappan, 1995; Jabar et al., 1998; Cassman, 1999) identified commensurate use of modern inputs as essential conditions for sustainable increase in productivity, in the absence of which intensification could lead to rapid land degradation.

Intercropping is traditionally noted for its role as a risk aversion measure. Its persistence in the Nigeria food sub-sector is further justified by the increasing threat of climate variability in recent times. However, the competition among component crops for space, soil nutrients and moisture in a cropping system characterized by low level of external inputs point to the challenges of such practices on productivity. Similarly, the structure of the cropping system also goes a long way in determining the opportunity for further intensification through multiple cropping, opportunity for enhanced productivity and income. The income potential is to a large extent, influenced by the market premium attracted by each commodity. This study therefore examined the implication of the choice of crop combination for intercropping, land-use intensity and agro-ecologies on productivity and income potential of food crop farmers in Southwest Nigeria.

 

The relative agronomic and economic potentials were then compared across crop combinations, fertilizer usage, agro-ecologies and land-use intensification categories using Analysis of Variance (Anova) to explore the influence of these factors on the potentials. Following Alabi and Esobhawan (2006) and Chukwuji (2008), the agronomic and economic potential indices were estimated as shown respectively:

 

 

Where: Q_pij = Average yield of crop j when grown by farmer i as intercrop (or polyculture); Q_mj = Average yield of crop j when grown by farmers as sole (or monoculture). R_pij = Values of crop j when grown by farmer i as intercrop estimated at farmgate price and R_mj = Average value of crop j when grown by farmers as sole crop estimated at farmgate price.

Prominent cropping systems and crop combinations

 

The   distribution   of   farmers  by  cropping  system  and prominent crop combination (Table 1) shows that intercropping was commonly practiced by about 67% of the farmers with prominent crop combinations as maize/cassava (40.2%), maize/cassava/yam (9.4%), maize/yam/sorghum (4.1%), maize/yam (2.6%), maize/sorghum (2.5%) and cassava/yam (2.6%) as shown in Table 2. The results also show that maize was the commonest crop grown under monoculture by 12.9% of the farmers followed by sole yam (10.3%), sole cassava (2.4%) and sole sorghum (2.4%).  

 

 

 

 

 

 

The results indicate the dominance of crop combination that are usually described as soil nutrient miners (cereals and tubers) in the farming system. The need to create security against potential risk of monoculture has been identified as one of the driving forces behind intercropping as a form of diversification among smallholder farmers (Muhammad et al., 2003; Preston, 2003). However, the absence of legume crops in the combination points to the impending challenges as regards the maintenance of soil fertility and consequently, productivity of the crops. The basic challenge in such multi-cropping systems is associated with the inherent competition for space, soil nutrients, moisture, light and air among the component crops, and when the cultural practices adopted by the farmer do not cater for such competitions adequately, reduction in soil fertility, land degradation and consequently, low productivity result (Makinde et al., 2007).

 

Fallow rotation intensity and cropping intensity in food crop production

 

The classification of the farmers on the basis of the fallow rotation intensity and cropping intensity indices using the framework advanced by Ruthenberg (1980) and Dayal (1978) respectively is shown in Table 3. The distribution shows that 75.4% of the food crop farmers have engaged their land in continuous cropping while 18.5 and 6.1% engaged in bush fallow and shifting cultivation respectively. Similarly, the distribution according to the level of cropping intensity showed that 79.1% of the farmers engaged their land in high cropping intensity while about 7.7 and 13.2% had their land under low and medium cropping intensity respectively (Table 3). Consequently, land-use in food crop production in southwestern Nigeria is characterized mainly by continuous cropping under high cropping intensity and the pattern is similar across agro-ecologies.

 

 

 

 

Furthermore, the grouping of the farmers on the basis of the aggregate ranked score for the two indices shows that majority of the food crop farmers (65.4%) cultivated their farmland under very high land-use intensity while 21.1, 12.0 and 1.5% cultivated farmland under high, medium and low land-use intensities respectively. However, none of the farmers had their land under very low land-use intensity.

The distribution of food crop farmers by their use of modern inputs (fertilizer, herbicides) shows that the use of fertilizer was more prominent with cereal based cropping systems than with non-cereal based cropping systems (Table 4). Thirty percent of farmers who cultivated sole root crops used fertilizers while fertilizer was used by 50.7, 43.3 and 65.9% of farmers who cultivated cereal-root crop, cereal-root-tuber crops and sole cereal crop combinations respectively. The result also showed that fertilizer use was not common among farmers that cultivated stem tuber crops (8.8%) as sole crop.

 

 

 

 

The prominence of fertilizer usage among cereal-based cropping systems could be a pointer to the farmers’ realization of the high level of responsiveness of cereal to nutrient depletion and the intensive nature of most cereal crops notably the dominant maize. However, fertilizer use was more prominent among farmers who had their land under very high use intensity (33.2%) while about 11, 4 and 1% of the farmers who cultivated their land under high, medium and low intensities used fertilizer. Consequently, about 44% of the farmers (that is, those in high and very high categories who did not use fertilizer) could be said to be mining the soil (Table 5).

 

 

 

 

 

Potential of intercropping relative to monocropping systems

Although the previous sections of this study have shown the prevalence of intercrops relative to monoculture in the food crop farming systems of the study area, this section examine the inherent potential in the intercropping system relative to monoculture across crop combinations, fertilizer usage, and agro-ecology and land-use intensity categories.

The average value of crop output per hectare and the estimated LER and RVT are presented in Table 6. The results show that the total value of crops estimated at the farm gate price differed by crop combination. The average values of intercrops were significantly higher than the estimated value of sole crops. However, the average value of sole yam (N1, 105,467.80) was higher than the value of all the intercrops except cassava/yam intercrop with average value of N1,474,154.60. The results also showed that the average value of intercrops with yam as component crop was significantlysignificantly higher than the average values of other intercrop combinations thereby underscoring the high economic potential of yam in the farming system.

 

 

 

 

The LER estimates showed that generally, more than 1 ha of land would be required by the farmers to produce from sole cropping, yield that are equivalent to the aggregate yield from the component crop in the intercrops. However, the RVT values showed that more than 1 ha of land would be needed for intercrops to obtain revenue equivalent to the aggregate revenue from crops cultivated as sole except for cassava/yam intercrop which has RVT value of 1.17.

The results have shown that despite the fact that intercropping has greater agronomic potentials relative to monocropping in terms of yield obtained by farmers, the choice of the crop combinations did not generate enough economic returns to compensate for the possible loss of yield from intercropping. This is attributable to the prominence of cassava in the intercropping systems as a less intensive crop compared to maize, and yam which command greater market premium in the study area.

The comparism of the estimate of the relative potentials across usage of inorganic fertilizer, agro-ecology and land-use intensity class (Table 7) indicated that there were significant differences in the LER of the intercrops by fertilizer usage, agro-ecology and land-use intensity classes. The LER of farmers who applied fertilizer in their intercrops (1.59) was significantly higher than the estimate for farmers who did not apply inorganic fertilizer. Similarly, the LER of intercrops in the derived savannah (1.65) was significantly higher than the ratio in the forest (1.37) and southern guinea savannah (1.13) agro-ecologies respectively while LER values of 2.67 and 1.57 under low and medium land-use intensity were significantly higher than the estimates of 1.50 and 1.37 for high and very high land-use intensity respectively. However, there were no significant difference in the RVT estimate by usage of inorganic fertilizer and land-use intensity group. The RVT however differed across agro-ecologies with the RVT value of 0.77 for the derived savannah been significantly higher than the values of 0.66 and 0.41 estimated for forest and southern guinea savannah respectively.

 

 

 

 

These results have shown that farmers may continue to gain by intercropping and this gain is higher for farmers in the derived savannah agro-ecology and might further be enhanced when they apply inorganic fertilizer and/or reduce the level of land-use intensity possibly through longer fallow. However, situating the production system within the objective of maximizing income, the results show that the farmers would need to incorporate more intensive crops (crops that command higher market premium) for their income to be enhanced. The relative potential of yam in this regards is shown in the greater RVT generated.

The low economic potential of the intercrops is largely due to the prominence of cassava which is bedeviled with frequent market price fluctuation arising from market glut at the peak of the season. However, yam production is a very lucrative but at the same time requires high investment in its production. The high economic premium placed on it by famers is evident in the greater priority accorded the crop in land allocation by farmers who engages in its production. The usual practice among farmers in the zone is to allocate the most fertile (usually fallow land) to yam production although fertilizer application is very uncommon in its application. Although cassava attracts lower premium compared to yam, the crop is very strategic in food security of the farming households while it is less sensitive to environmental stress compared to other crops such as maize and yam. Hence, it is widely grown and therefore prominent in most cropping systems in the zone.

The desire to balance household need with economic returns therefore requires combinations of crops that attract greater market premium (e.g. yam and maize) with those that are strategic to household food security (e.g. cassava). Engaging in sustainable production of these crops however requires adequate consideration to soil fertility maintenance more so when there are indications than land-use intensification would continue to increase due to scarcity of fertile land as also pointed out by Oyekale (2007).

 

The results of this study have shown that farmers may continue to gain higher yield by intercropping but higher productivity could be attained when farmers apply inorganic fertilizer and/or reduce the level of land-use intensity possibly through longer fallow. The potential under this scenario could also be higher in the derived savannah agro-ecology. The farmers’ choice of crop combination is largely devoid of leguminous crops thereby indicating the inability of the farmers to take advantage of soil fertility replenishing potential of legume through natural nitrogen fixation in the intercrop.

The RVT however points to the low economic potential of the choice of crop combinations relative to monocropping. Largely, the composite value of the component crops in the intercrops, are not enough to compensate adequately for income loss traceable to yield loss arising from competition and low market premium of the component crops when compared to their respective values under monocropping. 

Although the inclusion of more intensive crops like yam displays some potential, such combination should be combined with soil fertility maintaining practices such as fertilizer application and legume rotation or fallow. The possibility of farmers engaging in long fallow as an alternative strategy is constrained by land unavailability in the midst of increasing population and the evident high land-use intensity characterizing food production in the region.

The authors have not declared any conflict of interests.