Growing crops in mixed cropping is the conventional practice of traditional farmers in the tropics. It is an ancient agricultural practice and has been identified as the crop production system that appeals most to small scale farmers with limited resources (Okigbo, 1982; Machado, 2009). According to Olojede et al. (2002), mixed cropping system may have evolved in attempts by farmers to mimic the multistory/multispecies concept that characterized the tropical agro-ecology. The most important reason behind the wide adoption of mixed cropping in the farming systems of the tropics is that it offers greater yield stability than mono cropping (Rao and Morgado, 1984). Greater yield stability obtainable in the production of food crops is important to subsistence farmers who require sustainability and insurance against total crop failure. Mixed cropping brings diversity of crop species in the systems. This makes the systems more resilient against environmental perturbation and thus enhances food security (Frison, et al., 2011). It also helps the farmers to maximize waste-use efficiency (Yang et al., 2011) and reduce disease and pest incidence (Lithourgidis et al., 2011) which are serious drawback of mono cropping.
In the traditional cropping system, crops are grown in mixture without proper arrangements to utilize growth resources properly (Negash and Tewodoros, 2014). Farmers adopt no definite planting pattern when planting their crops. For instance, cassava is often left scattered in the field to mature after the other crops have been harvested (Ayoola and Agboola, 2004). The fields become very weedy and few farmers carry out weeding after harvesting the early season crops (Isola, 1998). Francis (1989) attributed the poor crop yields among small scale farmers to improper crop arrangement. Ayoola and Agboola (2004) pointed out that cropping could be intensified with appropriate plant arrangement in the field as this will not only increase the productivity of the land, but will also prevent weed from taking over the field.
In crops, yield is strongly depended on leaf area index, leaf area duration and leaves efficiency for absorption of solar radiation for photosynthesis process (Mouhamed and Ouda, 2006). Reduction in the photosynthetic activity of a plant due to reduction in leaf area could result in yield losses that vary depending on the crop, stage of growth and intensity of defoliation.
The growth and yields of crops are limited by a wide range of abiotic factors which include flood, frost, hail, drought, wind and biotic factors such as defoliating insect pests, diseases, animals and man. The deliberate removal of leaves for fodder and leafy vegetables for feeding, natural senescence and age related leaf shedding also affect crop yield. Severe tuber yield loss of sweetpotato due to defoliation by sweetpotato butterfly (Acrea acerata) was reported by Lugojja et al. (2001). According to IITA (1990), the grasshopper, Zonocerus variegatus defoliates cassava in prolonged dry seasons leading to 20 to 60% reduction in yield. Deliberate defoliation of crops by man such as harvest of cassava leaves for human consummation in some African countries (Dahniya, 1994; Achidi et al., 2005) and maize leaves for feeding animals where pastures are scarce are becoming more common (Hassen and Chauhan, 2003). The defoliated maize culms are used as live-stakes for yam and other creeping crops (Igwilo, 1994). There are agronomic benefits to deliberate defoliation. Yield of cowpea was increased by judicious defoliation of older leaves or by topping the growth apices at the onset of flowering (Ezedinma, 1973). The author suggested that a reduction in leaf area coupled with better display of the remaining leaves made cowpea more productive. Artificial defoliation was associated with high yield and quality of cotton. Defoliation improved pickers’ efficiency in cotton and tea fields (MSUcares, 2003).
An artificial defoliation study is a simulation of natural environmental hazards on the field, and provides information on plant responses to environmental stress. Such information has enhanced the development of computer simulation models for use in evaluating pest management decisions (Wilkerson et al., 1984). Furthermore, simulation studies from defoliation serve as the basis for chart used in the developed countries by crop hail insurance adjustor in determining grain losses due to leaf blade removal by hail as well as other destruction of viable tissue (National Crop Insurance Services, 1998). The response of plants to defoliation could be used to manipulate source-sink relations by removing lower and senescing leaves to obtain higher photosynthetic capacity and efficient carbon and nitrogen metabolism under optimal and stressful environment (Iqbal et al., 2012).
Appropriate fertilizer management is central to the profitability and sustainability of crop production. Fertilizer application increase the yield potential of crops as optimal crop performance is usually limited by inadequate availability of plant nutrients. According to Nwaoguala et al. (2015), fertilizer application can ameliorate the effects of defoliation since nitrogen fertilization favours growth and leaf production while phosphorus and potassium improve yield and quality in grain and tuber crops.
Cassava/maize intercrop is a common practice in the cropping system of the humid rainforest of southern Nigeria. According to Obajimi (2001), it is an old and very common cropping system in the humid and subhumid regions of Africa. Obajimi (2001) summarized the benefits of growing cassava and maize in mixture to include; it allows for a more efficient use of total annual rainfall and solar radiation, it keeps a continuous cover over the soil surface for the greater part of the cropping, thereby helping in soil conservation, it has in-built crop insurance scheme in that the risk involved in farming is spread between two crops, it provides a handy and continuous supply of fresh food and helps prevent the buildup of pests and diseases. In this combination, cassava is usually regarded as the main crop. Thus, the objective of the farmer is to obtain a good tuber yield in addition to a supplementary maize crop to augment his income.
1.1 Justification of the Study
In the traditional cropping systems, crops in mixture are arranged at random. This method reduces the efficiency of crops to utilize growth resources. According to Ghosh (2004), spatial arrangement and plant population in an intercropping system have important effects on the balance of competition between component crops and their overall productivity. Francis (1989) attributed the poor crop yields among small scale farmers to improper crop arrangement in mixture. Ghosh (2004) pointed out that the arrangement of crops in mixture must be considered because it is one of the most important factors for better yield advantage. It is therefore worthwhile to develop better cropping arrangements through research.
There is dearth of defoliation studies associated with mixed cropping. Most studies on defoliation were carried out on sole crops (Remison, 1982; Iremiren, 1987; Lugojja et al., 2001; Thomison and Nafziger, 2003; Nwaoguala et al., 2015; Osaigbovo and Law-Ogbomo, 2016). The results obtained from defoliated sole crop cannot be directly applicable to defoliated mixed crops. It is therefore logical to conduct defoliation studies for mixed cropping systems.
Leaves are vital for photosynthetic and respiration activities and are also indices of dry matter production in plants. Harvest of leaves or defoliation during active plant growth may negatively affect crop yield. However, researchers differ on the issue of what extent defoliation can reduce yield, especially when crops are grown in mixtures. This is another reason why more researches are required to elucidate the effects of defoliation on different crop types and time of defoliation of crops in mixture.
Crops may be defoliated by herbivores, hailstorm, windstorm, drought, insect pests, herbicides and farm machinery (Heidari et al., 2013). Studies on defoliation have attempted to ascertain the effects on crop yield when leaves are defoliated at different growth stages. Generally, results showed that the impart of defoliation on yield is depended on the crop growth stage at defoliation, the severity of defoliation and climatic factors (Iremiren, 1987; Thomison and Nafziger (2003). Studies by Muro et al. (2001) on sunflower (Helianthus annus L.) and Siebert et al. (2005) on cotton (Gossypium hirsutum L.) which reported similar results cannot be substituted for cassava/maize intercrop. It is evident that a defoliation study on cassava/maize intercrop is required since this is a common cropping system among farmers in Nigeria and has not yet been widely reported in literatures.
1.2 Objectives of the Study
The overall aim of the study was to determine the best planting pattern with maximum economic benefit and the impart of defoliation on cassava and maize in mixture.
The specific objectives were to determine the:
- appropriate planting ratio for growing cassava and maize in mixture;
- planting ratio with optimum economic advantage and cost benefit;
- effects of defoliation on the performance of cassava/maize grown in intercrop;
- effects of defoliation on quality attributes of cassava/maize grown in intercrop and
- effects of defoliation and NPK fertilizer on the performance of cassava/maize