EFFECTS OF DIFFERENT COMPANION CROPS AND INCREASING NPK FERTILIZER LEVELS ON THE PERFORMANCE OF MAIZE ( ZEA MAYS L . ) IN SOUTHWESTERN NIGERIA B . OSUNDARE

Field experiments were conducted at the Teaching and Research Farm of the University of Ibadan, Nigeria, during 2002 – 2003 cropping seasons to appraise the effects of different companion crops, increasing NPK fertilizer levels, and the interaction between these two treatments on the growth and yield indices of maize. The design was a split-plot arrangement, laid out in a randomized complete block with three replications. Companion crops constituted the main-plot treatment, namely: sole cropped maize (control), maize/melon, maize / cassava and maize / cassava / cocoyam. NPK fertilizer was the sub-plot factor, applied at five levels, namely: no fertilizer (control), 100, 200, 300 and 400 kgNPK ha. The results obtained indicated that there were significant differences (P<0.05) between the treatments in the growth and yield parameters of maize. The mean effects of companion crops on maize leaf area were 0.61, 0.60, 0.60 and 0.52 m/plant for sole maize, maize / melon, maize / cassava and maize / cassava / cocoyam, respectively. The two – year average values indicated that NPK fertilizer significantly increased maize leaf area from 0.33 m/plant for control to 0.60, 0.86, 1.05 and 1.19 m/plant for 100, 200, 300 and 400 kgNPK ha, respectively. The mean effects of companion crops on maize grain yield were 1.87, 1.92, 1.89 and 1.75 t ha for sole maize, maize/melon, maize/cassava and maize/cassava / cocoyam, respectively. NPK fertilizer significantly increased maize grain yield from 0.72 t ha for control to 1.92, 2.11, 2.62 and 2.89 t ha for 100, 200, 300 and 400 kgNPK ha, respectively. Companion crops significantly interacted with NPK fertilizer in the growth and yield components of maize. Treatment combination (400 kgNPK ha + sole maize) gave the highest values of growth and yield indices of maize in both years.


INTRODUCTION
In the humid tropics, maize is scarcely grown as a sole crop, rather, it is traditionally grown in association of certain arable crops such as cassava, yam, melon, sweet potato etc (Atilola, 2005;Agbede, 2007).Maize is the principal cereal, usually combined with cassava in the humid tropics, and the reason behind the popularity of cassava / maize mixture is that of the high productivity and compatibility of the mixture.The faster growing maize exploits the microenvironmental resources earlier in the growing season than the slow growing cassava (Agbede, 2007).Atilola (2005) noted that maize grain yield in a cassava / maize mixture was not significantly affected by the associated cassava, but the growth and yield indices of cassava were significantly reduced by maize in a cassava / maize mixture.
Maize (Zea mays L.) requires relatively high soil fertility, particularly nitrogen, phosphorus and potassium for optimum performance (Aitu, 2004;Veen, 2007).In the past, high soil fertility used to be maintained through bush fallow, however, the fallow period has been shortened, following increasing population pressure and a higher demand for land for agricultural and non-agricultural human activities, which have consequently resulted in a sharp decline in soil fertility.This has necessitated the use of fertilizers to improve soil fertility, and hence, crop yield.
Significant responses of maize to NPK fertilizer had been demonstrated by many studies (Brauzel, 2003;Aitu, 2004;Caitt, 2005 andVeen, 2007).These authors noted significant increases in the growth and yield parameters of maize with increasing NPK fertilizer levels.However, too liberal application of NPK to maize, results in excessive vegetative growth and increased lodging (Veen, 2007).Been et al;(2006), however, noted that the degree of responsiveness of maize to NPK fertilizer, depends on the nature of the preceding crop(s).They concluded that where maize is preceded by soybeans, the latter is likely to have contributed about 30 -50 kgN ha -1 to maize crops.Although, in the southwestern Nigeria, many aspects of the Agronomy of maize had been researched, with a view to raising the present level of maize yield on farmers' farms.However, the growth and yield of maize as influenced by companion crops, increasing NPK fertilizer levels, and the interaction between these two treatments have not been accorded enough research attention.Thus, this paper reports a two-year trial, aimed at appraising the influence of different companion crops, increasing NPK fertilizer levels, and the interaction between these two factors on the growth and yield components of maize.

Study site
The experiments were conducted at the Teaching and Research Farm of the University of Ibadan, Nigeria, in 2002-2003 cropping seasons.The soil of the study site belongs to the broad group alfisols (USDA, 1975) of the basement complex, though, locally classified as Ibadan series (Smyth and Montgomery, 1962).The soil is highly leached, with low to medium organic matter content, deep -red clay profile with top sandy loam texture, slightly acidic to neutral.The study site had earlier been cultivated to a variety of arable crops such as cassava, yam, melon, maize, sweet potato etc, but was under fallow for four years prior to the commencement of this study.The fallow vegetation was manually slashed, and the residues were burnt.The land was ploughed and harrowed.

Collection and analysis of soil samples
Prior to planting, ten core soil samples, randomly collected from 0 -15 cm top-soil were mixed to form a composite sample, which was analyzed.The soil samples were airdried, ground and passed through a 2 mm diameter sieve.The sieved samples were then analyzed.The pH was determined by glass electrode pH meter.Bray P-1 extractant was used to extract available P, organic C and total N were determined by Walkey -Black oxidation and Kjeldahl digestion techniques, respectively.Exchangeable K, Ca, Mg and Na were extracted by neutral normal ammonium acetate.K, Ca, Mg and Na were determined by flame photometry, while Mg was by the Atomic Absorption Spectrophotometry.Effective Cation Exchange Capacity was obtained by summation method (i.e.sum of K, Ca, Mg, Na and exchangeable acidity).The determination of exchangeable acidity was by extraction -titration method described by Mclean (1965).Particle size distribution was done by the hydrometer method of soil mechanical analysis as outlined by Bouyoucous (1951).

Experimental design and treatments
The design was a split-plot arrangement, laid out in a randomized complete block with three replications.Companion crops constituted the main-plot treatment, namely: sole cropped maize (control), maize / melon, maize / cassava and maize / cassava / cocoyam.NPK fertilizer was the sub-plot factor, applied at five levels, namely: no fertilizer (control), 100, 200, 300 and 400 kgNPK ha -1 .The NPK fertilizer was applied in two split doses, at three weeks after planting (WAP) and 6 WAP.The gross plot size was 6 m x 4 m, with 2 m margin round each plot.

Planting
In 2002 and 2003, planting was done on March 8 and March 6, respectively.The maize variety, Oba Super 1, dressed with Apron Plus was planted on the flat at a spacing of 100 cm x 50 cm (20,000 plants ha -1 ).Melon seeds, cassava stem-cuttings and cocoyam corm setts were planted at a spacing of 1 m x 1 m apart.Weeds were controlled manually using the hoe.

Collection and analysis of data
Data were collected from five randomly selected maize crops from the two central rows of each plot in accordance with information for maize trial management in IITA' s maize research programme pamphlet on growth and yield parameters. Leaf area was determined by finding the product of the length and breadth of the leaf, and multiplying by a factor of 0.75 (Saxena and Singh, 1965).Stem girth was measured by using Venier calipers.Dry seed weight was measured on a metler weighing balance.Analysis of variance was carried out and means were compared using the Duncan' s Multiple Range Test (DMRT).

RESULTS
The physical and chemical analyses of soil of the study site indicated that the soil was sandy loam in texture, with a pH of 5.5 .Organic C and total N were 2.88 and 1.30 gkg -1 , respectively.The available P was 1.81 mgkg -1 .

Maize leaf area
Table 2 shows the effects of different companion crops and increasing NPK fertilizer levels on maize leaf area.The mean effects of companion crops on maize leaf area were 0.61, 0.60, 0.60 and 0.52 m 2 /plant for sole maize, maize / melon, maize / cassava and maize / cassava / cocoyam, respectively.The two-year mean values indicated that NPK fertilizer significantly increased maize leaf area from 0.33 m 2 /plant for control to 0.60, 0.86, 1.05 and 1.19 m 2 /plant for 100, 200, 300 and 400 kgNPK ha -1 , respectively.Companion crops significantly interacted with NPK fertilizer in maize leaf area.

Maize stem girth:
The effects of companion crops and NPK fertilizer on maize stem girth are presented in Table 3.The two-year average values indicated that companion crops decreased maize stem girth from 2.45 cm for sole maize to 2.44, 2.43 and 2.32 cm for maize / melon, maize / cassava and maize / cassava / cocoyam, respectively.In contrast, NPK fertilizer significantly increased maize stem girth from 2.24 cm for control to 2.44, 2.66, 2.77 and 2.88 cm for 100, 200, 300 and 400 kgNPK ha -1 , respectively.Companion crops significantly interacted with NPK fertilizer in maize stem girth.

Grain yield and fresh stover yield of maize
The effects of companion crops and NPK fertilizer on grain and fresh stover yield of maize are presented in Table 4.The mean effects of companion crops on maize grain yield were 1.87, 1.92, 1.89 and 1.75 t ha -1 for sole maize, maize / melon, maize / cassava and maize / cassava / cocoyam, respectively.NPK fertilizer significantly increased maize grain yield from 0.72 t ha -1 for control to 1.92, 2.11, 2.62 and 2.89 t ha -1 for 100, 200, 300 and 400 kgNPK ha -1 , respectively.The mean values of maize stover yield adduced to companion crops were 2.43, 2.46, 2.42 and 2.32 t ha -1 for sole maize, maize / melon, maize / cassava and maize / cassava / cocoyam, respectively.NPK fertilizer significantly increased maize stover yield from 1.86 t ha -1 for control to 2.35, 2.60, 2.93 and 3.15 t ha -1 for 100, 200, 300 and 400 kgNPK ha -1 , respectively.Companion crops significantly interacted with NPK fertilizer in the grain and stover yield of maize.S S S S Values followed by the same letter in the same column under each treatment are not significantly different at P = 0.05 (DMRT).C = companion crops, F = fertilizer, S = significant.

DISCUSSION
The non-significant difference in the growth and yield indices of maize interplanted with cassava and those of the sole cropped maize agrees with the findings of Atilola (2005) and Agbede (2007).This suggests compatibility of cassava/maize mixture, especially that the faster growing maize exploits the micro-environmental resources earlier in the growing season than does cassava.Consequent upon this, there was no prolonged period of competition between maize and cassava for the growth resources because maize and cassava make maximum demand on growth resources at different times, and maize would have completed its life cycle before cassava canopies closed (Atilola, 2005 andOtelo, 2006).Hence, the compatibility and high productivity of maize and cassava mixture explain its dominance in many mixed cropping systems in southwestern Nigeria (Otelo, 2006).The numerical, although, non-significant difference in the grain and stover yield of maize between sole cropped maize and maize interplanted with melon agrees with the findings of Aribe (2004); Otelo (2006) and Agbede (2007).The higher grain and stover yield of maize interplanted with melon than sole cropped maize can be ascribed to the provision of additional nutrients and improvement of soil conditions for maize by melon (Viny, 2004 andSaito, 2006).Similarly, the higher cormel yield of cocoyam in cocoyam / melon mixture than the sole cropped cocoyam was adduced to the addition of residual nitrogen to the soil by melon (Ale, 2003 andUdeata, 2006).Much as the difference in grain and stover yield of maize between sole cropped maize and maize /melon mixture can be ascribed to the addition of more nutrients and improvement of soil conditions for maize by melon, another factor that can be implicated for the difference in the grain and stover yield of maize is the less incidence of weed interference in maize/melon plots, compared to sole cropped maize plots.This is because melon, being a cover crop, smothered weeds considerably (Zinta, 2004).In addition, the inclusion of melon in the mixture drastically reduced leaching losses, run-off and erosion, with resultant preservation of organic matter in the topsoil for maize use.The lowest values of growth and grain yield of maize recorded in maize / cassava / cocoyam mixture, suggests the existence if inter -and intra -specific competitions among the three crops for below -and abovesoil growth resources (air, water, light, nutrients etc).These prevented maize crops from putting forth much vegetative growth, which perhaps, led to decreased supply of photosynthates to the sink, with resultant reduced grain yield (Tai, 2006).
The significant increases in the growth and yield parameters of maize with increasing NPK levels are in agreement with the reports of Brauzel (2003); Aitu (2004); Caitt (2005) and Veen (2007).These authors noted significant increases the growth and yield parameters of maize on increasing NPK fertilizer levels.The increases in the growth and yield indices of maize associated with increasing NPK levels point to the indispensability of nitrogen, phosphorus and potassium in the nutrition of maize.Nitrogen has been known to play crucial roles in the formation of vegetative structures in plants because N is an active constituent of protoplasm, enzyme and chlorophyll, plays a role of catalytic agent in various physiological processes, accelerates cell division and speeds up photo-assimilation process, which in turn, boost up plant growth, thus, hastening the attainment of reproductive phase (Pande, 1992).Similarly, phosphorus strengthens the skeletal structure of plants, increases plant resistance to diseases, and accelerates flowering, fruiting and seed formation processes in plants (Pande, 1992;Been et al;2006 andVeen, 2007).Potassium activates various plant enzyme systems, particularly those that are connected with growth.Potassium also plays important roles in plant metabolism, in carbohydrate formation and translocation of starch to all plant parts.Potassium strengthens straw and stalk of plants, thereby helping the plants to resist fungal and bacterial attack, as well as to resist lodging (Tankou, 2004 andHea, 2006).The significant interactions between companion crops and NPK fertilizer, imply that the magnitude of the differences in the growth and yield indices of maize under different companion crops was affected by NPK fertilizer.

CONCLUSION
The results of this study have indicated that the growth and yield parameters of maize increased linearly with increasing NPK fertilizer.Melon and cassava did not significantly reduce the yield of maize, contrasting a significant reduction in the growth and yield of maize by cocoyam.