Weed Species Distribution of Juvenile Oil Palm Tree ( Elaeis guineensis ) Intercropped with Maize ( Zea mays ), Okra ( Abelmoshus esculentus ) and Pepper ( Capsicum anuum var. abbreviatum )

This field experiment was carried out to evaluate the weed species distribution in the experimental plots of an intercrop of juvenile oil palm trees ( Elaeis guineensis Jacq.) with maize ( Zea mays Linn.), okra ( Abelmoschus esculentus Moench) and pepper ( Capsicum annuum var. abbreviatum). This was carried out during the cropping season between July and October 2012. The crops were intercropped with the juvenile oil palm trees of about 3-years-old. The experiment was laid out in a completely randomized block design with five (5) replicates. The treatments comprised of intercropping distances of 1 m, 2 m and 3 m each for the three (3) crops (maize, okra and pepper) and a plot for each of the three (3) arable crops without oil palm trees as control. Weed species distribution was carried out in each of the plots to determine the Simpson’s Diversity Index (D), Simpson’s Index of Diversity (1-D) and Simpson’s Reciprocal Index ( 1 / D ). Weed species’ frequency, density, relative density, relative frequency, importance value, abundance, dominance and relative dominance were also computed from data collected at 3WAP and 6WAP. The results showed that the control plot has the highest weed species distribution at 3WAP having the lowest Simpson’s Diversity Index (D) of 0.0930. Okra plot has the least weed species distribution with the highest Simpson’s Diversity Index (D) of 0.2726. At 6WAP, the pepper plot has the highest weed species distribution having the lowest Simpson’s Diversity Index (D) of 0.1741. Control plot has the least weed species distribution with highest Simpson’s Diversity Index (D) of 0.2831.


Introduction
Species distribution in terms of species richness is measured as the number of species in a community. Distribution could be within or between communities. Two communities with an identical number of species can differ in terms of evenness, and hence it is also useful to know the proportional or relative abundance of species within the community. Intercropping is a predominant cropping system in developing countries which involves the practice of growing two or more crops at the same time, during the same season in the same piece of land (Geiler et al., 1991;Willey, 1979). Intercropping has been reported to increase crop diversity, biological stability of the ecosystem and labour efficiency (Okigbo, 1977). Many tree crops notably oil palm, cocoa, coffee have been successfully intercropped with other trees and food crops (Ofoli and Lucas 1988;Okpala Jose and Lucas 1989;Famaye, 2004). Intercropping of compatible plants also encourages biodiversity, by providing a habitat for a variety of insects and soil organisms that would not be present in a single-crop environment. This biodiversity can in turn help to limit outbreaks of crop pests (Altieri, 1994) by increasing the diversity or abundance of natural enemies, such as spiders or parasitic wasps. Increasing the complexity of the crop environment through intercropping also limits the places where pests can find optimal foraging or reproductive conditions (Adeyemi, 1988). Greater crop yield and less weed growth can be obtained more frequently in intercrops than in sole crops. In East Africa fruit crops are usually intercropped with annual crops; for example, banana is intercropped with food and/or fodder crops (Clark and Francis, 1985), while in India bananas are intercropped with potato which had resulted in good returns (Okigbo, 1979).
Indices have been developed to combine species richness with proportional abundance within a single value. Examples include the Shannon index, the Simpson index and 'a' of the log series. Recent studies have shown that weed shift occur in continuously cultivated land, which may be as a result of bush burning, high tillage practice, cropping systems, weed control methods and other changes in the habitat (Smith and Akinde, 2000;Olorunmaiye and Olorunmaiye, 2008). In order to determine weed control strategy for a successful weed control programmes in oil palm tree cropping systems, it is worthy to know the weed type and species composition in any ecology. It is therefore the objective of this study to investigate the weed flora and species diversity in the plots where the research was carried out.
The field work was conducted during the cropping season between July and October 2012 at the oil palm plantation of Matkis Farms and Agroservices Ltd. located at km 3 Ilado road, Lanlate, Ibarapa-East Local Government, Oyo state, Nigeria.
Six plots (juvenile oil palm trees/maize intercrop plot, juvenile oil palm trees/okra intercrop plot, juvenile oil palm trees/pepper intercrop plot, sole maize plot, sole okra plot and sole pepper plot) were selected within the oil palm tree plantation. It is an oil palm plantation with a layout of 6 m X 6 m triangular spacing. The maize, okra and pepper were intercropped at 1 m, 2 m and 3 m from the juvenile oil palm trees separately within each plot. The weed survey was carried out using the quantitative survey method of Thomas (1985). 0.5 m x 0.5 m quadrat was thrown randomly within each plot three times at 3WAP and at 6WAP. Weeds within each quadrat were uprooted, sorted into species, identified, counted and recorded.

Statistical analysis
Data collected were computed and important quantitative analysis such as density, frequency, relative frequency, relative density, importance value (IVI), abundance, dominance and relative dominance of tree weed species encountered were determined as per Curtis and McIntosh (1950) Simpson's Index (D): It measures the probability that two individual weed species randomly selected from a sample will belong to the same species (or category other than species).
Where n = the total number of weeds of particular species; N = the total number of weed of all species. With this index, zero (0) represents infinite diversity and 1, no diversity. That is, the bigger the value of D, the lower the diversity. This is neither intuitive nor logical, so to get over this problem, D is often subtracted from 1 to give Simpson's index of diversity.
Simpson's index of diversity (1-D): The value of this index also ranges between 0 and 1, but now, the greater the value, the greater the diversity. In this case, the index represents the probability that two individual weed species randomly selected from a sample will belong to different species. Another way of overcoming the problem of counterintuitive nature of Simpson's Index is to take the reciprocal of the Index.
Simpson's reciprocal index (1/D): the value of this index starts with 1 as the lowest possible figure. The figure would represent a community containing only one species. The higher its value, the greater the diversity. The maximum value is the number of species (or other category being used) in the sample. For example if there are five weed species in the sample, then the maximum value is 5.

Discussion
The results showed that in the maize plot, Tridax procumbens has the highest number of species both at 3WAP and 6WAP. This may be because this plant produces a lot of seeds that are easily dispersed by wind. This observation is in accordance with an earlier report by Olorunmaiye et al. (2011) who reported high colonizing power of this family, readily brought about by the high fruit production and the efficient dispersal of fruits and seeds. Similar observation was recorded in the okra and pepper plots, both at 3WAP and 6WAP. In the control plot at 3WAP, Axonopus compressus has the highest number of species, closely followed by Tridax procumbens. This shows that Tridax procumbens is very abundant in this study area. However, at 6WAP in the control plot, Euphorbia heterophylla has the highest number of species present, recording the highest number of abundance in the control plot (Tab. 8). Euphorbia heterophylla also recorded high abundance in maize, okra and pepper plots. This confirms earlier report of Olorunmaiye et al. (2011) who reported high relative weed density of Euphorbia heterophylla in juvenile citrus plot in National Horticultural Research Institute (NIHORT) Ibadan, Oyo state, Nigeria. High light requirement, aggressive growth, short life cycle, large seed production with potent explosive seed dispersal mechanisms were identified as attributes that may be responsible for the high relative weed density observed.
Broad leaf weeds were also found to dominate all the studied plots. This observation was also in accordance with an earlier report by Olorunmaiye et al. (2011) who reported high concentration of broad leaf weeds under the canopies of mature citrus trees. However, more weeds were observed in the control plots where there was no intercropping (plots with sole oil palm trees) than in the intercropped plots. This also confirms an earlier report of Yakubu et al. (2006) who reported site specificity in both crops and weeds. Similarly, this observation is also in accordance with earlier report by Karaye et al. (2007) who reported that some weeds and crops are site specific, while others will thrive over a wide range of habitat.
Generally, the number of weeds found in the plots at 3WAP and 6WAP were the same in maize plot and okra plot. It was higher at 6WAP than at 3WAP in pepper plot, but was significantly reduced in the control plot at 6WAP compared to 3WAP. This observation shows that the intercrop helped to reduce the number of weeds in the plots and it is in accordance with an earlier report by Egbe (2010) and Karkanis et al. (2011) who reported that the enhancement of barley and fenugreek weights in the weed free treatments might be attributed to the high efficiency of weeds elimination, which consequently decreased the competitive ability of weeds against crop plants.
The control plot has the highest weed species distribution at 3WAP. This plot has the lowest Simpson's Diversity Index  Table 9). The high weed species diversity in the control plot might be because the weeds there were not regularly cleared like the intercropped space between the palm trees has attained stable biodiversity, as which is in accordance with earlier report of Yakubu et al. (2006) who reported site specificity in both crops and weeds. This is also in agreement with earlier report of Olorunmaiye et al. (2011) who reported higher number of weed species in the main citrus orchard plot than other plots. This may also be due to the absence of shade, which probably improves the growth of weeds; this confirms earlier report by Obadoni et al. (2009) who reported that shade effect from trees did not encourage undergrowth regeneration or establishment of weed.
The pepper plot has the highest weed species distribution at 6WAP having the lowest Simpson's Diversity Index The decrease in the diversity in the control plot might be due to the decrease in the number of weeds observed at 6WAP compared to those observed at 3WAP, which further confirms earlier report by Egbe (2010) and Karkanis et al.  Tab. 9. Weed species diversity at 3WAP and 6WAP (2011) who reported that the enhancement of barley and fenugreek weights in the weed free treatments might be attributed to the high efficiency of weeds elimination, which consequently decreased the competitive ability of weeds against crop plants.

Conclusions
This work has been able to provide a list of weed species and their distribution in the studied area. It has shown that broad leaf weeds were found to dominate all studied plots and Tridax procumbens Linn. has been found to have the highest number of species both at 3WAP and 3WAP, in most of the plots studied. The control and the pepper plots were also found to have the highest weed species distribution at 3WAP and 6WAP respectively.