INTRODUCTION

Forage quality and quantity play a significant role in abundance, distribution and seasonal movements of herbivores in both wet and dry seasons in conservation areas such as National Parks, as reported by (Aremu, 2005; Murray, 1995). During the dry season, most grasses are in a senescent phase and forage quality and quantity are limited, while in the wet season, the forage quality meets herbivores nutritional requirements since vegetation production and mineral concentration in the vegetation in Africa savannas are not only determined by rainfall and soil nutrients (Prins, 1996) but also by herbivore itself. Indeed much research has shown that grazing can keep the vegetation in an open, young, productive stage of growth and can improve plant nutrients availability, soil nutrients and water status, thus contributing to the maintenance of a high above ground production of good quality (Milihunas et al., 1995; Hobbs, 1996; Belskey et al., 1993; Mc Naughton, 1999).

Plant communities evolved under the grazing pressures reflect the type of plants and their density, which largely reflect the grazing habits and food preferences of the animals present. The physiological ability of a plant to withstand gazing is considered only with respect to the major species in determining proper use factor which is the percentage use that is made of a forage species which depend upon the ability of the range as a whole to withstand grazing and not necessarily upon the ability of a particular plant species to withstand grazing which may vary according to associated plant species, season, past grazing use and preference among others.

Wild grazing ungulates generally encounter food resources that temporally as well as spatially vary in quality and quantity. Hence, most hypotheses which have been put forward to explain the seasonal movements that are related to differences in food quality, quantity and availability in both dry and wet seasons (Murray, 1995; Kreulen, 1995). Forages have seasonal characteristics related to quantity such as biomass, grass height and quality such as mineral concentrations and digestibility for which large grazers can select (Prins, 1996; Heitkonig and Owen-Smith, 1998). Selection of those specific characteristics can lead to differential use of grasses as rangeland resources. The forage potential in the Park is been limited due to habitats destruction through indiscriminate burning, over grazing and encroachment of ungulate habitats (Aremu, 2005).

In this study the effects of grazing as stimulated by clipping on forage biomass production, nutrients status and forage availability in both dry and wet seasons to herbivores in the Park were investigated. The study also intended to prescribe measures to maintain a balance between herbivore populations and forage resources in the Park.

MATERIALS AND METHODS

Kainji Lake National Park is located between latitudes 9° 40¹ and 10° 30¹ N and between longitudes 3° 30¹ and 50¹ E with total land area of 5, 340.83 km² (The Borgu sector is 3,970.83 km² while Zugurma sector covers an area of 1,370 km²), while (Afolayan, 1978) gave the three main vegetation communities in Borgu sector of the Park as Detarium microcarpum woodland savanna, Isoberlinia woodland and riparian forest and woodland. The Oli River flows from the Republic of Benin through the Borgu sector into the Niger River. In the dry season, the river breaks into pools, which hold water throughout the year and serves as the only source of water for the wild animals long term average annual rainfall is between 900 and 1100 mm, which are usually between May and September.

The Park is blessed with diverse fauna resources including Papio anubis, Kobus kob, Hippopotamus amphibious, Syncerus caffer, Alcelaphus buselaphus, Hippotragus equines, Redunca redunca, Panthera leo, Panthera pardus, kobus defassa among other. Flora resources in the Park include Burkea africana, Terminalia avicennoides, Diospyros mespiliformis, Anogeissus leiocarpus, Entanda africana, Viftex doniana, Hyparrhenia rufa, Hyparrhenia dissoluta andropogon gayanus, Braciliaria brachyiopha, Hyparrhenia cynascens andropogon perligulatus, Beckeropsis uniseta and Andropogon tectorum among others (Ayeni et al., 1982).

Methods: Three plots of (2.5x2.5) m were established in the three main vegetation zones in the Park which were Burkea africana/Detaium microcarpum woodland savanna, Isoberlinia woodland and riparian forest/woodland. The plots were in three replicates in each vegetation zone. Each plot was separated by a fire trace of 5 m to prevent fire occurrence. Also, the plots were fenced with chain-link enclosures and were labeled A, B, C, each of them received different clipping treatment. Plot A-medium treatment, which was clipped to 15 cm, plot B-heavy treatment that was clipped to 3 cm and plot C-control treatment that was left unclipped. The experiment was started in June 2003 in the beginning of the wet season and lasted until May 2004 at the end of dry season. The heavy and medium treatments were clipped every 4 weeks and the clipped biomass was collected. All plant materials were hand sorted into green leaf, green stem and dead materials, dried to a constant weight and weighted.

To describe the soil properties of the different vegetation, soil samples were collected from each site using a metal pipe with a diameter of 4.2 at 0-15 cm soil depth. All soil samples were taken in duplicate and mixed to account for spatial variability. Samples were sieved through 2 mm mesh screen to remove stones and root materials. Samples were dried to a constant weight and stored for chemical analysis. Plant materials and soil samples were digested using a modified Kjeldahl procedure with selenium as a catalyst (Novozamsky et al., 1988).

Total nitrogen and phosphorus concentrations in plant and soil materials were analyzed colorimetrically using a continuous flow analyzer. Total calcium and Sodium concentrations were analyzed with Atomic absorption Spectrophotometer. Soil organic matter content was determined via combustion of soil samples at 55°C for 3 h. Soil pH was determined in the extraction residue from soil using a 0.01 m CaCl₂ solution (Houba et al., 1986).

Monthly changes in total above ground biomass, production of leaves and proportion of live biomass were compared between treatments and sites. The above ground biomass of medium clipping treatment was calculated as the re-growth from each period plus an estimated value of the biomass between ground level and 15 cm. The proportion of leaves was calculated as leaf biomass divided by the sum of the leaf and stem biomass. The proportion of live materials was calculated as live biomass divided by the sum of the live and dead biomass as recommended and described by Voeten (1999) and Hobbs (1996).

Annual above ground production was calculated for both total biomass and live biomass. For the control treatment, annual above ground production was calculated at the end of experiment when the biomass was clipped to ground level as the sum of the positive biomass increment between harvests (Mc Naughton, 1999). For the heavy and medium clipping treatments annual production was calculated as the sum of the removed regrowth plus, for the medium treatment, the biomass harvested at the end of the experiment (Voeten, 1999). All data collected were subjected to two ways analysis of variance (ANOVA) at (p = 0.05) significant level and Duncan’s multiple range test contrasts as recommended by (Alika, 2006).

RESULTS AND DISCUSSION

The chemical properties of each vegetation community at 0-15 cm soil depth indicated that organic matter varies between 3.94 and 8.73%. Also, soil nitrogen content ranged between 0.03 and 0.82 mg kg^-1 soil, there was a significant different (p<0.05) between the soil nitrogen content on each vegetation zone. While, soil phosphorus content ranged between 0.01 and 0.11 mg kg^-1 soil, the values of soil phosphorus and potassium recorded in each of the vegetation zone were significantly different at (p≤0.05). Equally, the values of soil calcium and sodium ranged between 0.30-0.52 meq/100 g soil and 0.28-0.42 mq/100 g soil respectively. The soil pH values ranged between 5.92 and 6.48, which tended towards neutral soil medium (Table 1) Higher total annual above ground biomass was recorded in clipped plots which ranged between 1,642 and 2,458 g m^-2 year^-1 when compared with unclipped plots with total annual above ground biomass of between 1,357 and 1435 g m^-2 year^-1. These observations supported earlier views of (Belskey et al., 1993; Milihunas et al., 1995; Hobbs, 1996) that grazing can keep the vegetation on an open, young productive stage of growth and can improve plants availability, thus contributing to the maintenance of a high above ground production of good quality forage. However, the increase in production may be a temporal or immediate response to disturbance (clipping/grazing). It is unlikely however, that high production can be sustained over longer periods of grater than one year (Prins and Olff, 1998) (Table 2).

Highest mean annual live biomass was recorded in medium clipped plots which ranged between 280 and 450 g m^-2 year^-1, followed by heavy clipped plots with mean annual live biomass of between 210 and 442 g m^-2 year^-1, while the least value of annual live biomass was recorded in control/unclipped plots which ranged between 172 and 197 g m^-2 year^-1.

Table 1:	Soil chemical properties of three main vegetation communities in the Park
Rfw-Riparian forest and woodland, Badmws-Burkea africana/Detarium microcarpum woodland savanna, Iw-Isoberlinia woodland. Mean values with the same letter(s) are not significantly different

Table 2:	Average total annual above ground biomass (g m-2 year-1) in each plot
Rfw: Riparian forest and woodland, Badmws: Burkea africana/Detarium microcarpum woodland savanna, Iw: Isoberlinia woodland, mc: medium clipping, hc: heavy clipping, c: control/unclipped. Mean values with the same letter (s) are not significantly different

This indicated that through grazing simulated by clipping forage quantity could be enhanced; this observation was in consonance with that of (Georgiadis and Mc Nanghton, 1990; Belsky et al., 1993; Voeten, 1999). There was a significant difference in mean annual live biomass recorded in clipped an unclipped plots at (p≤0.05). Equally, highest mean total production was recorded in the medium clipped plots which ranged between 365 and 491 g m^-2 year^-1, followed by heavy clipped plots which values ranged between 250 and 471 g m^-2 year^-1, while the least value of mean total production was recorded in control/unclipped plots which ranged between 210 and 254 g m^-2 year^-1 (Table 3).

There was a significant difference (p≤0.05) between vegetation types and total mean annual live/total production. Quality parameters related to composition of vegetation materials such as the proportion of leaves and live materials in vegetation were higher in clipped plots due to a longer build up of stem and dead materials in the unclipped plots while regrowth after clipping primarily consisted of leaf material. These observations were also reported by (Belskey et al., 1993; Hamilton et al., 1998; Voeten, 1999).

It was observed that the quantity of nitrogen phosphorus, calcium and sodium found in grasses in clipped plots were above minimum quantity required by herbivores for body maintenance, pregnancy and lactation. While, the quantity of these nutrients recorded in unclipped plots were below the minimum quantity required for body maintenance, pregnancy and lactation (Table 4). This observation was in conformity with those of (Van de Vijver et al., 1999; Voeten, 1999). The grasses in clipped plots which included Andropogon gayanus andropogon schirensis, Hyparrhenia rufa, Hyparrhenia dissoluta, Panicum maximum, Cybopogon giganteus, Beckeropsis uniseta and Braciliaria branchylopha contained higher crude protein and fat which ranged between 5.0-8.9 and 4.8-6.3%, respectively. While, lower crude protein and fat contents were recorded in grasses in unclipped plots which ranged between 4.7-7.2 and 3.4-5.1, respectively (Table 5).

Table 3:	Mean annual live and total production (g m-2 year-1) in each plot
Rfw: Riparian forest and woodland, Badmws: Burkea africana/Detarium micropcarpum woodland savanna, Iw: Isoberlina woodland, production, mc: medium clipping, hc: heavy clipping, c: control/unclipped treatment. Mean values with the same letter (s) are not significantly different

Table 4:	Average forage nutrient composition in each plot
Rfw: Riparian forest and woodland, Badmws: Burkea africana/Defarium microcarpum woodland savanna Iw: Isoberlinia tomentosa woodland mc: medium clipping treatment, hc: heavy clipping treatment, c: control/unclipped treatment, *N-minimum nitrogen required for maintenance, *P: minimum phosphorus required during pregnancy, **P: minimum phosphorus required for lactation, *Ca: minimum calcium required during pregnancy, **Ca: minimum calcium required for lactation, *N: minimum Sodium required during pregnancy, **Na: minimum sodium required for lactation. Values of *N, *P, **P, *Ca, *Na and **Na were taken from Voeten et al. (1999)

Table 5:	Average proximate analysis of common grass species in each plot
% cp: Percentage crude protein, %cf:: percentage crude fibre, %ee: percentage ether extract, Ag-Andropogon gayanus, As: Andropogon schirensis, Hr-Hyparrhenia rufa, Hd: Hyparrhenia dissoluta, Pm: Panicum maximum, Cg: Cymbopogon giganteus, Bu: Beckeropsis uniseta, Bb: Braciliaria brachylopha, mc-medium clipping treatment, hc: heavy clipping treatment, c: control/unclipped treatment

CONCLUSIONS

It could be generally concluded that grazing as simulated by clipping enhances forage quantity and quality in terms of rapid production of more above ground biomass, with more live and leaves materials which are more succulent, tender and nutritious which are preferred by herbivores of the Park because of their higher nutritive values. If grazing could be properly managed through introduction of efficient grazing management plan by the Park, the forage resources could continue to support the teaming population of ungulates in the Park, most especially during the dry season when forage quantity is usually limited in the Park.

ACKNOWLEDGMENTS

Permission to conduct this study was granted by Nigeria Parks Service, Abuja and Kainji Lake National Park, New-Bussa, Nigeria and is gratefully acknowledged. We also would like to thank the International Institute for Tropical Agriculture (IITA) Ibadan and the Department of Computer Science, University of Lagos, Nigeria, for allowing us to use their facilities.