Soil type and grazing management practices : effect on vegetation composition and distribution in selected Communal Property Associations (CPAs) of Bela-Bela Municipality
Abstract
The objective of this study was to investigate the effect of grazing management on soil
properties, vegetation composition and nutritive value of species found in selected CPAs of
Bela-Bela Municipality. The CPAs were as follows: Mawela (Hutton- clay loam: HCL), Bela-
Bela (Hutton-clay: HL), Moretele (Hutton-loamy sand: HLS) and Ramorula (Ecca sand-clay
loam: ESCL). Three 200 m line transects served as replicates and were laid 50 m apart from
each other in each of the three camps per CPA. In each of the three line transects, sub-transects
were marked at 50 m intervals (50, 100,150 and 200 m) to create four 10 m x10 m homogenous
vegetative units (HVU) making a total of 12 HVUs per camp. For physical and chemical
properties, the soil samples (top and sub-soil) were analysed for particle size distribution,
acidity, resistance, pH, organic carbon, macro and micro minerals. The abundance, height and
nutritive value were determined for both types of grass and woody species. For chemical
composition and in vitro dry matter degradability of both types of grass and browse leaves were
bulked, respectively. All data for soil properties were subjected to a two-way factorial analysis
of variance (SAS 2010) while data for species composition were subjected to a one-way
analysis of variance (SAS, 2010). The highest (P < 0.05) pH (7.1) recorded on the sub-soil was
in HLS. The topsoil had the highest (P <0.05) nitrate-nitrogen (N-NO3) concentration (2.4
mg/kg) and ammonium nitrogen (N-NH4) concentration (4.5 mg/kg) in the HC soil type. Soil
organic carbon for both topsoil (0.66 %) and subsoil (0.41%) was significantly low (P <0.05)
in the HLS soil type and ESCL soil type, respectively. The ESCL and HCL had the highest (P
<0.05) chlorine (Cl) concentration (42.2 and 66mg/kg, respectively) in the top and sub-soil
respectively. Phosphorus (P) and iron (Fe) concentrations were significantly high (P <0.05) in
ESCL soil type for both topsoil and sub-soil. Sub-soil, manganese (Mn) concentration was
found to be higher (P <0.05) in ESCL soil type (7.6 mg/kg). Copper (Cu) and zinc (Zn)
concentrations were high (P < 0.05) in the HC soil type for both the topsoil and sub-soil. The
HLS soil type had the lowest (P <0.05) acidity (0.01) in topsoil while HC had the lowest (P
<0.05) acidity (0.02) in sub-soil. The HCL soil type had the highest (P <0.05) soil resistance
(2880 Ω) in topsoil, while ESCL had the highest (P <0.05) soil resistance (3640 Ω) in sub-soil.
Across all soil types, Aristida congesta and E. rigidor were the two dominant (P <0.05) grass
species. Hutton-clay (HC) soil type had the highest (P < 0.05) biomass (823.3 kg/ha) and also
had higher (P <0.05) basal cover (55.8%). The HCL had taller (P <0.05) Digitaria eriantha
and Aristida congesta. For the tuft diameter, Setaria sphacelata in the ESCL soil type had the
largest (P <0.05) diameter (21.3 cm) when compared to the one found in the HC soil type
(4.3cm). Bothriochloa insculpta diameter was significantly larger (P <0.05) in ESCL soil type
(19.7 cm). The crude protein (CP) content (45.4 g/kg DM) was significantly higher (P <0.05)
in ESCL soil type. Bulked grasses in all soil types had similar (P >0.05) neutral detergent fibre
(NDF), acid detergent fibre (ADF), digestible energy (DE) and metabolizable energy content.
Grasses in the Hutton clay (HC) soil type had the lowest (P <0.05) acid detergent lignin (ADL)
content when compared to those from other soil types. The bulked grass species in the HLS
soil type had the lowest (P <0.05) ether extract (EE) content (20.8 g/kg DM). The in vitro dry
matter degradability (DMD) of bulked grasses at 24 (366.1 g/kg), 36 (478.4 g/kg) and 48 (629.8
g/kg) hours was highest (P <0.05) HC soil type. Hutton-loamy sand (HLS) soil type had high
(P<0.05) total plant density (TPD) (4300 plants/ha), canopy cover (CC) (55.8%) and total tree
equivalent (TTE) (5068.9 plant/ha) compared to other soil types. Bulked browse leaves from
all soil types had similar (P >0.05) CP, ash and NDF content. In all soil types, the condensed
tannins (CT) concentrations in browse leaves was similar (P >0.05). Browse leaves from the
HC soil type had the lowest (P <0.05) soluble phenols concentration (289.8 g TAE/ kg DM).
Browse leaves from all soil types had similar (P >0.05) DMD at 36 and 48 h. For most of the
soil types, mineral concentration is more in the topsoil than in the sub-soil. This trend explains
that the uptake of these minerals by plants took place due to the inconsistencies of grazing
management employed in these selected CPA farms. With HC soil type outcompeting others
in terms of both basal cover and biomass, which suggests that improved grazing management
would improve the productivity of other soil types. The CP content of bulked grasses was low,
and which suggests that animals should be supplemented with high protein sources such as the
current bulked browse leaves due to their high CP and DMD for optimum livestock
productivity. In order to encourage the growth of the herbaceous layer, the principal feed source
for ruminants, it is crucial to manage the matured tree population prevalent in some of these
CPAs as shown by the highest number of woody species (seedlings and adult trees). The
observed high CP and DMD in bulked browse leaves from all soil types shows their potential
as a solid, affordable source of protein for livestock.