| dc.description.abstract | A survey of the literature revealed that mineral imbalances
were likely to arise in livestock grazing tropical pastures;
that a variety of techniques involving soil, plant or animal
measurements could be used to assess imbalance but none could be
singularly relied upon; that no comprehensive study of mineral
imbalance had been made in Western Kenya.
2. One hundred and thirty-five samples of soil and herbage were
collected from 84 farms in Bungoma and Trans Nzoia Districts;
sites were classified in terms of their geology, topography,
altitude and management type and herbage by species. Soils were
assayed for extractable calcium (Ca), phosphorus (P), iron
(Fe), aluminium (Al), cobalt (Co), manganese (Mn), copper
(Cu), molybdenum (Mo) and zinc (Zn), and for total selenium
(Se). Herbages were analysed for the same elements plus
magnesium (Mg), sulphur (S) and silicon (Si) as total
concentrations on a dry matter (DM) basis.
3. Forage analysis revealed low mean concentrations for most
elements, notably P (1.35gjkg DM), Ca (1.48gjkg DM), S (1.46gjkg
DM), Cu (4.0mgjkg DM) and Se (97ugjkg DM). Comparison with the
Agricultural Research Council (1980) nutrient requirements
suggested that 25-98% of the herbage samples were inadequate
with respect to these elements.
4. The analysis of
and Rhodes grasses
species 'effects' showed that both Kikuyu
were superior to Napier grass and Nandi
setaria with respect to the concentrations of Ca, Mg, Se, P and
S. However, the advantage of Kikuyu grass was offset by the
likely occurrence of deleterious compounds and presence of
higher levels of Mo which, with S, may inhibit Cu utilization in
the ruminant. The legumes lucerne and sweet potato vines
generally had a higher mineral content which may offer an
alternative to supplementation of animal diets.
5. Low herbage concentrations of Ca, Mg, Sand P were not
confined to particular bedrocks, indicating that geology has a
negligible effect on the distribution of the macro-mineral
deficiencies in grazing livestock in the survey area. Opposing
influences of geology on the trace element composition of soil
and herbage, resulted in poor prediction of herbage composition
form the soil data. The trace elements most affected by geology
were Cu and Se, and Se deficiency is expected on soils overlying
igneous granites and alluvial deposits.
6. Low herbage phosphorus concentrations were obtained at low
altitudes « 5000ft above sea level); these were attributed to
fixation by high exchangeable Fe and Al levels in the soils and
low pH. Variations in altitude were associated with far larger
changes in extractable mineral concentrations in the soil than
in herbage and trends were often in opposite direction for Co,
Al, Mn and Fe. Higher altitude herbages were richer in S, Ca,
Cu, Fe, Mg, Mo and Zn and low in Se and Mn.
7. The prevalence of herbage mineral deficiencies was not
attributable to differences in landscape profiles or management:
good management may be nullified by overriding soil properties
of low pH and geology, and the strong effects of plant species
and altitude.
8. The evaluation of a variety of soil extraction methods
showed that availability in vitro may not be a good predictor of
herbage composition in the survey area. The poor soil-herbage
relationships were attributable to inconsistencies in the way in
which extraction methods simulated soil conditions at the root
surface.
9. Strategies and priorities have been suggested for further
response to supplementation in young, growing animals. In this
way any constraints on livestock production can be identified
and alleviated. | en |
