Tropical soil-plant interactions in relation to mineral imbalances in grazing livestock
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.