David Mbugua

Analysis of condensed tannins: a review

Plant condensed tannins (proanthocyanidins, PAs) have both positive and negative effects on feed digestibility and animal performance, depending both on the quantity and biological activity of the tannins that are present. In this review, the chemistry and analysis of condensed tannins (PAs) are examined. Our first focus is on the complexity of the structures of condensed tannins and our second emphasis is on the analytical methods used to evaluate tannins. The section on methods is subdivided into a discussion of methods to determine the amount of condensed tannins or total phenolics in a sample and a section on methods to measure biological activity. The methods to measure reactivity include assays involving protein binding and precipitation, as well as those that involve enzymatic and microbial inhibition. The last section of the paper discusses structure–activity relationships and provides information on how to select appropriate assays for measurement of the quantity and activity of condensed tannins.

Modeling the Impact of Natural Resource-Based Poverty Traps on Food Security in Kenya: The Crops, Livestock and Soils in Smallholder Economic Systems (CLASSES) Model

We investigate the interactions between natural resource-based poverty traps and food security for smallholder farms in highland Kenya using a recently developed system dynamics bio-economic model. This approach permits examination of the complex interactions and feedback between farm household economic decision-making and long-term soil fertility dynamics that characterize persistent poverty and food insecurity among smallholders in rural highland Kenya. We examine the effects of changing initial endowments of land and stocks of soil organic matter on smallholders’ well being, as reflected in several different indicators. We show that larger and higher quality land endowments permit accumulation of cash and livestock resources and conservation of soil organic matter relative to smaller or more degraded farms. This suggests the existence of asset thresholds that divide food secure households from food insecure ones.

The Effects of Some External Management Factors on the Nitrogen Composition of Cattle Manure on Smallholder Farms

Smallholder farmers in Kenya collect manure from confined cattle housing termed zero-grazing units. Zero-grazing designs may include urine collection, though the effectiveness of these designs in improving manure N content has not been established. The manure-urine mixtures produced in these units were simulated to determine urine effects on manure N composition. Manure and manure-urine mixtures were stored for 120 days during dry and rainy seasons in Kenya. Manure-urine mixtures leached 26% of their mineral N content during the dry season, but only 12% during the rainy season. After storage, manure-urine mixtures had less organic-N and fiber-N than manure alone during the dry season (𝑃<0.01), but not during the rainy season. Results suggest that the effect of cattle urine on manure N composition is greater during dry seasons than rainy. Manure should not be stored more than 30 days to minimize N loss to leaching. Farmers may take steps to reduce N loss by controlling leaching and protecting manure from rainfall.

The Impacts of Household Land Use and Socio Economic Factors on the Soil Fertility of Smallholder Farms in the Highlands of Kenya

Raising agricultural productivity in smallholder agriculture in sub-Saharan Africa requires an understanding of if and how farm household land use and socioeconomic factors affect soil fertility. Market access, population growth, socio economic characteristics and agro ecological zones have been proposed as important drivers of land use intensity and, consequently, soil fertility. We used diffuse reflectance infrared spectroscopy to measure soil fertility, and multivariate and exogenous switching regression statistical approaches to determine if soil fertility in the smallholder farms of the highlands of Kenya is associated by region, land use categories (cash crop, food crop, fodder and pastures), and selected household socio economic factors (household income, number of adults, farm size and number of cattle). Over 2000 fields on 236 farms were sampled in Embu (eastern Kenya highlands, primarily Andosols) and Madzuu (western Kenya highlands, primarily Ferrasols). Soil fertility variables, including total soil carbon (SC), total nitrogen (TN), pH, available Olsen phosphorous (P), extractable potassium(K), calcium (Ca), and magnesium (Mg), effective cation exchange capacity (ECEC) and texture, were measured using conventional laboratory techniques on 15% of the sampled soils. From these analyses, SC, TN, P and K were all greater in Embu compared to Madzuu soils. Soil fertility variables were significantly higher in pastures compared to other land uses in Madzuu, but were comparable with other land uses in Embu. This soil data was then used to calibrate soil reflectance results in order to predict soil fertility variables for all soil samples. Principle component analysis (PCA) of soil fertility variables developed from the spectroscopy data for each soil sample indicated similarities among sites in the three most important eigenvectors: the first (soil nutrient) vector had high positive loadings for K, Ca, Mg, ECEC and pH; the second (soil organic matter, SOM) vector had high positive loadings for soil organic carbon and total nitrogen; and the third (soil texture) vector had high positive loadings for clay plus silt. However, in Embu, P was associated with the soil organic matter vector while in Madzuu it was associated with the soil nutrient vector. In comparison to pasture all other land uses were associated with lower values of soil nutrient and SOM components in Madzuu, while in Embu, these other land uses were associated with higher values of the SOM component. Number of cattle per farm had no association with any of the three soil fertility components at either site. In Embu, farm income and adult population were both positively related to SOM. In Madzuu, farm size was positively associated with SOM but negatively associated with soil nutrients. More than twice as much P fertilizer is applied on average in Embu compared to Madzuu (27 vs. 11 kg ha-1 season-1). Our study supports the link between poverty dynamics and soil degradation in smallholder agriculture; wealthier households in the eastern Kenya highlands are able to invest in soil fertility management while the poorer households in western Kenya are mining nutrients in soils.

Modeling Feedback between Economic and Biophysical Systems in SmallholderAgriculture in Kenya: The Crops, Livestock and Soils in Smallholder Economic Systems (CLASSES) Model

We investigate natural resource-based poverty traps using a system dynamics model of smallholder farms in highland Kenya. System dynamics modeling is well-suited to examining the complex interactions and feedback between farm-household economic decision making and long-term soil dynamics which may be at the source of persistent poverty among smallholders in this region. We examine the effects of changing initial endowments of land, labour and stocks of on-farm soil organic matter on the long-term welfare of these households. We find that larger farms are better able to cope with both labour shocks and deteriorating natural capital than smaller farms, with smaller farms remaining poor and unable to invest into more diversified agricultural activities, like livestock. This suggests locally increasing returns to various combinations of economic and biophysical assets. Information obtained through such simulation model experiments may lead to better targeting of poverty alleviation programs as well as suggest a broader array of strategies that play off of the complex interactions between economic and biophysical assets. The flexibility to examine these different leverage points is partially provided by our use of system dynamics, rather than other modeling techniques, to develop a descriptive rather than prescriptive model of farm behaviour.