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The two years on-farm tillage research during the 1999 and 2000 cropping seasons in an Ethiopian highland Vertisol area demonstrated the importance of adapting cultural practices into participatory trials. The minimum tillage package could be an effective intervention for soil conservation due to the early-vegetative cover of the soil. Based on farmers' application of ash on Vertisols at Chefe Donsa, the incorporation of 50 kg ha −1 of potassium sulphate (K 2 SO 4 ) in the on-farm trial significantly increased grain and straw yields of wheat. The nitrogen levels in the grain and straw of wheat on plots given K 2 SO 4 were higher than those without. The nitrogen level for both wheat grain and straw was even higher when grown on plots previously growing legumes that received K 2 SO 4 . This showed that the availability of extra potassium in these soils improved the extraction of nitrogen by the wheat crop, thus improving the grain yield. The findings indicate the need to reassess the traditionally-practised system of not applying potassium fertilizer to Ethiopian soils. I N T RO D U C T I O NVertisols cover 10.3 % of the total landmass of Ethiopia, two-thirds of which are in the highlands at altitudes higher than 1500 m asl (Wakeel and Astatke, 1996). Despite the fact that they have high agricultural potential, these soils are generally regarded as marginal due to their hydro-physical properties leading to waterlogging in the main rainy season ( June to September) and cracking during the dry months. On Vertisols, crops are planted at the tail end of the rainy season, exposing the tilled soil to the heavy rains during most of the rainy period; this often results in high soil erosion.In order to solve the waterlogging problem on Vertisols, a research consortium has developed an animal-drawn cultivator called a broadbed maker (BBM) by modifying the local implement known as 'maresha'. The BBM creates 0.8 m wide beds separated by 0.4 m wide furrows that allow excess water during heavy rains to be discharged to a main drain or other outlet at the bottom end of a plot. This technology allows early planting to take advantage of a longer growing period, resulting in higher yields and less erosion as there is adequate vegetative cover during the rainy season (Astatke and Mohamed Saleem, 1998;Mohamed Saleem, 1995). † Corresponding author: a.astatke@cgiar.org

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Two hundred and eight blood samples were collected from four plains bison herds, one wood bison herd, and one herd that is assumed to be a hybrid between wood and plains bison. Haemolytic tests were made using 13 cattle blood typing reagents. Red cell carbonic anhydrase alleles were checked using electrophoresis. The frequencies of each blood trait within each of the six herds were determined. Comparisons of all possible pairs of herds for each trait were made and at least two significant differences (P < 0.05) were found in every case. Ten blood characteristics were used to discriminate among the six herds. Bison from Santa Catalina Island, California, were similar to those from the National Bison Range, Montana, and they jointly differed from the herd from Oklahoma and the three from Canada. The Canadian herds, which included wood bison, hybrid bison, and plains bison, were relatively similar to each other. Differences among the plains bison herds were at least as great as those among the two subspecies and their hybrids. These findings are discussed in relation to herd histories and to the variability among North American bison populations.