Biological nitrogen fixation (BNF), nitrogen (N), and phosphorus (P) imports-exports budgets were estimated at four locations, each with 20 farmer-managed fields for two years in a semi-arid Tanzania and Malawi. The 15 N isotope dilution method was used to quantify BNF by three pigeonpea (Cajanus cajan L. Millspp.) varieties intercropped with maize (Zea mays L.). The N and P accumulation in plant components of sole maize and intercrops of maize-pigeonpea systems were used to estimate the mean exports and imports of N and P. The proportion of N derived from air (%Ndfa) by the pigeonpea varieties ranged from 93.8% to 99.9% in Malawi and 65.6% to 99.3% in Tanzania. The amount of fixed N (BNF; kg N ha À1 yr À1 ) varied from 37.5 to 117.2 in Malawi and 6.3 to 71.5 in Tanzania. The mean values for BNF during the two cropping seasons were 64.3 for Nyambi, 85.3 for Ntonda, 34.1 for Gairo and À54.3 for Babati sites. The mean N budget (kg ha À1 ) was À26.1 in the sole maize plots and À40.3 for the intercrops at the two locations in Malawi, and À50.1 in the sole crop plots and À51.1 in the intercrops at the locations in Tanzania. In a scenario where all the aboveground material except the edible parts was returned to the soil, a positive value of 30.5 kg N for the intercrops was recorded compared with À8.9 kg N for the sole maize in Malawi. For the same scenario in Tanzania, the budget was more negative (À35.4 kg N) for sole maize compared with intercrops (À5.9 kg N). Including the roots in the calculations, did not change the differences between mono and intercrops. The P budget was negative irrespective of whether the aboveground biomass of maize and pigeonpea was incorporated or exported out of the fields, and the values were similar for intercrops and sole maize. The most negative N and P budgets were recorded in the two study areas where the extractable soil P status of the soils and the maize yields were high. These findings indicate that pigeonpea incorporated into maize-based cropping systems will maintain a very high %Ndfa ( > 90%) in all plant parts and thereby contribute to improved N budgets but not increase the proportion of P mined of the soil.

Common bean (Phaseolus vulgaris L.) is a dominant grain legume in eastern and southern Africa, where it constitutes a major source of protein and microminerals in peoples' diet. The current studies aimed at determining how initially promising genotypes of bean responded in terms of yield and grain element composition under farmers' cropping conditions. It was found that variations between genotypes in the proportions of elements in the grain dry matter across a wide range of conditions could be linear with an additional 20% iron (Fe) or zinc (Zn) for some genotypes. However, this linearity was only identifiable under relatively favourable conditions. Further, a favourable season could enhance the proportion of Fe in the grains of the same genotypes by up to 20%, whereas Zn did not respond. Fe and Zn correlated only to some degree with P (r 2 > 0.35). It is concluded that the supply of elements in the diet may best be secured by selecting for high-yielding cultivars as the amounts of phosphorus (P), Fe and Zn in the grains correlated strongly (r 2 > 0.93) to the dry matter grain yield.

Pigeonpea (Cajanus cajan L. Millsp.) is often intercropped with maize (Zea mays L.) in eastern and southern Africa. The studies aimed at determining how different genotypes of pigeonpea responded in terms of grain element composition under farmers' cropping conditions. Approx. 78 farmers participated. They came from four study sites in Tanzania (Babati and Gairo) and Malawi (Nyambi and Ntonda) that differed in terms of tradition for using pigeonpea as well as in environmental conditions. The individual grain weight of the pigeonpea crops from Malawi were 21% (P < 0.05) higher than those from Tanzania. However, only B, Cu, Mo, N, Ni, P and S were affected by grain weight (P < 0.05). Weak (r 2 < 0.10) negative correlations existed between grain yield and the grains' proportion of Ca, Mg, P, and Zn. The proportion of every element, with the exception of Cr, in the grain differed between sites (P < 0.05) but not between varieties (P > 0.05). The amounts of K, Mg, S and Fe accumulated per grain were slightly lower (P < 0.11) in ICEAP00040 compared to the more traditional varieties. Variations in DTPA-exchangeable Zn and Fe in the soil were not reflected in grain concentrations but grain P had a curvi-linear relation (r 2 = 0.44) to the soil NaHCO 3 --exchangeable P indicating P deficiency for several soils. The P and Zn content were correlated (r 2 > 0.41) as the only two grain elements. Unique fingerprinting by multivariate statistics was possible for each site when using the element proportion of the grain dry matter with or without soil characteristics. In all cases, different elements contributed with varying weight to the discrimination between the sites. However, it was not possible to distinguish between the varieties when considering all four environments. Reducing the models to include Fe, K, Mg, P, S and Zn only, did however allow some distinction between the two genotypes, which indicates that genotypic variability is expressed in a fairly limited number of elements. In the cases of Gairo and Nyambi, it was possible to distinguish between varieties. In the case of Gairo, the models distinguished between ICEAP00068 and the others, i.e. ICEAP00040 and Babati White where ICEAP00068 was associated with a higher proportion of Fe, P, S, and Zn in the grain. In the case of Nyambi, the models distinguished between ICEAP00040 and ICP9145 where ICEAP00040 was

SUMMARYAn experiment to investigate the effect of planting pattern, time of planting the cowpea, and insecticide application on the yield of intercropped cotton and cowpea showed that insecticide applied to the cotton controlled cowpea pests best when the crops were planted in alternating single rows. I n wet seasons, delayed planting of the cowpea reduced its yield but its performance was better in alternating single rows than in other planting patterns. In drier seasons, cowpea yield was unaffected by time of planting and yields were similar with alternating single or double rows. Cotton yield was reduced by 47% when the planting pattern was changed to a one cotton:two cowpea row configuration.