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We conducted field experiments over 2 years on two acid soils of southern Cameroon to test whether efficient uptake and use of phosphorus (P) from less available sources by grain legume genotypes could benefit subsequent rotational maize. We grew two crops each year. For the first crop we grew 4 genotypes of soybean and of cowpea, plus maize. For the second crop we grew maize. The first crops were fertilized with 0, 90 kg P ha )1 as phosphate rock (PR) or 30 kg P ha )1 as triple super phosphate (TSP). P application highly significantly increased shoot dry matter, P uptake, N 2 fixation and grain yields of the grain legumes with TSP generally more effective than PR. Two of the soybean and two of the cowpea genotypes were more efficient at using P. Only the P-efficient soybean and cowpea genotypes increased subsequent maize yields. Yields of the subsequent maize grown in rotation were significantly correlated with shoot P uptake for which the quantity of P applied with the crop residues of the pre-crop appeared to be a major factor.We also grew the grain legumes in nutrient solutions and measured organic acid-anion exudation from roots, root-surface phosphatase-activity, and root morphological characteristics. Enhanced exudation of organic acid anions from roots of Pdeprived plants might have contributed to the P acquisition efficiency under field conditions of the P-efficient cowpea genotypes and one of the Pefficient soybean genotypes. A higher activity of root-surface acid phosphatase might have been important for the other P-efficient soybean genotype. The results show, that the potential positive rotational effect of cowpea and soybean on the acid, highly P-sorbing soils of southern Cameroon depends on breeding and using P-efficient genotypes when sparingly soluble and suboptimal rates of soluble P fertilizers are used.

Stem borers are the most important maize pests in the humid forest zone of Cameroon. Field trials were conducted in the long and short rainy seasons of 2002 and 2003 to assess the level of damage and yield reductions caused by stem borers in monocropped maize and in maize intercropped with non-host plants such as cassava, cowpea and soybean. The intercrops were planted in two spatial arrangements, i.e. alternating hills or alternating rows. All intercrops and the maize monocrop were grown with and without insecticide treatment for assessment of maize yield loss due to borer attacks. The land-use efficiency of each mixed cropping system was evaluated by comparing it with the monocrop. The temporal fluctuation of larval infestations followed the same pattern in all cropping systems, but at the early stage of plant growth, larval densities were 21.3-48.1% higher in the monocrops than in intercrops, and they tended to be higher in alternating rows than alternating hills arrangements. At harvest, however, pest densities did not significantly vary between treatments. Maize monocrops had 3.0-8.8 times more stems tunnelled and 1.3-3.1 times more cob damage than intercrops. Each percentage increase in stem tunnelling lowered maize grain yield by 1.10 and 1.84 g per plant, respectively, during the long and short rainy season in 2002, and by 5.39 and 1.41 g per plant, respectively, in 2003. Maize yield losses due to stem borer were 1.8-3.0 times higher in monocrops than in intercrops. Intercrops had generally a higher land-use efficiency than monocrops, as indicated by land-equivalent-ratios and area-time-equivalent-ratios of >1.0. Land-use efficiency was similar in both spatial arrangements. At current price levels, the net production of mixed cropping systems was economically superior to controlling stem borers with insecticide in monocropped maize. The maize-cassava intercrop yielded the highest land equivalent ratios and the highest replacement value of the intercrop. At medium intensity cropping this system is thus recommended for land-constrained poor farmers who do not use external inputs such as fertilizer and insecticides.

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Crop management without fertilizer input, which is commonly practiced by most farmers in the humid forest zone of West and Central Africa, requires soil fertility replenishment during a fallow period. Hypothetical relationships between fallow length and crop yields assume, that after the cropping phase replenishment starts with high annual increments, leading to an early recovery of most soil fertility, then slowly approaching a maximum level. The few available empirical data, however, indicate that this assumption is wrong. Within the first 8 years of fallow, biomass and nutrient accumulation is either progressive (low initial increments) or linear. Planted fallows are supposed to replenish soil fertility faster or to higher levels than natural regrowth and should thus lead to higher crop yields. Two major types of planted fallow are distinguished: tree-based and herbaceous fallows. Data from West and Central Africa do not confirm that tree based fallows are generally capable of attaining higher crop yields than natural regrowth or other planted fallows. The majority of experiments with tree-based fallows showed no differences to the control (60.0%). Crop yield declines were found in 15.7% of cases, and only 24.3% resulted in significant yield increases. Changes in soil properties were more frequently positive (34.3%) than negative (9.8%), yet, most often (55.9%), there was no effect. Herbaceous fallow had dominantly positive effects on crop yields (52.5% of cases), with only 3% of cases in which significant reductions were observed. Positive features of some herbaceous fallows, such as easy establishment, rapid weed suppression, and labor efficient slash-and-burn crop establishment make the technology more likely to be accepted and adopted by farmers. It appears that fallows have to be specifically designed for responsive crops, i.e. maize. It is unlikely that one type of fallow can serve the multitude of crops and intercrops grown in the region. Depending on the major constraints to crop production or income generation, planted fallows have to be specifically designed to address these constraints. This may require de-emphasizing the soil fertility aspect and focusing on marketable fallow by-products, weed and pest suppression and reduced labor requirements. Thus, future impact through research on planted fallows will depend on exact targeting of specific fallow types and species to the most responsive crops and to explicit farmer circumstances.Deceased.

This study was conducted in the humid forest zone of Cameroon, in 2002 and 2003. The main objective was to investigate the effects of intercropping on infestation levels and parasitism of the noctuid maize stem borer Busseola fusca Fuller. Two trials were planted per year, one during the long and one during the short rainy season. Maize monocrops were compared with maize/legume or maize/cassava intercrops in two spatial arrangements: maize on alternate hills or in alternate rows. Spatial analyses showed that the stemborer egg batches were regularly dispersed in the maize monocrop and aggregated in the intercrops, as indicated by b, the index of dispersion of Taylor's power law. Depending on the crop association and planting pattern, intercrops reduced the percentage of plants with stem borer eggs by 47.4-58.4% and egg densities by 41.2-54.5% compared to monocropped maize. Consequently, larval densities were 44.4-61.5% lower in intercrops compared to monocrops. Intercropping maize with non-host plants did not affect larval parasitism. Up to two-fold higher levels of egg parasitism by scelionid Telenomus spp. were recorded in inter- compared to monocrops during the short rainy seasons of 2002 and 2003. No differences were found among the mixed cropping treatments and parasitism was lower during the long compared to the short rainy seasons. It was proposed that differences in levels of parasitism were due to density dependence effects rather than the effect of the presence of non-host plants in the system.

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