A study was conducted to determine the population sizes of indigenous pigeonpea (Cajanus cajan)-nodulating rhizobia and responses of the crop to rhizobial inoculation in soils under smallholder management. Rhizobia populations were determined in 21 soils from three different agro-ecological regions of Zimbabwe using the plant infection most-probable-number technique. Pigeonpea response to rhizobial inoculation was tested in five soils representative of low, medium and high rhizobia populations. Pigeonpea rhizobia ranged from undetectable to 121 cells per g soil compared with 16 to 159 cells per g soil for cowpea (Vigna unguiculata) which was used for reference. Soils with high cowpea rhizobia counts had relatively low counts of pigeonpea rhizobia and vice versa, showing that the two legumes associate with different subgroups of rhizobia. Poor soil organic matter, low soil moisture at sampling, low pH and low clay content of the soils had a significant negative effect on rhizobial counts. Organic matter appeared critical for maintenance of high populations of indigenous rhizobia in the mostly sandy soils sampled. Lack of pigeonpea response to inoculation in all the soils tested despite the low initial rhizobial populations could be the result of within-season proliferation of indigenous populations which are competitive and effective. There was evidence of rapid build-up of pigeonpea-compatible rhizobia within one growing season when the crop was first introduced. It was concluded that effective pigeonpea rhizobia occur in many arable soils of Zimbabwe. However, to fully exploit biological nitrogen fixation and maximize yields of pigeonpea, highly efficient, adapted and competitive indigenous rhizobial isolates must be identified and evaluated.

This study, designed to investigate trade-offs between forage yields, feed quality and labour use in fitting a newly introduced crop, Lablab purpureus, into an existing farming system, adopted distinct applied and adaptive research approaches. The applied research component compared forage yields, nutrient contents and feeding qualities of L. purpureus harvested at 100, 114, 128 or 142 d after planting in an on-farm trial. The adaptive research investigated how farmers incorporated L. purpureus cultivaton into their farming calendars given the constraints of labour for planting and harvesting.

Harvested dry matter yields from the on-farm experiment were 1.28, 1.98, 1.74 and 1.44 t ha−1. The resultant hays were fed to four groups each of five Bunaji lactating cows in an on-farm feeding trial over a 10-week period. Ranking the various harvests for yield, the hays for feed value, animal responses to the feed consumed and the quality of animal products on a linear scale (1 to 4) to estimate a composite utility index, showed that harvesting at 114 d after planting gave the most desirable overall benefits.

The survey for adaptive research showed bimodal rainfed and irrigated cultivation patterns for L. purpureus. Peaks were observed in September (31% of farmers) and November (24%) for planting, and in December (39.4%) and March (15%) for harvesting. Forage yields were, however, highest for the 3% of farmers who afforded labour to plant in July and harvest in October. The lack of congruence between highest forage yield periods and the peaks of cultivation of L. purpureus by farmers in the study area was mostly explained by very significant differences in the cost of hired and household labour in different months. Thus, socio-economic and cultural factors, including stakeholders' participation, need to be considered along with applied research results when recommending forage species for farmers' adoption.

Seeds of watermelon (Citrullus lanatus) with a moisture content of 7.2% were heat treated at 60°C for 15 or 20 h, and their performances compared with that of untreated seeds. Radicle emergence of treated seeds was only slightly lower than that of the control when germinated at 25°C. To ascertain the effect of the treatments on hypocotyl growth (emergence) per se, radicle emergence was synchronized in all treatments and emergence from deep plantings in sand determined. Small differences in emergence were observed when seeds were germinated at 25°C, but seed treatment resulted in large reductions in emergence at 15°C. This demonstrates that the hypocotyls developing from heat-treated, germinated seeds are less able to withstand low germination temperatures, which could be an important factor in emergence failure. Levels of free abscisic acid (ABA) were higher at 15°C than at 25°C, with hypocotyls of treated seeds containing more free ABA and its breakdown products than the control at the lower temperature.

A study was conducted to examine the effects of the time and severity of cutting lablab (Lablab purpureus), both in sole crops and intercropped with maize (Zea mays). The effects of cutting management on the dry matter (DM) yield, leaf area index (LAI), nodulation, the nitrogen (N) yield, and N fixation by lablab were measured, as were the total DM yield, grain yield, and a yield component of maize.

Two dates of lablab cutting (40 and 60 d after sowing (DAS)), and two heights of cutting (30 and 20 cm above the soil surface) were applied in factorial combinations as well as a no-cutting treatment. Maize was not cut either in monoculture or intercrops.

Intercropping lablab with maize increased the grain yields of maize from 3.2 t ha−1 to 3.8 t ha−1 if lablab was cut at 40 DAS, but delaying cutting until 60 DAS had no effect, while uncut lablab reduced the yield of maize by 10%. Intercropping lablab with maize reduced the total N yield of lablab by over 50%, from 302 to 132 kg N ha−1. Regardless of cropping system, the total N yield of lablab at 130 DAS was 24% less when lablab was cut at 60 DAS compared with 40 DAS. Cutting height did not significantly affect the total N yield of lablab. Although both cropping system and cutting time significantly affected the total N fixation by lablab at 130 DAS, there were no significant effects of cropping system, cutting time or severity of cutting on the proportion of N in lablab derived from N fixation.

Values of Land Equivalent Ratio (LER) which ranged between 1.2 and 1.6, and of Area Time Equivalent Ratio (ATER) which ranged between 1.0 and 1.4 for intercrops of maize with lablab indicated a definite advantage of intercropping. LER and ATER values were greatest when lablab was uncut, and least when lablab was cut at 60 DAS.

Infection of groundnut by pathogens causing early and late leaf spot diseases is strongly affected by accumulated daily leaf wetness periods and, in the rainy season, temperature is unlikely to severely limit infection. Earlier work relating patterns of leaf wetness to infection, was used to define a daily Wetness Index (WI) which was compared with infection on inoculated plants exposed in the crop for periods of 7 d. Infection was only severe when the 7-d WI total exceeded a value of 2.3. The proportion of leaves with one or more lesions on the main stem was used to assess the amount of inoculum in the crop. When the proportion of diseased leaves exceeded 10% and the WI total exceeded the threshold, application of a fungicide was advised. Successive sprays were separated by at least 14 d and a maximum of three sprays were applied in the growing season. Field trials showed that three sprays gave limited benefit where the disease pressure was severe, but substantial increases in pod and haulm yield were possible with only one or two fungicide applications in locations with less disease pressure.

A procedure was used to calibrate the DSSATv3 CERES–Maize (Zea mays) model and to evaluate its performance in simulating growth and development of maize using input data collected from campesino farmers instead of using data obtained from on-station experiments or from the literature. The problems encountered in the calibration process are illustrated, particularly the failure of the model to simulate the growth and development of local highland maize cultivars (HMC). It is argued that the low ambient temperatures to which HMCs are exposed in the Toluca Valley are responsible for this failure, because HMCs respond differently to temperature and have a different optimum temperature range from temperate and tropical maize cultivars. It was concluded that the model needs to be adjusted to allow for consideration of the effects of constant low temperatures on the prediction of plant phenology and production, and partition of biomass in HMCs, since grain yield is not the main criterion used by smallholders when selecting maize cultivars.

The effect of seven planting densities on yield and witches' broom disease incidence in hybrid cacao (Theobroma cacao) trees in the Brazilian Amazon was investigated. Data collected over a three-year period showed that it was possible to optimize regional cacao yield by high planting densities (2500 and 1666 trees ha−1) and that there was no density × year interaction. However, high planting density also favoured witches' broom incidence. A growing regional interest in planting cacao clones at high planting density should be pursued with caution, since high planting densities of clones in environments under strong pressure of selection for the witches' broom pathogen, such as in the Amazon region, are still a risky strategy. Alternatives to the high planting density system are presented and discussed.

Since shading often occurs under normal growing conditions in various cropping systems (for example, intercropping, alley cropping), field experiments were carried out to determine the effects of shading on stomatal density, leaf size, leaf dry matter, and leaf lamina thickness in the major tropical root and tuber crops, tannia (Xanthosoma sagittifolium), sweet potato (Ipomoea batatas), yam (Dioscorea esculenta), cassava (Manihot esculenta), and taro (Colocasia esculenta). Shading decreased stomatal density in the lower epidermis of tannia, sweet potato, yam and cassava, and in the upper epidermis also in tannia and sweet potato; the upper epidermis of yam and cassava were devoid of stomata. In contrast to the other species, taro under shade had an increased stomatal density in both the upper and lower epidermis, a finding which was confirmed in subsequent pot experiments. This response of taro was postulated as a possible manifestation of greater shade adaptation by the species.

For all the species, shading generally resulted in the production of larger (in terms of surface area) but thinner leaves, with a decreased dry matter concentration. For the two species (yam and cassava) that had stomata only on the lower epidermis, normal sun-grown plants had about twice as many stomata per unit area of lower epidermis than did corresponding plants of the other three species. The agronomic and physiological significance of the findings are discussed.