A conceptual model of organochlorine fate from a combined analysis of spatial and mid- to long-term trends of surface and ground water contamination in tropical areas (FWI)

In this study, we investigated the management of long-term environmental pollution by organic pollutants such as organochlorine pesticides. We set out to identify conditions that are conducive to reducing pollution levels for these persistent molecules and then propose a conceptual model of organochlorine fate in water. Our approach looked at spatio-temporal changes in pollutant contents in surface water (SW) and groundwater (GW) on a large scale, in order to decipher the respective roles of soil, geology, hydrology and past treatment practices. The case of chlordecone (CLD) on the island of Martinique (1100&thinsp;km2) was selected given the sampling campaigns carried out since 2007 over more than 150 sites. CLD, its metabolite chlordecone-5b-hydro (5bCLD) and the metabolite-to-parent-compound ratio were compared. As regards the spatial variability of water contamination, our results showed that banana cropping areas explained the location of contaminated SW and GW, whereas the combination of soil and geology factors explained the main spatial variability in the 5bCLD∕CLD ratio. For temporal variability, these conditions defined a high diversity of situations in terms of the duration of pollution, highlighting two groups: water draining old geological formations and ferralsols or vertisols vs. recent geology and andosols. A conceptual leaching model provided some key information to help interpret downward trends in CLD and 5bCLD observed in water. Lastly, a conceptual model of organochlorine fate is proposed to explain the diversity of the 5bCLD∕CLD ratio in water. Our conclusions highlight the combined role of soil and groundwater residence time for differentiating between conditions that are more conducive, or not, to the disappearance of CLD from the environment. This paper presents a model that provides an overall perception of organochlorine pesticide fate in the environment.</p

Limits of conservation agriculture to overcome low crop yields in sub-Saharan Africa

Conservation agriculture (CA) has become a dominant paradigm in scientific and policy thinking about the sustainable intensification of food production in sub-Saharan Africa. Yet claims that CA leads to increasing crop yields in African smallholder farming systems remain controversial. Through a meta-analysis of 933 observations from 16 different countries in sub-Saharan African studies, we show that average yields under CA are only slightly higher than those of conventional tillage systems (3.7% for six major crop species and 4.0% for maize). Larger yield responses for maize result from mulching and crop rotations/intercropping. When CA principles are implemented concomitantly, maize yield increases by 8.4%. The largest yield benefits from CA occur in combination with low rainfall and herbicides. We conclude that although CA may bring soil conservation benefits, it is not a technology for African smallholder farmers to overcome low crop productivity and food insecurity in the short term

An on-farm study of Striga as constraint to improved sorghum cultivar production in Mali

Sorghum (Sorghum bicolor) is the most important food crop in savanna areas of the West and Central Africa (WCA) region, including Mali, where grain yield averaged 0.71 t in 1999 (FAO 2001). Improved caudatum sorghum cultivars have not been widely adopted in Mali (Yapi and Debrah 1998). However, some of these cultivars such as ICSV 1063 and ICSV 1079 were introduced in the Kolokani area (about 130 km north of Bamako) by Catholic missionaries in the late 1980s. They have since spread and are being cultivated under the name “Gadiabani” by many farmers in over 100 villages (SEPD 1995)

Combined on-farm effect of plot size and sorghum genotype on sorghum panicle-feeding bug infestation in Mali

The reaction of four sorghum cultivars to panicle-feeding bugs was assessed in small (15 m2) and large (0.5–1.0 ha) plots for 2 years in three villages of the Kolokani region (Mali). The aim was to explain the somewhat contradictory earlier observations of pest infestation and damage in small experimental plots (on-station and on-farm) as well as in farmers’ field surveys. Irrespective of the plot size, the local guinea sorghum cultivar Bibalawili was consistently the least infested and damaged, followed by bug-resistant compact-headed cv Malisor 84-7, whereas the improved caudatum cultivar Gadiabani, which had been disseminated for nearly a decade in the region, and the improved hybrid ICSH 89002, were the most heavily damaged. When located along the border of large plots of a susceptible cultivar, small plots of the four cultivars overall were less infested and damaged than when located along the border of plots of resistant cultivars. However, they were more infested and damaged when located in the centre of large plots of susceptible cultivars than when they were in the centre of resistant cultivar plots. In large plots, bug populations and damage decreased from the border to the centre. These results suggest that, in addition to the mere plot size, plant breeders should take the genotypic environment of their experimental plots into account, namely the vicinity of large plots of pest-susceptible or -resistant cultivars, and the position of the test plots (border or centre) relative to these large plots

Effects of slash-and-burn practices on soil seed banks in secondary forest successions in Madagascar

Forest seed bank assessments are scarce in Madagascar and thus little is known about the relationships between the seed banks and vegetation dynamics on which secondary succession processes rely. The objective of this paper is to analyse the effects of slash-and-burn agriculture on soil seed banks and their dynamics along secondary forest successions. The aim of this work is to determine the effects of agricultural practices (cropping duration, tillage regime and fallow age) on seed bank density (SBD) and diversity (SBSR) during secondary succession periods (<25 years). Cropping duration was divided into three classes: (i) short (1&#8211;2 years), (ii) medium (3&#8211;5 years) and (iii) long (6&#8211;11 years). Tillage regime during crop successions was divided into three categories: (1) no tillage (only direct sowing is observed), (2) light tillage (after direct sowing, local shallow tillage is observed) and (3) heavy tillage (after direct sowing followed by light tillage practice, complete tillage practice is observed). Fifteen fallows subdivided into three regrowth age classes (2&#8211;6, 10&#8211;12 and 14&#8211;22 years) were surveyed. Fallow age is the main driver of SBD and SBSR: they increase with fallow age, regardless of cropping duration and tillage regime. SBD and SBSR recovery always occurs but is slower under long cropping duration (3&#8211;11 years) and heavy tillage regime because cropping duration helps to maintain a high level of wind-dispersed seeds during secondary succession and tillage regime contributes to reducing the proportion of viable seeds

Comparative performance of conservation agriculture and current smallholder farming practices in semi-arid Zimbabwe

Conservation agriculture (CA) is currently promoted in sub-humid and semi-arid areas of sub-Saharan Africa as a means to increase crop water use efficiency and stabilize yields. In this study, conducted during three consecutive seasons in a semi-arid area of Zimbabwe, the short-term performance of CA and current farming practices (CP) were compared in two multi-locational experiments: (1) unfertilised on-farm trials with a cotton-sorghum rotation during three consecutive seasons, and (2) farmers' cotton fields receiving fertiliser provided on credit by cotton companies during two consecutive seasons. In both cases, residues for mulch were produced in situ. In addition to biophysical measurements, farmers' perceptions of the technology were appraised. CA did not affect cotton productivity during the first 2 years of the experiments, which received average or above average rainfall. During the drier 2009-2010 season CA had a negative effect on crop yield both in the on-farm trials (average yield of 730 and 820 kg ha(-1) under CA and CP, respectively) and in the farmers' cotton fields (average yield of 1220 and 1440 kg ha(-1) under CA and CP, respectively). There was no difference in water runoff between CA and CP on a relatively fine-textured soil, but significantly more runoff with CA on a coarser-textured soil (14 mm during the wetter 2008-2009 season), due to soil surface crusting and soil compaction. Most soils in the study area fall into this latter category. For this reason, farmers perceived ploughing as necessary during drier years to maximize water infiltration, but perceived CA as beneficial during wetter years as a means to 'shed water' and avoid water-logging. This is rather counterintuitive vis-a-vis the common description of CA as a water-harvesting technology. Soil crusting and compaction may be avoided by the production and retention of quantities of biomass greater than what was realised in this study (on average, only 770 kg ha(-1) of residues were retained as mulch in the on-farm trials). This may be achieved through better crop management (e.g. adequate fertilisation, timely planting, crop protection) in combination with intercropping. Increasing crop primary productivity (e.g. through adequate fertilisation, timely planting and crop protection) is a pre-requisite for the principles of CA to benefit smallholders under semi-arid conditions. Our results indicate that certain legume intercropping combinations may contribute to such an end

Utilization of biological insecticides for the protection of stored yam chips

The processing of yam into dehydrated chips is used extensively in certain areas of West Africa to stabilize the product. However, the chips are often severely attacked by borers, which can reduce whole stocks to powder within a few months. The level of protection provided by various biological products derived from neem (Azadirachta indica) and Crotalaria caricea was compared with that of a synthetic pesticide, Sofagrain™(1.5% deltamethrin + 0.5% pirimiphos-methyl), a reference chemical for stored products in Benin. Insect damage was assessed over a period of 10 months in the store room. Infestation was well controlled by all treatments for up to six months, after which only Sofagrain provided any protection. After 10 months, the chemical still provided a good level of protection, whereas the untreated control was severely attacked by the following insects: Dinoderus porcellus and D. bifoveolatus, Rhyzopertha dominica, Lasioderma serricorne, Palorus subdepressus and Tribolium castaneum. L. serricorne was the most abundant pest, followed by P. subdepressus and T. castaneum. Among the biological products used, the three neem-based treatments (oil, seed powder and leaf powder) gave the best level of protection, whilst Crotalaria seeds and periodic turning over of storage bags had little effect on reducing damage