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A survey of stored product pests, their natural enemies, grain storage and pest management approaches in northern Namibia was conducted. The survey revealed five types of crop commodities (pearl millet, sorghum, cowpea, maize, feed mixture) stored in four types of grain stores (baskets, bag stags-flat stores, concrete bin, metal containers). No synthetic pesticides or plastic sheeting was recorded. Wooden-ash natural pesticide is the only protectant used. Ten insect pest species were found affiliated to the orders of Coleoptera (Attagenus fasciatus, Callosobruchus subinnotatus, Cryptolestes ferrugineus, Oryzaephilus surinamensis, Rhyzopertha dominica and Tribolium castaneum), Lepidoptera (Corcyra cephalonica, Sitotroga cerealella), Psocoptera (Liposcelis paeta) and Blattodea (Blattella germanica). Out of these, seven species were recognized as new stored-products pests for Namibia. Although listed for southern Africa region, no storage pest-mites (Acari) and Sitophilus spp., Prostephanus sp. or Trogoderma sp. beetles were traced. We found four new species of natural enemies of storage pests recruiting from insects (Habrobracon hebetor, Cephalonomia wattersoni, Brachymeria sp.) and mites (Blattisocius tarsalis). The occurrence of natural enemies indicates a potential for pest bio-control in Namibian grain stores.

An appropriate combination of rice cultivar and cropping system that maximizes water use efficiency (WUE) may improve yield of rainfed lowland rice. In the paddy field, a large amount of water is consumed by evaporation during the early growth period, and it can be reduced by canopy coverage especially in semi-arid regions. Therefore, we evaluated the role of canopy coverage in WUE of rice in the early growth period in semi-arid region. A pot experiment was conducted in Namibia to investigate the genotypic and species difference in WUE, and another pot and a field experiment were conducted to investigate the effects of planting density on WUE. Although no significant difference was observed among species, the mean WUE was in the decreasing order of Oryza sativa, and Oryza glaberrima followed by the interspecific progenies including NERICA. In contrast, there was a significant difference in WUE at the genotypic level. Highly tillering genotypes such as WAB1159-2-12-11-5-1 and WITA 2 showed a high WUE. Furthermore, WUE was significantly correlated with the number of tillers (R 2 = 0.453), and higher planting density resulted in a higher WUE. In contrast, stomatal conductance had no significant correlation with WUE (R 2 = 0.081). Thus, the physical conditions affected by number of tillers and planting density had greater impacts on WUE than physiological characteristics such as stomatal conductance. The suppression of surface water evaporation by coverage was significant, probably contributing to WUE improvement. To increase WUE in semi-arid regions, we recommend the increase of canopy coverage and higher planting density.

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Maize (Zea mays L.) genotypes exhibit differential tolerance to metazachlor (2-chloro-N-(pyrazol-1-ylmethyl)acet-2'-6'-xylidide) in the greenhouse and in the field. However, since little is known on the effect of metazachlor on the ultrastructure of maize seedlings, laboratory experiments were conducted to determine this effect.Metazachlor caused ultrastructural changes in both root and leaf cell organelles of maize seedlings of susceptible genotypes. In susceptible genotypes root cell nucleoli were found to be abnormally large, empty and more abundant than those in untreated control plants. In susceptible plants the chromatids appeared disorganized in cell nucleoli, and both the nuclear and plasma membranes showed signs of disintegrating. There were more and larger vacuoles in the susceptible plants. Leaf cells from susceptible plants had more empty vacuoles and more chloroplasts with generally disorganized content. The bundle sheath chloroplast membranes were dilated in susceptible plants, and the orientation of the grana was disrupted. In the herbicide-tolerant plants, the ultrastructure was not affected. These findings show that metazachlor causes changes on the ultrastructure of susceptible maize genotypes.

Mine water effluents are a danger to the environment as they contain toxic materials such as heavy metals. In some cases though, efforts are made to neutralise the effluents so that the mine water is used for crop irrigation. However, suspected herbicide injury to crops has been reported in areas where gypsiferous water is used for irrigation, suggesting interaction between herbicides and gypsiferous water. Utilization of gypsiferous water for irrigation of crops is one major method promising to reduce the problem of effluent mine drainage disposal and also the shortage of irrigation water. Gypsiferous water is already used for irrigation of crops such as maize and wheat. The influence of gypsiferous water on key behavioural aspects of three important herbicides: atrazine, 2,4-D and metolachlor were assessed. Bioassay experiments were done to assess the bioactivity of the three herbicides in the presence or absence of gypsum in soil. The results showed that the activity of atrazine and 2,4-D was significantly increased in the presence of gypsum, while that of metolachlor was significantly reduced. These differential effects on herbicide activity would have important practical consequences for herbicide performance. Thus weed control efficacy, selectivity, and behaviour in the environment could be negatively affected.