son crops such as pulses can leave greater residual soil water, particularly below the root zone (Merrill et al., Characterization of plant uptake of soil water at different points 2003). Therefore, a rotation consisting of alternating in time and space are important in evaluating seasonal water use as well as rotational dryland cropping strategies. The objective of this shallow-rooted and short-season crops with deep-rooted study was to characterize root length density (RLD) and soil water crops may improve overall yield and likely reduce yield depletion patterns of cowpea [Vigna unguiculata (L.) Walp.], grain variability associated with continuous cropping of high sorghum [Sorghum bicolor (L.) Moench], and sunflower (Halianthus water-using crops such as sorghum and sunflower. Stratannuus (L.)] under no tillage (NT) and stubble mulch tillage (SMT) ified use of soil water and nutrients may also improve the systems in a Torrertic Paleustoll. Root length density of crops was overall water use efficiency in cropping systems (Bunting measured from scanned images of washed root samples obtained from

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Flood recession farming locally known as molapo farming in the Okavango Delta is practiced along the edges of the river channels or seasonally flooded depressions on its fringes. It relies on residual moisture and natural fertilization of the floodplains, and is promoted as being more productive than dryland farming. However, the productivity of this low-input farming system has not been extensively investigated. The objective of this study was to evaluate the yield potential of hybrid maize (Zea mays L.) in molapo farming. Two molapo field experiments and one dry-land experiment were conducted at Matsaudi and Lake Ngami, and Shorobe, respectively, in a randomized complete block design with four replications. Maize grain yield and plant height were measured from six consecutive plants from the centre row. The results showed that maize grain yield and plant height were significantly (P < 0.05) higher at Lake Ngami than at either Matsaudi or Shorobe. Late planting at Lake Ngami coincided with rainfall in November providing even and abundant soil moisture which contributed to better productivity of maize. Similarly, soil fertility was also better at Lake Ngami. Maize grain yield although lower (2.41 t ha-1) at Matsaudi and Shorobe (2.58 t ha-1) than at Lake Ngami (3.4 t ha-1), it is much higher than yields normally obtained by smallholder farmers in semi-arid zones in sub-Saharan Africa, where actual yields are often less than half of the potential yields. Similarly, plant height was significantly (P < 0.05) higher at Lake Ngami (235 cm) than either Matsaudi (194 cm) or Shorobe (192 cm). It could be concluded that timely planting of maize in molapo fields supplemented by rainfall is capable of producing better yields in semi-arid regions where water is the most limiting resource to crop production

Weed management is an ongoing constraint in southern Africa for conventional farming systems and in emerging conservation agriculture systems, which are more heavily reliant on herbicides for primary weed control. The challenge of rising labour costs and decreasing availability creates a greater need to develop effective and efficient weed management methods in key crops such as maize. Field experiments were conducted at Sebele Agricultural Research Station, Botswana in the 2011/12 and 2012/13 cropping seasons to evaluate pre-emergence application of atrazine at 1,000 and 2,000 g a.i. ha-1 and S-metolachlor at 1,440 and 2,880 g a.i. ha-1, and a tank mixture of atrazine at 1,000 and S-metolachlor at 1,440 g a.i. ha-1. Atrazine at either rate alone, effectively controlled annual broadleaf weeds: Acanthospermum hispidum, Datura ferox and Sesamum alatum, but failed to control annual grass weeds (Tragus berteronianus and Urochloa spp.). Conversely, sole application of S-metolachlor at either rate provided complete control of annual grass weeds, but poorly controlled annual broadleaf weeds except small-seeded Amaranthus hybridus and Amaranthus thunbergii. A tank mixture of atrazine and S-metolachlor provided broad-spectrum weed control and successfully controlled both annual broadleaf and grass weeds. Atrazine alone and in tank mixture with S-metolachlor significantly reduced annual broadleaf weed density and biomass and increased maize grain yield by more than 80% when compared with the weedy treatment. High weed density and biomass of annual broadleaf weeds in S-metolachlor treatments significantly reduced maize grain yield to levels similar to the weedy treatment. A pre-mixture of atrazine and S-metolachlor is recommended for broad-spectrum weed control. Using a combination of herbicides with different modes of action may reduce selection pressure for herbicide resistance.