Surveys of research reports and farmers' opinions in semi-arid Botswana suggested that poor stand establishment of cereals was common and a major cause of low yields. Of 146 researcher-managed trials conducted since 1979, 40% failed to germinate, emerge or establish properly. Analysis of a subset of 84 trials for which more detailed data were available showed that the most common reason for failure was 'soil too dry' at, or just after, sowing. Therefore we tried to improve post-seeding conditions by agronomic means. Sorghum seeds were sown on nine planting opportunities during the 1990-1991 wet season, all after substantial rainstorms, Adding manure did not affect seedling emergence but enhanced growth over the first 25 days after sowing (DAS) when the soil did not dry out rapidly. Two varieties (Segaolane and 65D) differed significantly and consistently throughout the experiment, with Segaolane emerging more quickly and growing more vigorously. The effect of depth of sowing was inconsistent between sewings. Establishment was more successful if rain fell or if the soil dried out slowly after sowing. Consequently, both stand count and dry matter (plot(-1) and plant(-1)) at 25 DAS were directly proportional to the rate at which seedlings emerged. Individual plant vigour in relation to rates of emergence was explained in terms of the interaction between sorghum morphology and rapid soil-drying, Successful establishment was achieved when the soil dried slowly or when seeds germinated and emerged quickly. Soaking seed in water immediately prior to sowing (seed-priming), as a way of speeding up germination, was explored in detail in controlled environments. The time taken for seeds to germinate at 30 degrees C decreased as the soaking time increased from 0 to 10-12 h, a treatment in which a 50% saving in time could be achieved. Germination of seeds soaked for 16 h or more was found to continue even after soaking ceased, suggesting that they would be susceptible to damage in the event of any delays in sowing. Emergence from soil at 30 degrees C was significantly hastened by 23% when seeds were pc-soaked for 6 h or longer. The rate of imbibition during soaking was found to be proportional to temperature. The practicality of seed-priming for improving establishment in the field is discussed.
