PHYSIOLOGICAL DETERMINANTS OF COWPEA SEED YIELD AS AFFECTED BY PHOSPHORUS-FERTILIZER AND SOWING DATES IN INTERCROP WITH MILLET

The interaction of cowpea (Vigna unguiculata (L.) Walp.) cultivars wit h management factors and environments was analyzed in terms of paramet ers of a simple physiological model. In one set of experiments seven c ultivars were sown at three rates of phosphorus (P) fertilizer (0, 8 a nd 16 kg P ha-1). In another set, five cultivars were sown on two date s relative to sowing of millet (Pennisetum glaucum (L.) R.Br.). All ex periments involved factorial combinations of management and cultivar t reatments, and were conducted in 1988 and 1989 at two sites in western Niger. Seed yield, shoot dry matter, vegetative (D(v)) and reproducti ve (D(r)) durations were determined and crop growth rate (C) and parti tioning (p) to seed estimated. Application of P increased seed yield b y increasing C. As sowing was delayed from 1 to 3 weeks after the mill et was sown, there was a reduction in seed yield due to decreases in C . D(v) and D(r). Variation in cultivar performance across years was ma inly a function of C and partitioning. In both experiments the C of th e landrace Sadore Local was greater than that of other cultivars, alth ough the partitioning to seed of this cultivar was unstable due to var iable control of insect pest damage. It is concluded that the primary causes of G X E interactions were differential canopy development and insect damage effects. Therefore in the development of cowpea cultivar s for intercropping with millet, emphasis should focus on light captur e capabilities. Improved yields would then seem possible with most cul tivars, providing insect damage is controlled.