YIELD STABILITY OF COWPEA CULTIVARS IN RICE-BASED CROPPING SYSTEMS - EXPERIMENTATION AND SIMULATION

There is potential in cultivating cowpeas (Vigna unguiculata (L.) Walp .) in rice-based cropping systems in the humid tropics as a source of proteins for humans, forage for ruminants, and for improvement, of soi l fertility. Seasonal and yearly variability is, however, a major cons traint. A statistical stability model and a mechanistic simulation mod el were used to quantify yield variability of early and medium maturit y groups of cowpea cultivars in pre- and post-rice environments at Los Banos, Philippines. Four field experiments, with 24 cultivars in each experiment, were conducted in pre- and post-rice environments, to det ermine the response of the cultivars to varying moisture and to water table depth regimes, and a stability analysis was performed across the regimes using a statistical model. In the post-rice environment, irre spective of moisture and water table depth regimes, the medium maturin g cultivars tended to outyield early maturing cultivars, whereas in th e pre-rice environment, early maturity demonstrated a distinct yield a dvantage. The statistical stability model indicated that medium maturi ng cultivars performed well in high-yielding environments, whereas in low-yielding environments their yields were comparable with those of e arly maturing types. However, results of the statistical stability mod el are limited because of the short time series of basic data. To broa den the analysis, a simulation model of the cowpea crop was calibrated and validated for its performance. Measured and simulated dry weights of crop parts (leaves, stems and pods) and soil moisture contents at 10-cm soil depths throughout the growing season indicated satisfactory performance of the simulation model. Long-term simulation results usi ng the validated model indicate that TVX1948-012F (medium maturity) pe rforms betters (800-2250 kg ha-1) than IT82D-889 (early maturity) (430 -1620 kg ha-1) with a fluctuating shallow water table in the post-rice environment, whereas IT82D-889 is superior (80-1150 kg ha-1) to TV194 8-012F (30-1150 kg ha-1) in the pre-rice environment. Yields are highe st and least variable for the 15 January planting in the post-rice env ironment, and for the 15 April planting in the pre-rice environment.