WATER-TABLE GRADIENT EFFECTS ON THE PERFORMANCE OF DIVERSE COWPEA CULTIVARS

Cowpea sown in tropical monsoon climates is often constrained by soil saturation and a high water table during growth. We hypothesized that a strong interaction exists between cowpea maturity class and the soil water table depth regime experienced during the growing season, and t hat the occurrence of shallow water table depths in the dry-wet transi tion period before rice and the rapid decline in water table depth in the wet-dry post-rice period favor early maturing cultivars. The effec ts of three water table regimes on the performance of 24 diverse cowpe a [Vigna unguiculat (L.) Walp] cultivars was studied on a Typic Tropud alf toposequence. The water table depths in the shallow (SWT), medium (MWT), and deep (DWT) water table regimes varied between 0.06 to 0.54 m, 0.28 to 0.96 m, and 0.68 to 1.44 m in the 2 yr of dry season (DS) e xperiments. In the wet season (WS), the three regimes were 0 to 0.27 m , 0.30 to 0.60 m, and 0.55 to 1.05 m. Most of the early maturing culti vars exhibited their best performance in the DWT regime, but the mediu m maturing cultivars were superior in seed yields in all water table r egimes. The mean yields of the early maturing cultivars were reduced b y 49% (1986-1987 DS) and 57% (1987-1988 DS) in SWT compared to DWT. Th ey were reduced by 50% in the 1987 wet season. The mean yield reductio ns for the medium maturing cultivars were 40%, 54%, and 51%, respectiv ely, in SWT compared to DWT. Pods per plant was the yield component mo st affected by excess moisture. Flowering in the SWT was delayed by 4 vs 5 d (DS) and 5 vs 7 d (WS) between the early and medium maturing cu ltivar groups, respectively. The superiority of the medium maturing cu ltivars was consistent among sites with a water table within the root zone for a major portion of the crop season.