Season Effects on Forage and POD Yield of Cowpea Grown on Heavy Clay Soils of the Eastern Caribbean Island of St. Croix

Cowpea [Vigna unguiculata (L) Walp] cvs. Iron clay, Coolona, and Meringa performance on the heavy clay soils with the rainfall conditions of St. Croix, U.S. Virgin Islands is unknown. Studies were conducted in 2002 to assess season effects (wet versus dry), seedling vigor (plant height), forage dry matter (DM) yield, and pod yield (PY) of three commercial cowpea cultivars (Iron clay, Coolona, and Meringa). Field studies consisted of randomized complete blocks with four replicates. Field plantings were during July [dry season (DS)] and November [wet season (WS)]. Soil types were of the mildly alkaline Fredensborg clay (fine carbonatic, isohyperthermic, Typic Rendolls, Mollisol) with pH above 7.5. Mean seedling vigor differed (P<0.0001) across cultivars by season. Plant height during the DS at 2 wk post planting averaged 12.7, 7.3, and 7.1 cm for Iron clay, Coolona, and Meringa, respectively. This trend (P<0.02) was consistent among cowpea cultivars up to its physiological maturity at 12 wk post planting (46.3, 42.6, 36.5, cm for Iron clay, Coolona, and Meringa, respectively). For the WS, seedling vigor did not differ (P>0.05) for cowpea cultivars and averaged 46.3 ±9.4 at 12 wk post planting. Mean plant height by season for cultivar Iron clay was highest during the dry season and decreased during the wet season. During the DS, plant height averaged 30, 24.3, and 22.1 cm, and during the WS 25.4, 28, and 28 cm for Iron clay, Coolona, and Meringa, respectively (PO.Ol). Forage yield also differed (P<0.05) for the DS, but not for the WS planting (P>0.05). Forage DM yields in the DS averaged 2.3, 1.0, and 1.5 Mg ha"1 for Iron clay, Coolona, and Meringa, respectively. In the WS, mean DM yield of the three cultivars averaged 2.2 Mg ha"1. Pod yield differed (P<0.05) among cowpea cultivars for the DS planting. Highest pod yield was observed for Coolona cowpea (1.5 Mg ha"1). Pod yield for WS did not differ (P>0.05) for cowpea cultivars and averaged 0.68 Mg ha"'. These results indicate that in the DS, Iron clay is far superior to other cultivars for both plant vigor and yield, but in the WS exhibit decreased production to that of cultivars Coolona and Meringa. Yield performance during the WS was similar for all cultivars. Opportunities exist for the further development of cultivar Iron clay as a dry season forage legume in special purpose feeding and conservation systems in the U.S. Virgin Islands.