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Utilization of Cowpea Crop Residues to Reduce Fertilizer Nitrogen Inputs with Fall Broccoli

Dep. of Hort. and Landscape Architecture, Oklahoma State Univ., Stillwater, OK 74078-6027

J. Q. Lynd

Dep. of Agronomy, Oklahoma State Univ., Stillwater, OK 74078-6028

^* Corresponding author (bakahn{at}okway.okstate.edu).

Environmental concerns have led to renewed interest in use of sustainable green manure legume cropping systems. Field experiments were undertaken to determine whether cowpea [Vigna unguiculata L. (VValp.)] crop residues could be used to reduce fertilizer N inputs with fall broccoli [Brassica oleracea L. (Italica Group)]. Studies in 1992,1993, and 1994 involved broccoli preceded by cowpeas, with or without factorial combinations of preplant N and sidedress N from urea on the broccoli. Control broccoli plants were grown on fallowed ground and supplied with 168 kg ha^–1 of N from urea. Cowpea crop residues alone did not provide sufficient N for optimum production of marketable broccoli. Marketable broccoli yields sometimes showed no significant differences between controls and cowpea residue treatments receiving only 84 kg ha^–1 of fertilizer N. However, there was a tendency for increased broccoli transplant mortality in cowpea residue plots. A fourth study in 1994 indicated a negative main effect of cowpeas on broccoli stand establishment, and a complex interaction of cowpeas and trifluralin treatments on broccoli yield. Petiole N concentrations of broccoli plants with reduced fertilizer N inputs which succeeded cowpeas usually were lower than those of control plants. Soil nitrate levels at 15- to 30-cm soil depths at the time of broccoli planting were consistent indicators of apparent nitrate utilization by microbial decomposition of cowpea crop residues. We were unable to demonstrate a cowpea-broccoli succession cropping system which consistently reduced fertilizer N inputs without some type of adverse effect on the broccoli crop. Therefore, the use of cowpea crop residues to reduce fertilizer N inputs with fall broccoli is not recommended.

Received for publication March 27, 1997.