Can Leguminous Cover Crops Replace Nitrogen Fertilization in Mississippi Delta Cotton Production?

Instability of petroleum supplies has impacted nitrogen fertilization costs. A three year field study was conducted in Stoneville, MS to assess interactions of leguminous cover crops (Austrian pea or hairy vetch) on nitrogen availability and cotton yield grown under reduced-tillage management. Cover crops were seeded in October and paraquat-desiccated in early April. At planting, nitrogen fertilizer was applied at 0, 60 or 120 lb/acre. Plant residues before desiccation averaged 3600, 7400, and 7100 lb/acre in no cover crop, Austrian pea, and hairy vetch plots, respectively.  Nitrogen in residues averaged 43, 196, and 163 lb/acre, in no cover crop, Austrian peas, and hairy vetch plots, respectively. Based on isotope discrimination mass spectroscopy, approximately 158 lb/acre of the nitrogen in Austrian pea and 136 lb/acre in hairy vetch was due to biological nitrogen fixation. In the first year of the study, both cover crops decreased cotton yield, with no effect of fertilizer N. In the second year, cover crop had no effect on cotton yield and the highest yield was with N applied at 120 lb/acre.  In the third year (2007), in no N plots, cotton yields were 65% higher in both cover crops than no cover crop. However the highest yield was achieved in no cover crop and 120 lb of N/acre. Lack of synchronization between N release and plant uptake can depress cotton yield in fertile Mississippi Delta soils. Partial economic analysis of the 2007 data indicated that a net return of 322, 481, and 619 $/acre in no cover crop plots receiving 0, 60 and 120 lb N/acre, respectively. Under Austrian peas, a net return of 474, 501, and 450 $/acre with the same nitrogen levels. Despite environmental benefits of leguminous cover crops, the extra seed cost is a challenge in developing sustainable low-input cotton production systems.