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Petroleum prices impact cotton nitrogen (N) fertilization cost. A field study was conducted from 2005 to 2007 to assess the interactions of cover crop (none, Austrian winter pea (Pisum sativumspp.arvense) or hairy vetch (Vicia villosaRoth)) and N fertilization (0, 67 or 134 kg N/ha applied at planting) on N availability and cotton yield under reduced-tillage management. Nitrogen content in desiccated residues averaged 49, 220, and 183 kg N/ha, in no cover crop, Austrian winter pea, and hairy vetch, respectively. Seventy percent of N in the above ground cover crop was derived from biological N fixation. In 2005, cover crops decreased cotton yield, while fertilizer N had no effect. In 2006, cover crops did not affect yield, but yield was positively correlated with N rate. In 2007, in no N plots, cotton yields were 65% higher in cover crops than in no cover crop. However, yield from N fertilized cover crop plots were similar to N fertilized no cover plots. These results indicate that leguminous cover crops can provide over 150 kg N/ha, but this N may not be as effective as fertilizer N for lack of synchronization between cotton N requirements and N release from residues.

During the 14-month-long fallow that arises when moving from summer to winter crops, stubble breakdown can denude the soil surface and leave it vulnerable to erosion. Cover crops of millet have been proposed as a solution, but this then raises the question, how often is there sufficient water in the system to grow a cover crop without reducing the soil water reserves to the point of prejudicing the following wheat crop? An on-farm research approach was used to compare the traditional long fallow (TF) with a millet fallow (MF) in a total of 31 commercial paddocks over 3 years. Each treatment was simulated using the simulation-modelling framework (APSIM) to investigate the outcomes over a longer timeframe and to determine how often a millet fallow could be successfully included within the farming system. The on-farm trials showed that early-sown millet cover crops removed before December had no effect on wheat yield, but this was not true of millet cover crops that were allowed to grow through to maturity. Long-term simulations estimated that a spring cover crop of millet would adversely affect wheat yields in only 2% of years if planted early and removed after 50% cover had been achieved.