Sugarcane is commercially produced on 340,000 ha in the US and is valued at over $1 billion US annually. Cultural practices that improve sugarcane sustainability are needed to maintain yields in fields with degraded soils. Historically, leguminous rotation crops provided organic matter and biologically fixed nitrogen (N) for subsequent sugarcane crops. Currently, sugarcane is usually grown as a monoculture with only a short, 6-month fallow period. The objective of these field studies was to determine how growing cowpea (Vigna unguiculata (L.) Walp.) and sunn hemp (Crotalaria juncea L.) as cover crops during fallow affected the yield of subsequent sugarcane crops. A companion laboratory study investigated the decomposition rate of cover crops in soil at different temperatures. Cowpea and sunn hemp production produced 12.8 t/ha dry matter and 250 kg N/ha. Cowpea generally improved plant cane yields, but the effects of sunn hemp varied. However, neither cowpea nor sunn hemp reduced cane or sucrose yields consistently, and mineral N additions may have a role in mitigating yield gains or losses. Based on laboratory data, the average half-life for cowpea and sunn hemp would be 3 months. Overall, using legume cover crops should be viewed as an important component of sustainable sugarcane practices.