Sunn Hemp Biomass and Nitrogen Production for Different Planting Dates and Seeding Rates.

Elevated nitrogen (N) fertilizer costs have renewed interest in alternative N sources, such as legumes. Sunn hemp (Crotalaria juncea L.) is a tropical legume capable of producing considerable biomass in a short period of time.  A randomized complete block design with a split-plot restriction and four replications was conducted to assess the performance (plant biomass, N content) of a new sunn hemp cultivar for two planting dates (main plots) following corn (Zea mays L.) and wheat (Triticum aestivum L.) harvest across different seeding rates (subplots) (16.8, 33.6, 50.4, and 67.2 kg ha^-1) at the Plant Breeding Unit near Tallassee, AL on a Wickham sandy loam during 2007 and 2008.  In 2007, sunn hemp planted immediately after wheat (PD1) compared to a two week delay (PD2) resulted in 51% more sunn hemp biomass.  Sunn hemp biomass from 2008 was 32% greater than PD2 after wheat harvest, however PD1 biomass contained 32% more N than PD2.  Sunn hemp biomass collected after corn harvest showed similar results.  In 2007, there was a 49% increase in sunn hemp biomass for PD2 compared to PD1 following corn harvest and PD1 biomass also contained 29% more N than PD2 biomass after corn.  In 2008, sunn hemp produced 54% more biomass from PD1 than PD2 after corn, but PD2 contained 30% more N than PD1.  Results for planting dates were not consistent and appeared to be influenced by year, but seeding rates had little effect on sunn hemp productivity.  Future research will continue to evaluate how sunn hemp can fit into existing crop rotations across the Southeast.