Herbage Accumulation and N Fixation of Sunnhemp in South Florida.

Sunnhemp is a warm-season annual legume that has potential to be used as forage in the southeastern USA. The objective of this study was to evaluate herbage accumulation (HA), nutritive value, N fixation, and nematode reduction of different cultivars of sunnhemp in South Florida. The research was conducted in Ona, FL (27º N) from March to November 2016. Treatments were the factorial arrangement of two harvest periods (60-d after seeding or flowering) and four sunnhemp cultivars, ‘AU Golden’, ‘Blue Leaf’, ‘Crescent Sun’, and ‘Ubon’, distributed in a randomized complete block design with four replicates. Plot size was 4.5 x 4.5 m with a 2.0-m alley between plots. Plots were seeded with 28 kg seed ha^-1 in a prepared seedbed in March 2016. There was a cultivar × harvest period effect on herbage accumulation (HA), crude protein (CP), and in vitro digestible organic matter (IVDOM) concentrations. Herbage accumulation was greater at flowering than 60-d harvest for all cultivars; however, the magnitude of increase was greater for Blue Leaf and Crescent Sun than AU Golden and Ubon. At 60-d harvest period, Crescent Sun had the greatest HA (3.8 Mg ha^-1), followed by AU Golden and Blue Leaf (2.8 Mg ha^-1), and Ubon had the least HA (1.5 Mg ha^-1). AU Golden and Ubon flowered 56 and 66 d after seeding, while Blue Leaf and Crescent Sun flowered 156 d after seeding. AU Golden had the greatest CP at 60-d harvest (175 g kg^-1), followed by Blue Leaf (150 g kg^-1), and Crescent Sun and Ubon had the least CP concentrations (125 g kg^-1). Forage CP trends were the opposite of HA and AU Golden and Ubon had greater CP concentrations than Blue Leaf and Crescent Sun when harvested at flowering. There was no difference in IVDOM among cultivar at 60-d harvest (500 g kg^-1); however, AU Golden and Ubon had greater IVDOM than Blue Leaf and Crescent Sun at flowering (480 vs. 370 g kg^-1). Plots harvested at flowering had greater atmospheric N fixation than at 60-d (95 vs. 50 kg N ha^-1) and there was a significant difference in atmospheric N fixation among cultivars. Crescent Sun had the greatest N fixation (99 kg N ha^-1), followed by AU Golden and Blue Leaf (73 kg N ha^-1), and Ubon had the least N fixation (45 kg N ha^-1). Blue Leaf and Crescent Sun had the greatest reduction in root-knot nematode (Meloidogyne incognita) count (81%), followed by Ubon (45%), and AU Golden had the least (20%) reduction in nematode count. Plots harvested at flowering had greater reduction in nematode count than at 60-d (73 vs. 34%). In order to utilize sunnhemp as forage, HA, N fixation, and nematode reduction will have to be compromised by harvesting forage with decreased maturity and greater nutritive value.