Biomass Yield and Flower Production in Sunn Hemp: Effect of Cutting the Main Stem

Abstract

A field experiment was conducted at the Tropical Research and Education Center, University of Florida, Homestead, to determine the effects on plant morphology, biomass yield, and flower production. of cutting the main stem of sunn hemp (Crotalaria juncea Aref A. Abdul-Baki is Plant Physiologist with the United States L.) plants at different heights. Seeds treated with cowpea (Vigua unguicalata)-type rhizobium were sown on 15 April 1999. The main stems were cut at 30, 60, and 90 cm above soil surface 100 days after seeding when the plants were about 1.5 m tall. Control plants were left uncut. Biomass that had been cut from plants was included in the total biomass yield. Seventy days following stem cutting, individual plants were evaluated for: plant height; main stem diameter; fresh and dry weights of roots, main stems, primary branches, secondary branches, leaves, open flowers, and unopened flowers. Leaf area and nutritional analyses of the plant parts were determined. Cutting the main stem at 30 and 60 cm above soil surface reduced total plant biomass, whereas cutting at 90 cm height increased biomass yield. Cutting at 30 cm produced the highest quality of biomass by increasing the leaf yield and reducing the weights of root and main stem both of which are low in N and high in C/N. Cutting the main stem at 90 cm produced the highest biomass yield, increased the number and weight of primary and secondary branches and, consequently, increased the number of flowers per plant. Nitrogen was highest in flowers and lowest in roots and main stems. Flowers were highest in K, P, Zn and Cu, whereas roots were highest in Fe content. We conclude that cutting the main stem at 30 cm height and allowing the plants to grow for an additional 70 d result in the highest quality biomass for use as green manure, windbreaks, and mulch. Cutting at 90 cm produced the largest biomass yield and increased flower production.

Phenotypic Characterization of 16 Accessions of Sunn Hemp in Florida

“…The increase in branching may be an influential factor for the higher number of flowers in accessions from Group 2 (Table 3). Abdul-baki et al (2001) proposed that there is a positive correlation between branching and flower number and reported an increase in flower number when the main stems of sunn hemp were cut at 90 cm.…”

“…Recently, renewed interest in sunn hemp as a cover crop in the United States created an imbalance between supply and demand of sunn hemp seed. Th is imbalance also contributed to the high cost of sunn hemp seed due to limited seed sources (Abdul-baki et al, 2001). Th e limitation in seed availability has been reported since 1946 as being the major factor in the prevention of widespread use of sunn hemp as a soil-improving crop (McKee et al, 1946).…”

“…Th e limitation in seed availability has been reported since 1946 as being the major factor in the prevention of widespread use of sunn hemp as a soil-improving crop (McKee et al, 1946). Currently, seed production areas limit the amount of available sunn hemp seed and control the market price of seed (Abdul-baki et al, 2001). Developed by the University of Hawaii and the USDA-NRCS, the cultivar Tropic Sun has historically been the focus for commercial seed production (Abdul-baki et al, 2001;USDA, 1983).…”

“…Currently, seed production areas limit the amount of available sunn hemp seed and control the market price of seed (Abdul-baki et al, 2001). Developed by the University of Hawaii and the USDA-NRCS, the cultivar Tropic Sun has historically been the focus for commercial seed production (Abdul-baki et al, 2001;USDA, 1983). Although Tropic Sun grows well in the southeastern United States, the environmental conditions for seed production cannot be met in subtropical regions where the fi rst fall frost prevents eff ective seed set (White and Haun, 1965 photoperiods that are inductive to flowering are abundant and frost is not an issue.…”

“…Although Tropic Sun grows well in the southeastern United States, the environmental conditions for seed production cannot be met in subtropical regions where the fi rst fall frost prevents eff ective seed set (White and Haun, 1965 photoperiods that are inductive to flowering are abundant and frost is not an issue. Expansion of sunn hemp seed production to a domestic location, such as southern Florida, could create a local seed source to meet market demands while simultaneously lowering the cost of seed (Abdul-baki et al, 2001). Identifying sunn hemp germplasm that is better adapted for seed production than Tropic Sun in the continental United States could also encourage greater use of sunn hemp as a cover crop and potentially as a seed crop.…”