Plant population effects on stalk growth, yield, and bark fiber content of Sunn hemp

As worldwide consumption of newsprint and wood-based products grows, potential demands for supplemental nonwood fiber sources like sunn hemp (Crotalaria juncea L.) increase. Studies were conducted in 1994 and 1995 at Weslaco, TX to determine the influence of four plant populations (161300, 225800, 290320, and 354840 plants ha(-1)) and two sunn hemp cultivars (PI248491, a Brazilian introduction and Tropic Sun) on final stand, stalk height, basal stalk diameter, total stalk yield, and bark fiber percentage from two stalk locations. A split-plot experimental design with four row plots, 6.7 m long, and 1.0-m row spacing was used. Results from the 2 years indicated that plant population did not significantly influence stalk height or total yield; however,basal stalk diameter of plants in the two lower plans populations was significantly greater than for plants in the highest population. Final stand and percent plant stand reduction (initial vs. final population) differed between each of the four populations. Bark fiber percentage of the lower 0-30 cm stalk section did not differ between populations. Differences in the bark percentage occurred between populations for the middle 120-150 cm stalk section, with the highest and second highest plant populations having bark percentages of 24.0 and 21.8, respectively, compared to 16.6 for the lowest population. Stalk yield differed among cultivars, with Tropic Sun producing 16.4 Mg ha(-1) of total stalk fiber compared to 15.1 Mg ha(-1) for PI248491. Cultivars had similar stalk height, basal stalk diameter, final stand, and percent stand reduction. Plant height and basal diameter averaged across the four plant populations were positively correlated, r = 0.41; P less than or equal to 0.01 and r = 0.39; P less than or equal to 0.01, respectively, with stalk yield. Correlation coefficients calculated within each individual plant population indicated that basal diameter (r = 0.49; P less than or equal to 0.05), not stalk height (r = 0.31; P less than or equal to 0.15), was more closely correlated with stalk yield at the lowest population; whereas, plant height (r = 0.43; P less than or equal to 0.05) was more closely correlated than basal diameter (r = 0.21; P less than or equal to 0.33) with yield in the highest plant population. Results indicated that increasing the plant population above 161300 plants ha(-1) would not necessarily produce significant increases in total stalk yield. The effect of plant population on bark percentage suggests the possibility of manipulating bast and core fiber yields through specific plant densities. The ability to manipulate core and bark fiber yields could be an important crop management tool when growing fiber for core and bark fiber-specific products: Additional studies are needed to determine the best sampling techniques for estimating bark percentage of sunn hemp. (C) 1998 Elsevier Science B.V. All rights reserved.