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

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Abstract

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 (161 300, 225 800, 290 320, and 354 840 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 plant 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≤0.01 and r=0.39; P≤0.01, respectively, with stalk yield. Correlation coefficients calculated within each individual plant population indicated that basal diameter (r=0.49; P≤0.05), not stalk height (r=0.31; P≤0.15), was more closely correlated with stalk yield at the lowest population; whereas, plant height (r=0.43; P≤0.05) was more closely correlated than basal diameter (r=0.21; P≤0.33) with yield in the highest plant population. Results indicated that increasing the plant population above 161 300 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.

Introduction

Worldwide deforestation has resulted in the search for renewable nonwood fiber sources to supplement the paper and wood product industry. Sunn hemp, a widely grown green manure crop of tropical and subtropical regions (Kundu, 1964, Lai et al., 1967, Barros Salgado et al., 1972, Rotar and Joy, 1983), has good potential as a supplemental fiber crop (White and Haun, 1965, Duke, 1981, Cook and White, 1996). Other desirable characteristics of sunn hemp include nitrogen fixation ability, resistance to root-knot nematode (Meloidogyne sp.), and drought tolerance (White and Haun, 1965).

Because sunn hemp has generally been grown as a green manure crop, plant stand establishment has been achieved by broadcast or close-drilled plantings and the crop terminated before reaching maturity. Reports vary on optimum seeding rates for green manure cropping and fiber production. As a green manure crop, McKee et al. (1946)recommended seeding rates of 22.4–44.8 kg ha−1 planted broadcast and 5.6–7.8 kg ha−1 drilled on 1.1-m spaced rows. Suggested seeding rates for production in Hawaii were 45–60 kg ha−1 for broadcast plantings and 34–56 kg ha−1 when drilled on 15-cm spaced rows (Rotar and Joy, 1983). Lovadini et al. (1970)recommended seeding rates of 60 kg ha−1 for rows spaced 50 cm apart in Brazil. Ghumary and Bisen (1967)observed that optimum seeding rates for fiber production in India were 66.5–88.5 kg ha−1 when planted broadcast and 6.6 kg ha−1 when drilled on 30-cm spaced rows. They also noted that the drilled planting produced 10% more yield than the broadcast planting. Kundu (1964)reported seeding rates in India to be as high as 134 kg ha−1 in Bengal and Orissa and as low as 28 kg ha−1 in Madhya Pradesh. For production in the United States, White and Haun (1965)recommended seeding rates of 16.8–22.4 kg ha−1 when planted on rows spaced 30.5–35.6 cm apart. A commercial planting in the Lower Rio Grande Valley of Texas in 1995 used a seeding rate of 17 kg ha−1, planted double-drill on 1.0-m row spacing (G. Kinney, Donna, TX, pers. comm., 1995). Differences in seed size, seed quality, planting conditions, and use of the crop are undoubtedly related to variations in seeding rate.

Seed size of most sunn hemp cultivars ranges from 30 000–35 000 seeds kg−1, so seeding rates of 7–12 kg ha−1, with germination percentage above 80% should produce stands similar to those recommended for kenaf (Hibiscus cannabinus L.), a similar growing fiber crop (Dempsey, 1975, Bhangoo et al., 1986, Scott and Teague, 1990). Although adequate plant populations are necessary to achieve maximum stalk yields in kenaf, studies have indicated that natural self-thinning increases as the population increases (Scott and Teague, 1990, Neill and Kurtz, 1994).

The yields of sunn hemp and kenaf have been compared (Kundu et al., 1959, Scott et al., 1992, Scott and Cook, 1994). Kenaf will generally produce slightly higher yields than sunn hemp under optimum growing conditions, whereas, sunn hemp is better suited than kenaf for production on nematode-infested or nitrogen-depleted soils (Scott and Cook, 1994, Cook and White, 1996).

The growing conditions and cultural practices of the Lower Rio Grande Valley differ from many areas of previous sunn hemp research and, unlike kenaf, little is known about the interrelationships of plant growth with yield, and bark percentage. The objectives of this 2-year study were to determine: (1) the influence of four plant populations on final stand establishment, stalk height, basal stalk diameter, total stalk yield, and bark fiber percentage (0–30 cm and 120–150 cm from soil level) of sunn hemp grown in the Lower Rio Grande Valley, and (2) the relationships between yield and the measured traits across the four plant populations and within each individual population.

Section snippets

Materials and methods

In 1994 and 1995, two sunn hemp cultivars, `Tropic Sun' and PI248491 (cv. Guizo de Cascavel), were planted in the Lower Rio Grande Valley, at the USDA-ARS Subtropical Agricultural Research Laboratory in Weslaco, TX. Cultivar origin of Tropic Sun is Hawaii, whereas PI248491 is an introduction from Brazil. Planting dates were 29 March 1994 and 27 March 1995. Soil type at the experimental location was a Hidalgo sandy clay loam (fine-loamy, mixed, hyperthermic Typic Calciustolls), with a pH of 7.8.

Results and discussion

The analysis of variance indicated that significant differences (P≤0.05) occurred between years for final stand, stand reduction, stalk height, basal diameter, and stalk yield. Final stand, percent stand reduction, and basal stalk diameter differed between plant populations. There was no significant year×plant population or year×cultivar interactions for any measurement. Cultivars differed for total stalk yield only. A significant plant population×cultivar interaction was detected for final

Conclusions

Results of the 2-year study indicated that final plant population had the most influence on basal stalk diameter and the percentage of bark fiber from the middle stalk section. Stalk yield and plant height were not affected by the four plant populations. The significant effects of plant population on basal stalk diameter, but not stalk height, indicated that diameter was under more nongenetic or environmental influence than was stalk height.

The average bark percentage of sunn hemp in this study

Acknowledgements

The authors thank R. Cantu, A. Bautista, and E. Rodriguez for their technical assistance, D. Peterson for statistical support, and G. Sakamoto and R. Joy for seeds.

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