Kenaf (Hibiscus cannabinus L.), a warm-season annual, was first identified as a potential jute substitute in the manufacturing of rope, twine, carpet backing, and burlap (Wilson et al. 1965). Research demonstrated that kenaf is also a very promising fiber source for production of paper pulp (Nieschlag et al. 1960; White et al. 1970). Kenaf fibers were processed into both newsprint and bond paper (Clark et al. 1971; Bagby et al. 1979). Other potential commercial uses for kenaf are being investigated, such as animal feed (Webber 1993), poultry litter (Tilmon et al. 1988), bulking agent for sewage sludge (Webber 1992), and for oil absorption (Tiller et al. 1994).
As with other crops proper fertility maintenance, especially for supplemental nitrogen application, is needed to optimize kenaf yields, and minimize production cost. Reports so far are inconsistent relative to the effects of N on kenaf stalk yields (White and Higgins 1965); researchers in Georgia have reported both positive (Adamson et al. 1979) and no benefits (Massey 1974). Studies in Florida demonstrated that the positive response to N applications on stalk yields were dependent on soil type (Joyner et al. 1965), where kenaf grown on a sandy soil responded to N and did not respond to N on a peat soil. Bhangoo et al. (1986) in California, and Sij and Turner (1988) in Texas, increased stalk yields with the addition of N to soils with low available nitrogen. Stalk yields in Missouri (Ching and Webber 1993) on a silty clay soil and in Nebraska (Williams 1966) on a silty clay loam soil did not benefit from N applications. Stalk yields have also responded differently to N at the same location and soil between years (Hovermale 1993). In this study the effect of five nitrogen application rates (0, 56, 112, 168, and 224 kg/ha) on kenaf yield components was investigated in order to increase available information in kenaf products.
Kenaf plots were hand-harvested on 21 Oct. 1989, 172 days after planting (DAP), and on 19 Oct. 1990, 149 DAP. A 2.25 m2 (1.50 m by 1.50 m) quadrate was harvested from the center rows. Plant population and moisture content were also determined in the harvest area. The harvested plants were cut at ground level and fresh weights were determined. Three plants were randomly selected at 105 DAP and at harvest to determine plant heights. The harvested plants used to determine heights were also measured with calipers to determine stalk diameters at 1 m above ground level.
Leaves, flowers, and flower buds were removed from the stalks and weighed separately before and after the samples were oven dried at 66°C for 48 h. Fresh and oven-dry weights of the three plants were used to determine the percent moisture of the plants and the percent stalks by weight. The plant moisture content and percent stalks were used to convert kenaf fresh weight to dry weight. Stalk yields were based on oven-dry weights. Precipitation was measured at the Lane, OK, research location.
In both years, treatments were arranged in a randomized complete block design with four replications. When the F-test indicated statistical significance at the P = 0.05 level, the least significant difference (LSD) test was used to separate means. In addition, coefficient of linear correlation (r) analysis was used to establish the correlation between various yield components. Since no significant year by treatment by cultivar interactions were indicated results are reported averaged across years.
Plant heights, plant populations, and stalk biomass percentages for the N application levels were not different from the control (Table 1). Stalk yield and diameter were affected by N application rates. Stalk yield tended to increase as N applications rates increased up to 168 kg N/ha, and at 224 kg N/ha, a significant reduction in stalk yield occurred compared to the 168 kg N/ha level. Thus, excess N application can be detrimental to stalk yield. Stalk diameter at 1 m was increased at the 112, 168, and 224 kg/ha N rates compared to the control (Table 1). Massey (1974) and Hovermale (1993) also reported that N significantly increased stalk diameter without increasing stalk yield.
Regressional analysis showed that the only consistent difference between the r values between rates was for the stalk biomass percentage (Table 2). As N application rates increased the correlation between stalk yields and stalk biomass percentages decreased (Table 2). At the 0 N rate, yields were significantly correlated, (r = 0.63) at P > 0.01 level, whereas at the 56 kg N/ha and 112 kg N/ha rate the positive correlation was at the P > 0.05 level. At the 168 kg N/ha and 224 kg N/ha rates there was no significant correlation between stalk yields and stalk biomass percentages.
Plant height, and stalk diameter were greater for 'Tainung 1' than 'Everglades 41', but no differences were detected between cultivars for plant population, stalk biomass percentage or stalk yield. 'Tainung 1' plants were 20 cm taller than 'Everglades 41' at 105 DAP and this trend continued until the final harvest when 'Tainung 1' was 23 cm taller than 'Everglades 41' (Table 1). The height advantage for 'Tainung 1' was achieve prior to 105 DAP and was maintained until the end of the growing season. These results were consistent with research in Missouri, Oklahoma, and Mississippi (Ching et al. 1993).
All yield components, except for plant population, were greater in 1989 than 1990 (Table 1). Plants in 1989 were 52 cm taller at 105 DAP, and 50 cm greater at harvest than in 1990. Stalk diameter was the only plant parameter significantly affected by N rates, cultivars and years, whereas stalk biomass percentage was only affected by years. Stalk yield was more than twice as great in 1989 (22.5 t/ha) than to 1990 (10.6 t/ha) (Table 1). Greater plant height, stalk diameter, stalk percentage and yield in 1989 may have resulted in part from an earlier planting date in 1989 (21 days) and greater early seasonal precipitation.
|Variable||Early||Harvest||Plant population (1000 plants/ha)||Stalk diameters (mm)||Stalk biomass (%)||Stalk yield (t/ha)|
|Coefficients of correlation (r)|
|N (kg/ha)||Plant ht.||Plant population||Stalk diam.||Stalk (%)|