Evaluation of Yield and Nutritive Value of Kenaf (Hibiscus cannabinus) at Various Stages of Maturity

MATERIALS AND METHODS

Kenaf variety Khon-Kaen 50 was planted at Mahasarakham University farm in Maha Sarakahm province located in northeastern Thailand, latitude 16°62’N, longitude 102^o45’E. The kenaf was planted in plots (10x12 m) with seed and row spacing of 30 and 50 cm, respectively. The soil was loamy fine sand and its pH was 6. Fertilizer, NPK (8:8:8), was applied shortly after seeding at 300 kg/ha. Eradication of weed in experimental plots was carried out weekly.

Kenaf was sampled at approximately 5 cm above ground level using a 1x1m quadrant (two replicates for each sampling) on 6, 8, 10 and 12 WAP. Plants within the quadrant were immediately weighed to determine fresh weight. Five plants were selected randomly from each quadrant and measured for plant heights, number of leaves and later, separated the leaves and stems to determine leaf to stem ratio.

Table 1:	Production characteristics of 6, 8, 10 and 12 week-old kenaf
a,b,c,d Superscripts that differ are significant at p<0.05

Table 2:	Chemical composition (%) of whole plant, leaf and stem of 6, 8, 10 and 12 week-old kenaf
a,b,c,dSuperscripts that differ are significant at p<0.05

The cumulative precipitation and the average temperature during the experimental period were 108.5 mm and 31.3^oC, respectively.

Whole plant, leaf and stem samples from each quadrant were chopped and dried at 60^oC to constant weight to determine DM. Dried samples were ground through a 1-mm screen and analyzed for the contents of Crude Protein (CP), ash (AOAC, 1990), Neutral Detergent Fiber (NDF) and Acid Detergent Fiber (ADF) (Robertson and Van Soest, 1981). Yield of DM per unit area of whole plant, leaf and stem fractions and CP yield per unit area of whole plant were estimated from fresh weight, DM and CP contents of each fraction.

Statistical analysis: Data were analyzed in Completely Randomize Design using the general linear model procedure (SAS, 1996) according the following statistical model: Y_ij = μ + A_i + e_ij, where A is effects of age of kenaf. The differences among means were compared by Duncan’s New Multiple Range Test (Steel and Torries, 1980).

RESULTS AND DISCUSSION

Fresh and DM yield of whole plant, leaf and stem of kenaf increased significantly (p<0.05) with increasing age of cutting (Table 1). These results are in agreement with Chantiratikul et al. (2006). However, the DM yield of 6-10 week-old kenaf (0.12-0.95 t/ha) in present study was dramatically lower than that of previous studies which reported DM yield of 6-9 week-old kenaf ranged from 2 (Phillips et al., 1999; Muir, 2002) to 5 t/ha (Killinger, 1969; Chantiratikul et al., 2006). The discrepancy among the different reports could be due to differences in plant variety, seeding rate and environment. The leaf-stem ratio decreased (p<0.05) when cutting age of kenaf increased (Table 1). Normally, as kenaf increased in maturity, the lower leaves senesced and resulted in decreasing proportion of leaves, but increasing proportion of stems (Webber, 1993).

Plant height and number of leaves increased (p<0.05) as maturity advanced (Table 1). Height of 6-8 week-old kenaf (33.22-47.87 cm) was shorter than that of the same age kenaf (93-159.5 cm) in foregoing study (Chantiratikul et al., 2006). Generally, rate of plant growth is mainly dependant on plant variety and other agronomic factors such as season of grown, quality of soil, fertilizer, temperature and rainfall (Rowell and Han, 1999). The number of leaves increased from 11/plant at week 6 - 22/plant at week 8 (Table 1). The result of number of leaves is consistent with previous study which reported that the number of leaves of various varieties of 6-8 weed-old kenaf varied from 10-26 leaves (Adamson et al., 1972). However, number of leaves of 6-8 week-old kenaf, variety Khon-Kaen 60, ranged from 79.2-144.4 leaves (Chantiratikul et al., 2006). The results indicated that variety of kenaf had significant impact on number of leaves and Khon-Kaen 60 variety is leafier than Khon-Kaen 50 variety.

CP content of whole plant, leaf and stem decreased with maturity (Table 2), with CP content in leaf (13.93 -19.37%) was higher than that in stem (4.78-9.02%). The similar results were found in previous studies (Swingle et al., 1978; Webber, 1993; Phillips et al., 1999, Chantiratikul et al., 2006). The reduced CP content in the stem as plant matured was mainly due to rapid accumulation of fibrous components (Table 2). Similar findings were reported previously (Suriyajantratong et al., 1973; Phillips et al., 1999; Wong and Vijasegaran, 2001). Maturity has a large influence on the CP content of kenaf and its reduction may be explained by two factors; I) fodder mature increase proportionally in the stem and ii) CP content fall in all fractions with maturity advanced (Minson, 1990).

Neutral detergent fiber and ADF of whole plant, leaf and stem increased (p<0.05) with increasing maturity stage (Table 2). The obtained fibrous contents of whole plant were similar to those in foregoing studies (Swingle et al., 1978; Phillips et al., 1996; 1999). In the those studies, NDF was reported to be between 28.6-42.9% while ADF varied from 23.6-32.6% (at between 9-12 WAP). Normally, fiber content increased with plant maturity but environmental factors, particularly rainfall and temperature play an important role in determining this component of plant (Muir, 2002; Minson, 1990).

Although proportionally CP content of each plant part decreased as harvesting stage, CP yield of kenaf increased (p<0.05) substantially. This is because of the progressive increased yield of the plant (Table 1). However, CP yield (0.02-0.14 t/ha) in this study was significantly lower than that of other researches. Chantiratikul et al. (2006) reported that CP yield of 8 week-old kenaf was approximately 1 t/ha. Webber (1993) revealed that 11 and 14 week-old kenaf produced 0.39 and 0.449 t CP/ha, respectively.

The objective of this study was to evaluate the quality of kenaf as fodder for ruminants, the current results indicated that immature kenaf Khon-Kaen 50 variety produced lower yield and CP content than Khon-Kaen 60 variety (Najid and Ismawaty, 2001; Chantiratikul et al., 2006). However, 10 week-old kenaf Khon-Kaen-50 variety with potential DM yield of 0.95 t/ha and contained 10% of CP could be used as supplemental fodder for ruminants when the shortage of high quality fodder occurs. Additionally, further studies are needed to determine in situ nutrient digestibility of kenaf at the different cutting ages.

Conclusion: Dry matter yield and CP yield of 6-12 week-old kenaf were 0.12-2.14 and 0.02-0.14 t/ha, respectively (p<0.05). Crude protein (14.34-6.58%) content of kenaf decreased significantly (p<0.05) as harvesting age advanced. On the other hand, NDF (41.99-48.74%) and ADF (27.20-30.57%) of kenaf increased statistically (p<0.05) with maturity advanced.

ACKNOWLEDGEMENT

The research project was financially supported by Mahasarakham University.