Effect of  effluent from low-cost plastic film biodigester on yield and chemical composition of cassava foliage and tuber yield

Duong Nguyen Khang and Thomas R Preston^*

Department of Animal Physiology and Biochemistry,
Faculty of Animal Husbandry and Veterinary Medicine, Nong Lam University, Vietnam.
duongnguyenkhang@yahoo.com
^*University of Tropical Agriculture Foundation, UTA - TOSOLY - Finca Ecológica,
Morario - Guapota - AA # 48, Socorro, Santander, Santander del Sur, Colombia.
trpreston@mekarn.org

Abstract

A 4*2 randomized complete block design with 4 replications was conducted from June 2002 to March 2003 at the experimental farm of the Nong Lam University, Ho Chi Minh City, Vietnam, to determine effect of  effluent from low-cost plastic film biodigester on yield of foliage and root tubers, and chemical composition of the foliage of cassava. Foliage of cassava of the variety KM 94, grown in plots of 10 m x 20 m at a planting distance of 30 cm x 50 cm, was hand-harvested according to respective treatments, starting 100 days after planting. All tubers were harvested at the final harvest 280 days after planting.

Mean dry matter foliage yields were 5.36 and 4.33 tonnes per ha with or without the  effluent, respectively. The leaf dry matter proportion was high, ranging from 60 to 66%. The proportion of leaf increased and the stem decreased with application of the  effluent.The CP production was 39% higher with the  effluent. The ADF and NDF contents of foliage varied between 24.3 and 28.4%, and 35.3 and 37.6% of DM, respectively. The fresh tuber yield in the control treatment was 25.5 tonnes per ha. The mean fresh weight of root tubers was 7% higher with the  effluent. It is therefore concluded that the  effluent from the biodigester could be used as fertilizer for foliage production as well as tubers under these conditions.

Key words: biodigester, cassava, foliage,  effluent, tubers

Introduction

Economic evaluation studies have shown the importance of using the digested  effluent after the anaerobic digestion process, as well as the biogas. The economic importance of the digested  effluent is becoming more acceptable in recent years in the developing countries, and this concept is presented in many publications of China, India and other countries. The  effluent discharged from a digester contains 1 - 12% solids and consists of refractory organics, new cells formed during digestion, and ash. The  effluent can be used in its liquid or solid fractions, dried or as total  effluent. In most countries where biogas plants were constructed, the  effluent was used as a fertilizer. According to studies in Sichuan province of China in year 1979, the nutrient contents of the effluent increased yields by 6 - 10%, regardless of kinds of soil.  In long-term experiments, it was shown that the chemical and physical properties of the soil were improved markedly, after a few years of applying digester effluent, while total yields of severa1 crops were 11- 20% higher than controls.

Cassava (Manihot esculenta Crantz), a tropical root crop widely cultivated in Vietnam, has great potential as a starch source for both human and animal consumption. The cassava plant also produces a lush crop of leaves, which are rich in protein, minerals and vitamins (Oomen and Grubben 1978) and are regarded as a good protein source (Lancaster and Brooks 1983). The crude protein content in cassava foliage ranges from 19 to 23% of dry matter (Khang and Wiktorsson 2000; Man and Wiktorsson 2001, 2002; Arvidsson and Sandberg 2003). Up to the present time, most of the experimental work reported has been focused on the production potential of cassava root for human and animal food, and only a few published reports have focused on cassava foliage production as a protein feed for livestock, together with tuber production (Hong et al 2003; Tung et al 2001; Khang et al 2004). However, the effects of  effluent from biodigester on foliage and tuber yields and nutritive value of cassava foliage have not yet been fully investigated.

The objectives of this experiment were to determine whether a high tuber yielding variety, KM 94, could yield reasonable amounts of foliage as well as tubers under these conditions.
 

Materials and methods

Location, land and climate

The study was conducted from June 2002 to March 2003 at the experimental farm of Nong Lam University, Ho Chi Minh City, Vietnam. The soil contained 54% sand, 39% silt, 7% clay, 0.62% organic carbon, with 6.87, 76.21, 3.69, 1.13 and 0.13 meq per 100 g of N, P, K, Ca and Mg, respectively, and a pH _KCl of 5.79 at 15 cm depth (Soil Chemistry Lab. Data, Nong Lam University, Ho Chi Minh City, Vietnam, 2002).

The climate in the area is a tropical monsoon, with the rainy season between May and December and the dry season from February to April. The monthly mean temperatures ranged from about 27.5 ⁰C to 34 ⁰C with minima in December and February and maxima during the dry season. The mean relative humidity was 76% (Figure 1).

Figure 1. Monthly rainfall and temperature at the time of the experiment

The land was first cleared from weeds, and then ploughed by tractor to a depth of 20 - 25 cm to loosen the soil. Weeds were also removed twice during the establishment period.

Experimental design and treatments

Cassava was planted in monoculture for all the plots. The plots were each 10*20 m (200 m²) arranged in a randomized complete block design with 4 replications. Allocation of treatments is shown in Table 1. The experiment was set up in a field of 1940 m², of which 1600 m² was used for planting and 340 m² was border area.

Table 1: The layout of the experiment
Blocks	Treatments
1 2 3 4	C CS C CS	CS C CS C
C:      Cassava without  effluent; CS:    Cassava with  effluent

 

Establishment and management

Cassava, variety KM 94, was planted early June 2002 and the final harvest was 280 days after planting. Planting materials were chosen from healthy and disease-free plants and cut into segments just before planting. Cassava stem was planted in continuous rows with 50 cm between rows, 30 cm between stem cuttings, and grown as a pure stand. The length of stem was 20 - 25 cm. The planting depth was 15 cm. Only  effluent from a biodigester charged with cattle manure was applied to the CS plots at 5 tonnes of DM per ha. The same rate of  effluent was applied after each harvest in these plots. No other fertilizers were applied during the experimental period.

Harvesting and chemical analysis

The first harvest was made when the cassava plants reached 100 cm in height (about 100 days after planting). All the foliage was cut at 30 cm above the ground. The same pattern was followed for the re-growth at 60 days interval. Cassava foliage (comprising young stems, leaves and petioles) was hand-harvested. Cassava foliage was harvested between 08:00 h to 10:00 h to avoid HCN fluctuation, which occurs later during the day due to the hot sun (Yeoh and Oh 1979). Cassava tubers from all treatments were harvested at the final harvest. All cassava foliage and tubers from each plot were weighed to determine the fresh yield. The fresh foliage was sampled and pooled from the 3 replicates (1.5 kg fresh weight each), and was placed in a porous paper bag for dry matter determination and chemical analyses. A similar sample was collected to determine the ratio of leaf, petiole and stem to total foliage on a DM basis. The contents of crude protein (CP), ether extract (EE) and total ash in the samples were determined according to the procedure of AOAC (1990). The contents of neutral detergent fiber (NDF) and acid detergent fiber (ADF) were determined according to the procedure of Van Soest et al (1991). Total condensed tannin was determined by the butanol-HCl method (Terrill et al 1992). The HCN content was determined by the alkaline titration method (AOAC 1990).

Statistical analysis

Data were statistically analyzed using General Linear Model Procedures of Minitab Statistical Software version 13.31. When the F-test was significant (p < 0.05), the Tukey's tests for paired comparisons was used to compare means. The relationship between foliage yields or tuber yield, respectively, and cutting intervals were determined using the linear or quadratic responses in Fitted Line Plot procedure of Minitab 13.31.

Results

Effects of  effluent from low-cost plastic film biodigester on yield of foliage and tubers

Data on foliage and tuber yields of cassava for the whole period of 280 days after planting are summarized in Table 2.  Compared with the control treatment, there was an average increase of 24% in total DM foliage yield when the foliage was fertilized with the  effluent from the biodigester. The mean fresh tuber yield increased by 7% in total when the foliage was fertilized with the effuent.

Effects of  effluent from low-cost plastic film biodigester on fresh and dried proportions of foliage

Fresh and dried weight proportions of cassava foliage applied with or without  effluent are presented in Table 3.  Percent of leaf increased with the  effluent from biodigester.  The results were the same for both fresh and dried weight proportions of cassava foliage.

Effects of  effluent from low-cost plastic film biodigester on chemical composition of foliage

The analyses showed that CP content in cassava foliage varied from 18.6 to 20.7% of DM (Table 4).

The mean DM conten increased by 12% when the foliage was applied with  effluent at 5 tonnes of DM per ha.  The mean HCN content of fresh foliage increased, while the mean tannin and ash contents decreased with the  effluent  (Table 4). There were no differences in the content of ether extract.

Discussion

Dry matter yield of cassava foliage in the present study increased from 4.3 to 5.4 tonnes per ha when cassava foliage was fertilized with  effluent at 5 tonnes DM per ha. The foliage yields were slightly lower than the figures reported by Tung et al. (2001). They conducted an experiment in which three cassava varieties (MM 92, Black Twig and Local) were grown for foliage yield estimation. The results showed that dry foliage yields of MM 92, Black Twig and Local cut at 45 day cutting intervals and about 15 cm harvesting height over the 5 harvests were 5.9, 5.7 and 4.3 tonnes ha^-1, respectively. Thus, differences in DM foliage yield could be due to the differences in variety (Gomez and Valdivieso 1984; Simwambana et al 1992), fertilizer (Molina and El-Sharkawy 1995), age at first cutting and interval between cuttings (Lockard et al 1985; Simwambana et al 1992; Tung et al 2001; Hong et al 2003). Although there are no data from the present study on the effects of season on cassava foliage yield, DM yield was reduced in all the treatments, strongly in the control treatment during the last three months of the experimental period, most likely due to the onset of the dry season.

The mean dry leaf proportion of the foliage was high (63%) but with a wide range, from 60 to 66% (Table 3). The mean was higher than found in an earlier study by Meyrelles at al (1977) where the leaf proportion of cassava foliage on DM basis was almost 52% of shoot yield. The yield difference between varieties is obvious, in addition to environmental and treatment factors. In the present study, the dry leaf proportions were higher with applying the  effluent.

Crude protein content of cassava foliage ranged from 18.6 to 20.7% on DM basis in all treatments (Table 4). These results were similar to the figure of 22.8% reported by Khang and Wiktorsson (2000), and 18.8% reported by Man and Wiktorsson (2001). The results showed that CP content on DM basis increased from 18.6 to 20.7% with applying the  effluent from biodigester. Estimated protein yield in the present study ranged from 0.8 to 1.11 tonnes per ha with applying the  effluent from biodigester during the growing period of 280 days. This was lower than the levels of 1 to 1.5 tonnes per ha with five cuts at 45 day interval for three cassava varieties reported by Tung et al (2001).

Fibre components of cassava foliage varied from 35.3 to 37.6% of NDF and from 24.3 to 28.4% of ADF. Applying the  effluent decreased NDF and ADF contents. The levels of NDF and ADF were equal to those found by Arvidsson and Sandberg (2003), but lower than those reported by Man and Wiktorsson (2001, 2002). The differences were probably due to differences in cassava variety, study site and seasonal conditions.

Root yield was affected by the  effluent, with a increase in tuber yield of 7% compared to the control treatment without the  effluent until 280 days after planting. A similar, but less pronounced affect has been reported by Dahniya et al (1981) with a high yielding cassava tuber variety. However, they only picked the leaves from the top 30 cm of each branch at 1, 2, and 3 month intervals.

Conclusions

• The  effluent from biodigester strongly influenced the DM and CP yields of cassava foliage, the fresh weight of root tubers and to a less extent the nutritional quality of foliage produced during a growing season of 280 days.

• The foliage DM , fresh weight of root tubers and CP yields were higher when the foliage was applied with  effluent at 5 tonnes per ha.

• It is concluded that  effluent from the biodigester could be used effectively as fertilizer for foliage production as well as tubers under these conditions.

Acknowledgements

The authors are grateful to the Swedish International Development Cooperation Agency, Department for Research Cooperation (Sida/SAREC) for funding this study and Mr. Tuan and Mr. Thanh for their technical help.

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