Intercropping cassava (Manihot esculenta Crantz) with Flemingia (Flemingia macrophylla); effect on biomass yield and soil fertility

Ngo Tien Dung, Inger Ledin^* and Nguyen Thi Mui^**

Goat and Rabbit Research Center, Sontay, Hatay, Vietnam
dzunggrrc@mekarn.org
^*Department of Animal Nutrition and Management, Swedish University of Agricultural Sciences
**National Institute of Animal Husbandry, Hanoi, Vietnam

Abstract

An experiment was carried out at the Goat and Rabbit Research Center, Sontay, Hatay, Vietnam, from February 2001 to November 2002 to study the effect of intercropping cassava and Flemingia on biomass yield and soil characteristics.

The yield of Flemingia in monoculture was 5.08 tonnes dry matter (DM)/ha in the first year and yields increased in the second year to 14.7 tonnes DM/ha. The yields of cassava in monoculture were 13.9 (roots) and 10.1 (foliage) tonnes DM/ha, respectively, in the first year, and yields decreased in the second year to 11.5 and 9.63 tonnes DM/ha, respectively. Intercropping with Flemingia reduced cassava yields by 13.8% (roots) and 4.1% (foliage), respectively, in the first year, but increased them by 40.6% and 30.6%, respectively, in the second year. The highest accumulated DM yield during two years was obtained when cassava for root production was intercropped with Flemingia (27.0 tonnes/ha). The lowest DM yield was obtained when Flemingia or cassava for foliage were planted in monoculture. The highest crude protein (CP) yield was obtained with cassava foliage intercropped with Flemingia (3.44 tonnes/ha) and lowest when cassava for root was planted in monoculture. Intercropping with Flemingia generally increased biomass yield (6.3% and 19.2% of DM yield, 86.4% and 20.7% of CP yield of cassava for root and cassava for foliage, respectively) compared to cassava planted in monoculture.

Cassava and Flemingia intercropping systems increased biomass yield and maintained soil fertility with decreased soil erosion.

Keywords: Bio-test, Cassava (Manihot esculenta Crantz), Flemingia (Flemingia macrophylla), foliage, intercropping, root, soil fertility

Introduction

Cassava or tapioca (Manihot esculenta, Crantz) is an annual tuber crop grown widely in tropical and sub-tropical areas. It thrives in sandy loam soil with low organic matter and can tolerate low rainfall and high temperatures. It is therefore a cash crop cultivated by smallholder farmers within the existing farming systems in many countries. Cassava can also be cultivated in combined forage/root systems with two or more harvests of the foliage prior to letting the root develop to maturity (Wanapat et al 1997). More recently, efforts have been concentrated on managing cassava as a perennial forage crop with repeated harvesting at 2 to 3 months intervals (Preston et al 2000; San Thy and Preston 2000). In this system, the roots are not harvested, but serve as a nutrient reserve to support the forage re-growth.

In Vietnam, cassava is an important crop, the second after rice, with about 350,000 ha/year planted (Khang et al 1999). Beside the roots, each hectare of cassava can produce a large amount of leaves. The potential yield of cassava leaves as by-products at root harvesting may amount to as much as 4.64 tonnes dry matter (DM)/ha/year (Ravindran and Rajaguru 1988). If cassava is grown for foliage in the dry season, it can give 41 tonnes/ha of fresh leaves, equivalent to about 12 tonnes/ha of cassava hay (Wanapat et al 1997).

After several years of planting cassava in monoculture the soil will be eroded and the fertility of the soil will be affected negatively (Polthanee et al 2001; Ngo Tien Dung 2003, unpublished data). One possibility to reduce the negative effect of growing cassava in monoculture would be to intercrop cassava with a legume. Flemingia macrophylla is a leguminous tree that grows well in acid soils. Flemingia can improve soil fertility (Dinh Van Binh et al 1998; Andersson 2002) and supply wood for fuel (Dinh Van Binh et al 1998). The foliage of Flemingia is qjuite high in crude protein (17 to 18%) but it can only replace about 15% of the concentrate in the diet of growing goats because of low palatability and low digestibility (Dinh Van Binh et al 1998; Mui et al 2002).

The objective of the present study was to compare biomass yield, soil fertility and soil erosion in systems with cassava or Flemingia grown in monoculture or in an intercropping system. The hypotheses to be tested were:

Biomass yield of cassava will be increased when intercropped with Flemingia
Growing cassava in association with a legume tree such as Flemingia will have beneficial effects on soil fertility.

Material and methods

Location and climate

The experiment was carried out from February 2001 to November 2002 on sloping land (17.5±0.51%) at 105^o25' E and 21^o06' N, 220 m above sea level in Bavi region, Hatay province, North Vietnam. The soil characteristics at the experimental site were pH_KCl: 4.12; OM: 4.48; N_total: 0.16%; K₂O_total: 0.46%; P₂O_{5 total}: 0.039%. In general the soil is acid with low fertility and the OM has been reduced to very low levels by erosion. Drought occurs frequently in the area. The climate in the area is monsoon-tropical, with a wet season between April and November and a dry season from December to March. Average annual rainfall was 2371 mm in 2001 and 1409 mm in 2002, with a mean daily temperature from 19 to 30^oC (Figure 1).

Figure 1. Climate data from Bavi station (Mean annual air temperature in year 2001: 23.3^oC;
year 2002: 23.7^oC. Mean annual rainfall in year 2001: 2371 mm; year 2002: 1409 mm)

Treatments and experimental design

The five treatments were:

CR: Cassava for producing roots
CR-FM: Cassava for producing roots intercropped with Flemingia
CF: Cassava for foliage
CF-FM: Cassava for foliage intercropped with Flemingia.

The experimental plots were each 10*20 m (200 m²) arranged as a randomized complete block design with 4 replications (Figure 2).

Figure 2: Layout of the experimental plots
Block 1	CR	FM	CR-FM	CF-FM	CF
Block 2	CF	CF-FM	CR	CR-FM	FM
Block 3	FM	CR-FM	CR	CF-FM	CF
Block 4	CF-FM	FM	CF	CR	CR-FM

Establishment and management

The experiment was established in February 2001. The cassava was planted with 50 cm between rows, 75 cm between stem cuttings for producing roots and 15 cm for foliage. Lengths of stem cuttings were 20 to 25 cm. For the treatment of cassava and Flemingia, the cassava was planted at 50 cm between rows of Flemingia (5 cm spacing between seeds). The cassava was planted in two rows intermixed with two rows of Flemingia. The pure stand of Flemingia was planted with 50 cm between rows (5 cm spacing between seeds). Flemingia was planted first and cassava 14 days later.

Fertilizer was applied in the form of organic manure (a mix of pig and cattle manure) before planting at the rate of 2 kg/m² fresh weight in the first year and the same amount again in March the second year. No other fertilizers were applied during the experimental period.

Harvesting

The first harvest was made when the cassava reached 100 cm in height (about 3 months after planting). All the foliage was removed at 30 cm above ground level. The same procedures were followed for the re-growth (from 56 to 75 days). In the intercropped plot and in pure stand Flemingia was harvested at 30 cm above ground level at the same time as the cassava.

In each plot, five randomised sub-plots (2 * 2 m = 4m²) were chosen to estimate edible yield, which was calculated as an average of the 5 sub-plots. The edible fraction consisted of the leaves and the part of the stem that had leaves. After sampling, all the foliage was removed and separated into edible and non-edible fractions. The biomass yield was weighed at each harvest and in the plots with cassava planted intercropped with Flemingia, biomass yield for each species was weighed separately. In the plots with cassava for production of roots, roots and foliage (by-product) were harvested at the end of the year and the plots were planted again in the spring season.

Samples were taken from the edible fraction of the foliage of both cassava and Flemingia at each harvest and at the end of the year, and samples of the root and foliage as by-product were taken in the plots with cassava for root production. The samples from 5 sub-plots were mixed and 1 pooled sample was taken for plots with only cassava or Flemingia, and 2 samples for plots with both cassava and Flemingia.

Soil sampling and biological test of soil fertility

Soil samples for the biological test (from 0 to 20 cm depth) were taken randomly from each experimental plot at the beginning and at 6 month intervals. Equal amounts of soil (3 kg) were put into plastic bags for a biological test of overall soil fertility (Mui et al 2000; Boonchan Chantaprasarn and Preston 2004). Four seeds of maize were planted and after 5 weeks the maize plants were removed from the soil, washed to remove soil from the roots and allowed to dry for 1 hour. Total fresh biomass and the maize roots were weighed.

Soil samples for analysis (from 0 to 20 cm depth) were taken as a mix of 5 randomised sub-samples in the plot at the beginning and the end of the experiment. The 5 sub-samples were pooled to 1 sample/replication giving 4 samples per treatment.

Soil loss estimation

At the lower part of the plot, a canal system was established following the slope in each plot. The canals were 60 cm deep, 60 cm wide and 10 m long, and were covered by plastic to catch the soil that was washed away. The soil loss was weighed after each rainfall according to the method proposed by Hudson (1981).

Chemical analysis

Samples of cassava foliage, Flemingia foliage and cassava roots were analysed for DM, crude protein, ash and acid detergent fibre (ADF), and starch plus sugar in cassava roots, were determined according to AOAC (1990). Neutral detergent fibre (NDF) was determined by the method of Van Soest (1991) using sodium sulphite and amylase and was expressed with residual ash. The cassava and Flemingia foliages were analysed for condensed tannins according to Burns (1971).

Soil samples were analysed for pH_KCl (KCl 0.1M), OM (Walkley-Black), N (Kjeldahl), P (Spectrophotometer) and K (flamephotometer) according to AOAC (1990).

For the calculations of yields of digestible DM and CP, 40.3% and 41.8% of digestible DM and CP, respectively, were used for Flemingia foliage (Mui 2001), 60% and 62.2%, respectively for cassava foliage (Yangklang et al 2001) and 96% DM digestibility for cassava starch (Therdchai and Choke 2001).

Statistical analysis

The data were subjected to Analysis of Variance using the General Linear Model procedure of Minitab Software (Minitab 2000). The treatment means which showed significant differences at P<0.05 were compared using Tukey's pair-wise comparison procedures.

The statistical model used in the analysis was:

Y_ij = µ + T_i +B_j + (TB)_ij + e_ij Where: Y_ij : Biomass yield; µ: Overall mean; Ti: treatment (I= 1,...5), B_j: block (j = 1; …4); (TB)_ij = Interaction between treatment and block; e_ij is the experimental error.

Results

Chemical composition of foliages and cassava root

Cassava root had a very low CP content of only 2.8% in DM and low values of ash, NDF and ADF but contained high levels of total starch plus sugar (90.9% of the DM) (Table 1). The CP of the cassava foliage (16.9% in DM) was lower than in most of the literature reports (24 to 35% in DM for a range of varieties in South Vietgnam, according to Bui Huy Nhu Phuc et al 2001). The foliages of Flemingia and cassava had moderate contents of condensed tannins, 5.1% and 3.9% of DM, respectively.

Table 1. Chemical composition of foliages and cassava root (means and SE)
	Flemingia foliage	Cassava foliage	Cassava roots
Number of samples	6	8	6
DM, g/kg	244±3.3	203±4.3	397±4.6
Composition of DM (g/kg)
Crude protein	152±2.3	169±3.1	28±3.0
Ash	46±0.8	64±1.1	21±1.2
NDF	630±7.6	487±9.9	20±1.1
ADF	510±6.7	371±8.8	9±3.9
Condensed tannins	51±4.4	39±9
Sugar+starch			909±1.7

Biomass yield

In the first year, biomass yield of Flemingia was low but increased in the second year. In contrast, the yield of cassava decreased in the second year in the monoculture plots. Intercropping with Flemingia reduced yields by 4.1% and 13.8% of cassava for foliage and for root production in the first year, but increased them by 30.6% and 40.6% in the second year, respectively.

Table 2. Effect of treatment on biomass yield (tonnes/ha)
	CR	CR-FM	FM	CF	CF-FM	SEm
Year 2001
DM, tonnes/ha
Flemingia foliage		1.53	5.08	-	2.92
Cassava foliage	1.2	0.71	-	10.09	6.75	0.1
Cassava roots	12.74	9.76	-		-
Total	13.94a	12.01b	5.08d	10.09c	9.67c	0.21
Crude protein, tonnes/ha
Flemingia foliage	-	0.26	0.71	-	0.42	0.01
Cassava foliage	0.19	0.11	-	1.65	1.19	0.17
Cassava roots	0.36	0.26	-
Total	0.56d	0.64c	0.71b	1.65a	1.61a	0.02
Year 2002
DM, tonnes/ha
Flemingia foliage	-	6.01	14.73	-	7.58	0.15
Cassava foliage	1.02	0.69	-	6.81	4.32	0.59
Cassava roots	10.46	8.3	-	2.81	1.93	0.17
Total	11.49c	15.01a	14.73ab	9.63d	13.83b	0.25
Crude protein, tonnes/ha
Flemingia foliage	-	0.9	2.18	-	1.06	0.02
Cassava foliage	0.17	0.11	-	1.14	0.72	0.01
Cassava roots	0.29	0.26	-	0.06	0.04
Total	0.47e	1.28c	2.18a	1.20d	1.83b	0.03
			Two years
DM, tonnes/ha	25.44b	27.04a	19.81d	19.72d	23.50c	0.353
CP, tonnes/ha	1.03d	1.92c	2.90b	2.85b	3.44a	0.036
abcde Means within rows without common superscripts differ at P<0.05 CR: Cassava for producing roots, CR-FM: Cassava for producing roots intercropped with Flemingia; FM: Flemingia, CF: Cassava for foliage, CF-FM: Cassava for foliage intercropped with Flemingia

Intercropping led to an increase in yield of DM and CP. The highest DM yield was obtained when cassava for root production was intercropped with Flemingia (27.0 tonnes/ha for two years). Lowest DM yield was obtained in Flemingia and cassava for foliage planted in monoculture. The highest CP yields were obtained with cassava foliage intercropped with Flemingia (3.44 tonnes/ha for two years) and lowest when cassava for root was planted in monoculture.

Intercropping cassava and Flemingia also led to increases in yields of DM and digestible CP. The low digestibility of DM of Flemingia and cassava foliage led to low digestible DM yield in the plots with Flemingia in monoculture and cassava for foliage intercropped with Flemingia, at 7.99 and 12.56 tonnes/ha/2 years, respectively. The low CP content in cassava root resulted in low yields of CP and digestible CP.

Figure 3. Effect of treatment on dry matter and digestible dry matter yield during 2 years

Figure 4. Effect of treatment on yield of crude protein and digestible crude protein during 2 years

Soil fertility and soil erosion

The results of the biological test of soil fertility (Table 3) showed that biomass yield of maize was highest in soil from the Flemingia plot after 18 months of planting. There was no effect of intercropping on the growth and yield of the maize in the first year, but there was a significant effect on biomass yield of maize in the second year, especially in the plots that were planted with Flemingia or cassava intercropped with Flemingia.

Table 3. Fresh weight of root and green biomass of maize plants grown in soil from the experimental plots
	CR	CR-FM	FM	CF	CF-FM	SEM
Initial	37.5	35.5	36.4	34.9	36.2	2.22
6 months	31.4	34.3	34.3	33.4	34.1	1.51
12 months	31.7b	35.7a	37.9a	30.2b	34.8b	1.32
18 months	30.6c	38.2a	41.4a	30.9c	36.2b	1.56

The soil samples showed that, during the two years of the experiment, there was a decrease in OM and N in the plots with cassava for roots and cassava for foliage. There was no difference in OM and N content of the soil during the same period for the plots with Flemingia in monoculture or cassava intercropped with Flemingia. pH was not affected by the cropping system, neither was the content of P₂O₅ or K₂O.

In the first year, there was a low growth of Flemingia leading to high soil erosion (57.8 tonnes/ha/year) (Table 4 and Figure 5). In the second year soil loss was lowest in the plot with Flemingia in monoculture and highest in the plot planted with cassava for root production in monoculture. Intercropping cassava with Flemingia had no effect on soil loss in the first year but had a significant effect on soil loss in the second year, with decreasing soil loss when Flemingia showed higher biomass yields, resulting in a good cover of the soil.

Table 4. Soil characteristics and soil erosion
	CR	CR-FM	FM	CF	CF-FM	SEM
Initial
pH-KCl	4.12	4.15	4.08	4.11	4.15	0.047
OM, %	4.39	4.09	4.44	4.68	4.78	0.291
N total, %	0.155	0.175	0.133	0.162	0.165	0.016
P₂O₅, %	0.036	0.039	0.04	0.039	0.041	0.0064
K₂O, %	0.402	0.47	0.48	0.475	0.455	0.4212
After 2 years
pH-KCl	4.15	4.17	4.17	4.13	4.14	0.018
OM, %	4.02	4.37	4.35	3.73	4.41	0.167
N total, %	0.131	0.182	0.132	0.125	0.15	0.012
P₂O₅, %	0.073	0.086	0.1	0.077	0.099	0.0591
K₂O, %	0.277	0.29	0.307	0.268	0.285	0.034
Initial compared to after two years
pH-KCl	NS	NS	NS	NS	NS
OM, %	*	NS	NS	*	NS
N total, %	*	NS	NS	*	NS
P₂O₅, %	***	**	**	*	**
K₂O, %	**	*	*	**	**
Slope (%)	17.7	17.2	17.2	17.7	17.5	0.51
Soil erosion (tonnes/ha)
Year 1	51.0b	53.07b	57.8a	39.2c	49.2b	8.11
Year 2	58.3a	26.3c	10.6d	37.1b	18.8c	4.1
2 years	109.3a	80.3b	68.4c	76.3b	68.0c	7.52
abcde Means within rows without common superscripts differ at P<0.05 NS: Non significant difference; : P<0.05, : P<0.01, **: P< 0.001

Figure 5: Effect of treatments on losses of soil by erosion

Discussion

The marked increase in biomass yield from Flemingia in monoculture in the second compared with the first year agrees with the findings of Nguyen Phuc Tien et al (2003) that DM yields were 4.68 and 11.2 tonnes DM/ha/year in the first and second year, respectively. Dinh Van Binh et al (1998) reported yields of Flemingia foliage of 3.9, 13.4, 9.8 and 8.9 tonnes DM/ha in the first, second, third and fourth year, respectively.By contrast, cassava grown in monoculture as a perennial foliage produced similar yields in the first and second year. This is in agreement with the report of Preston (2001) that, in both Vietnam and Cambodia, foliage yields from cassava grown in monoculture, and harvested at 2-3 month intervals, did not differ between the first and second year.

The important findings in the present study relate to the overall two year period in which there were benefits in the biomass yield, in soil fertility and in reduced soil erosion, when cassava was inter-cropped with Flemingia. The benefits were equally apparent when the cassava was managed principally for root or for foliage production. Similar findings were reported by Dung (2002) who showed that the root yield of cassava intercropped with two and three rows of peanut compared to cassava in monoculture increased by 11.6% to 20%. Nguyen Phuc Tien et al (2003) also reported that the DM and CP yields of cassava intercropped with Flemingia over 2 years were higher than from cassava in monoculture. Intercropping cassava with cowpea resulted in 20% to 100% greater land use efficiency than for either crop grown alone according to Leihner (1983).

Research on cassava intercropping is relatively more recent than on other aspects of cassava production. Intercropping systems often result in better land use efficiencies than do sole cropping systems and are usually associated with greater production of total DM and CP (Natarajan et al 1980a, 1980b; Sivakumar and Vermani 1980). Intercropping is often considered to offer greater yield stability than monoculture cropping (Baker 1980; Rao and Willey 1980; Rao and Morgado 1984). Polthanee et al. (2001) reported that for subsistence farmers, greater stability in food production in intercropping systems is particularly meaningful, since this characteristic of the production system tends to better insure their sustainability and substantially reduces the risk of total crop loss. According to Nguyen Phuc Tien et al (2003), after two years of cultivation the OM and total N in the soil were maintained when cassava and mulberry were intercropped with Flemingia. The beneficial effects of Flemingia on soil fertility in part may be due to nutrient recycling as Dinh Van Binh et al (1998) observed in a monoculture of Flemingia that every year the surface of the soil was covered with dead leaves ranging from 0.28 to 1.88 tonnes DM/ha.

According to FAO (2000) the soil cover is an important factor in control of erosion by water through interception and absorbing the kinetic energy in the rain. The soil cover prevents the direct impact of the raindrops and hinders the surface from becoming sealed and preserves the soil structure, as well as reducing the run-off velocity and transport capacity of the surface flow. Andersson (2002) reported that the soil loss ratio depends on soil structure and management conditions mainly the contour tillage and type of crop. In the case of cassava in monocujlture, he showed about 34 tonnes/ha of soil loss during the year 2001 while the plots with Flemingia in year 2 lost only 5.4 tonnes/ha. The cassava cultivation caused several times more soil loss than Flemingia, even at 10% steepness of the slope.

Conclusions

Compared with monoculture of cassava or Flemingia, intercropping them resulted in:

Higher and more stable yields of dry matter and crude protein
Improved soil fertility and decreased soil erosion

References

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