Improving biomass productivity of plants and soil fertility on sloping land  in Bavi mountainous area

 

 Nguyen Thi Mui, Nguyen Phuc Tien, Ngo Tien Dzung,

 Dinh Van Binh and T R Preston*

 

Goat and Rabbit Research Centre, Son Tay, Vietnam
binhbavi@netnam.vn
* University of Tropical Agriculture Foundation, Royal University of Agriculture,
Phnom Penh, Cambodia
trpreston@uta.edu.kh

 

Abstract

This study aimed to evaluate contrasting farming systems of perennial grass (Guinea grass – Panicum maximum) and mixed crops of sugarcane and multi-purpose trees (Trichanthera gigantea and Flemingia macrophylla) in terms of biomass yield which can be used as animal feed. Comparisons were made of two cropping systems on the sloping land with a total area of 10,000 m² in the station and 5000 m² on two farms. The treatments were guinea grass (Panicum maximum) (Pure grass system) and alternate strips planted with Flemingia macrophylla, sugar cane and Trichanthera gigantea (Mixed species system). The allocation of the treatments was:

·        Mixed species: Flemingia (contour 1), sugar cane (contour 2) and Trichanthera (contour 3)

·        Pure grass system: Guinea grass on each of the three contours

Species plots (contours) were evenly spaced within each of three blocks which were used as replications.

 

In the on-station experiment, after a 20 month growing period, the tendency was for a greater increase in soil carbon in the upper contour and improved nutrient status of the soil in the mixed system. The pure grass system gave higher yields of protein in the first 10 months (47% higher) and in the second 10 months (28% higher) compared to the mixed species system but the latter produced higher yields of dry matter and digestible dry matter (by 21 and 19%, respectively). In the second 10 months, dry matter yield and digestible dry matter yield of Flemingia and sugar cane were higher than for the pure grass system on the same contour by 29% and 31%, respectively. In the on-farm research, total dry matter and digestible dry matter yields/ha/year were higher by 71% in the mixed than in the pure grass system
 

Key words: Grass, mixed cropping, Guinea grass, Flemingia, Sugar cane, Trichanthera , biomass productivity, soil fertility, goats, growth, digestibility

 

Introduction

Cultivation of grasses has been given priority in research institutions and state farms in North Vietnam as feed for livestock. Many grass species have been studied but the most popular has proved to be Guinea grass (Panicum maximum) because of its tolerance to drought and high productivity with fertilization of from 70 to 100 tonnes fresh biomass/ha/year.

Sugar cane is a perennial crop which can be used for animal feed as well as sugar production (Preston and Murgueitio 1994). It has a high leaf area index and higher photosynthetic efficiency under strong sunshine than any other crop in the tropics (Bassham 1988). The individual and combined effect of certain management practices can have a great impact on the growth and yield of sugarcane. Decreasing the row spacing and returning the dead leaves to the soil have been shown to increase biomass yields by up 20-30% (Nguyen Thi Mui et al 1996, 1997a,b).

Trichanthera gigantea is a tree that is native to the Andean foothills in Colombia and Venezuela. It is not a legume but its vigorous regrowth even with repeated cutting and without fertilizer applications indicates that nitrogen fixation by Mycorrhiza or other organisms may take place in the root zone. The advantage of this tree is that the leaves are consumed readily by pigs, rabbits and chickens. Trichanthera gigantea has adapted well to different ecological zones in Vietnam.  The leaves are rich in protein and are of high digestibility (Keir et al 1977a,b). It grows better under partial shade than in full sunlight (Nguyen Thi Hong Nhan et al 1996).

Flemingia macrophylla is a leguminous shrub with high biomass yields that grows well on acid soil. It is reasonably tolerant to drought and has given good results when mixed with foliage of Trichanthera  gigantea as the protein source for lactating goats (Le Diep Long Bien 1998). It appears to have potential to improve soil fertility and provide protection against erosion.

 

The present study was a first attempt to evaluate contrasting systems of perennial grass (Guinea grass) and mixed crops of sugarcane and multi-purpose trees (Trichanthera gigantea and Flemingia macrophylla) in terms of biomass yield and effects on soil fertility.
 

Experiment 1: Biomass productivity of plants and soil fertility of forage crops in association with sugar cane on sloping land

Materials and Methods

Location

The research was done at the Goat and Rabbit Research Centre (GRRC) and on two nearby farms in Hatay province in North Vietnam.

Treatments

Comparisons were made of two cropping systems:

·        Guinea grass (Panicum maximum )

·        Alternate strips planted with Flemingia macrophylla, Sugar cane and Trichanthera gigantea.

 

The allocation of the treatments, which were arranged in three blocks (replicates), is shown in Table 1. At GRRC, each block was 30m wide along the contour and 60m from the top to the bottom edge. Treatment plots were evenly spaced within blocks (see Table 1).

 

Table 1: Location of experimental treatments
Top of slope
	Block 1		Block 2		Block 3
	Pure grass	Mixed species	Pure grass	Mixed species	Pure grass	Mixed species
I	Guinea grass 1	Flemingia macrophylla	Guinea grass 1	Flemingia macrophylla	Guinea grass 1	Flemingia macrophylla
II	Guinea grass 2	Sugarcane	Guinea grass 2	Sugarcane	Guinea grass  2	Sugarcane
III	Guinea grass 3	Trichanthera  gigantea	Guinea grass 3	Trichanthera  gigantea	Guinea grass 3	Trichanthera  gigantea
Bottom of slope

 

On the farms the topography was the same but the plot sizes were smaller. On farm 1, each of the three blocks was 20m wide along the contour and 45m from the top to the bottom edge; on farm 2 the respective distances were 25 and 30m.

Plant establishment

The Guinea grass was planted from cuttings in single rows 50 cm apart. Flemingia was planted by seed in single rows separated at 50 cm distance and with 5cm spacing within the row. Sugar cane (Bavi local variety) was planted with stem cuttings in double lines at a row distance of 100 cm. Trichanthera was planted with single rows of stem cuttings with 50 cm between rows and between plants.

Fertilization

The soil was prepared uniformly by ploughing and harrowing and cattle manure was applied evenly to all plots at the rate of 20 tonnes/ha.

Measurements on-station

Biomass

Biomass yields were made over a total period of 20 months. The Guinea grass was harvested the first time two months after planting and subsequently at intervals of approximately 35 days for 6-8 harvests in each year. The sugar cane was harvested the first time 10 months after planting and the second time at 20 months. The biomass was separated into green leaves, tops (the growing point) and stalk. Dead leaves were not recorded. Flemingia and Trichanthera were harvested the first time 5 months after planting and subsequently at 65 to 80 day intervals according to season. Five biomass measurements were made on each treatment / block on an area of 5m². The biomass was weighed using a 100 kg scale. Samples from the harvested material of each plot were dried using a micro-wave oven (Undersander et al 1993) and total nitrogen was determined by the Kjeldahl technique and crude protein calculated as N*6.25. Differences between cropping systems were analyzed by combining all the harvests of all the components of each system within each block (eg: Guinea grass 1+2+3 vs Flemingia + Sugar cane + Trichanthera) based on dry matter yield ( DMY), crude protein yield (CPY) and digestible dry matter yield (DDMY).

Soil fertility (Maize biotest)

Soil samples (from 0-30 cm depth) were taken from each experimental plot at the beginning and after 3, 8 and 18 months. An equal amount (3 kg) was put into clay pots (about 5 litre capacity). Three seeds of maize were planted in each pot. After 5 weeks the maize plants were removed from the soil, washed to remove soil from the root and allowed to dry for 1 hour in the shade. The total “above ground” biomass and the roots were weighed.

The maize “biotest” was used to compare the soil fertility between each component of the mixed system and the corresponding plot of Guinea grass (eg  Flemingia vs Guinea grass 1; Sugar cane vs Guinea grass 2; Trichanthera vs Guinea grass 3) for each sampling date. The “biotest” method is based on the relative growth rates over a 35 day period of maize planted in soil taken from the experimental plots. As such, the absolute growth rates are affected by the climatic conditions at the time of making the test. Thus is not possible to compare the absolute growth rates between different sampling dates. In this experiment, the Guinea grass was considered to be the control treatment against which the other crops were compared. These comparisons were done separately for the combinations of Guinea grass and the particular crop from the mixed system located on the same contour. It was assumed that if maize growth was greater in soil from a particular crop in the mixed system than from the corresponding plot of Guinea grass than this could be interpreted as due to improvements in soil fertility relative to the soil fertility registered with Guinea grass.

Differences between cropping systems were analyzed by combining all the harvests of all the components of each system within each block (e.g. Guinea grass 1+2+3 vs Flemingia. + Sugar cane + Trichanthera )

 

Soil analysis:

Samples of soil (from 0-30cm depth) were taken from each plot, before and 20 months after planting. The 9 samples (3 from each treatment / block) were bulked and one sub-sample taken for analysis of pH, N, P, K and C  by standard methods (AOAC 1990).

 

Measurements on-farm

Measurements were made of fresh biomass yield and the dry matter and N composition of the biomass. 

Results

Soil nutrient status

Data on the pH and nutrient status of the soil before and after the study period are shown in Table 2. There were improvements in most of the parameters comparing values before and after the study period. As might be expected, there was a trend in the samples taken before planting for the carbon content to increase from the upper to the lower contour (from plots of Flemingia to those of Trichanthera and from Guinea 1 to Guinea 3). After 20 months the tendency was for a greater increase in soil carbon in the upper contour so that all plots reached similar levels of carbon. However, no conclusions can be drawn concerning the effect of the cropping system as samples were bulked by treatment for chemical analysis making it impossible to do any statistical analysis.

 

Table 2: Nutrient status of soil before planting and 20 months later
Parameters	Flemingia		Guinea 1		Sugar C		Guinea 2		Trich.		Guinea 3
pH
Before	3.91	3.91		4.00		4.00		4.01		4.01
After	4.00	3.95		4.01		3.98		4.01		4.02
Carbon (%)
Before	2.68	2.68		2.80		2.80		2.97		2.97
After	3.28	2.97		3.05		3.00		3.03		3.02
N (%)
Before	0.170	0.170		0.175		0.175		0.180		0.180
After	0.235	0.196		0.224		0.212		0.207		0.218
P2O5 (%)
Before	0.041	0.041		0.039		0.039		0.038		0.038
After	0.067	0.078		0.091		0.085		0.088		0.087
K2O (%)
Before	1.02	1.02		1.00		1.00		0.98		0.98
After	1.13	1.09		1.10		1.04		1.08		1.07

 

Soil bio-test

In the biological test of soil fertility, the data are presented as weights of total fresh green biomass of one maize plant and of the root fraction (Table 3). There were no significant differences between the systems (pure grass and mixed species) for the growth of the maize in samples taken at the beginning and after 3 and 8 months.  But there was a significant effect after 20 months in favour of the mixed system. Maize growth in soil on the mixed system increased relative to that grown on soil from the grass plots,  indicating a positive effect  of the mixed cropping on soil fertility. In view of the siting of the experimental plots on a hillside, valid comparisons are restricted to treatments located on the same contour. Thus each component of the mixed cropping system was compared with the corresponding plot of Guinea grass grown on the same contour. As the tests were carried out under different climatic conditions the absolute values reflect more the effect of time of year than the intrinsic fertility of the soil.

 

 

 

 

 

Biomass production

Total biomass yield during the first and the second ten month periods was compared on the basis of the individual crops in the mixed cropping system versus Guinea grass planted on the same contour (Tables 4a,b). Guinea grass gave significantly higher yields of fresh matter than Flemingia and Trichanthera but a lower yield than sugar cane. Analysis of variance of the components of the edible biomass,  in term of dry matter and digestible dry matter which can be used as animal feed (digestibility coefficients for goats fed the different sources of biomass were obtained from a concurrent experiment; Nguyen Thi Mui, unpublished), showed that the yields of Flemingia and Sugar cane in the contours 1 and 2 were higher than that for corresponding plots of Guinea grass. On the same contour, Flemingia had a higher crude protein yield than Guinea grass while Trichanthera  yielded less than Guinea grass.

 

The yield from the two systems is compared in Tables 4a,b and Table 5. Guinea grass gave significantly higher yields of protein (47%) in the first 10 months compared to the mixed species system but was inferior (21% and 19%) for yields of dry matter and digestible dry matter, respectively. In the second 10 months,  DM and DDM yields of Flemingia and Sugar cane were higher than pure grass system by 29% and 31%, respectively, The pure grass gave higher (28%) yields of protein compared to the mixed system.

 

 

 

 

 

 

Similar results on biomass yield were obtained in the farm (Tables 6 and 7), except that total crude protein yield in the mixed system was 45% higher than that in the pure grass system due to the contribution of the higher yield obtained from Flemingia in the mixed system.

 

Discussion

The three species chosen for the mixed cropping system (Flemingia, Sugarcane and Trichanthera) have different characteristics. Flemingia is a leguminous shrub, sugarcane is a perennial grass known for its high capacity to produce sugar-rich biomass and Trichanthera  is a non-leguminous tree with high biomass yields. All three are perennial crops which re-grow vigorously after being harvested and which are tolerant of acid soil.

 

There are indications form the comparison of soil on the contours planted with Flemingia (contour 1) and Sugar cane (contour 2), that both these cropping systems had beneficial effects on soil fertility. The results in this study are in agreement with the findings of Dinh Van Binh (unpublished data) for Flemingia and there are supporting data from work of Phan Gia Tan (1995) and Nguyen Thi Mui et al (1996;1997a,b) concerning the beneficial effect of sugar cane on soil fertility especially when the dead leaves are returned to the soil. There are few reports for Flemingia, which is a relatively new introduction to Vietnam. However, on the basis of this assessment, of the three species in the mixed system, only in the case of Flemingia were there indications of improvement in soil fertility with time (compare the “biotest” data at 8 and 20 months with that recorded before and 3 months after planting). This apparent improvement could be attributed to the fact that Flemingia is a legume, able to fix nitrogen in acid soil of low fertility. Improvements in soil fertility, measured by the maize “biotest “, were reported by Gomez  and Preston (1996 ) in succeeding years for the legume tree Gliricidia sepium, the aerial part of which was harvested repeatedly for livestock feed at three months intervals.

 

It is surprising that there were no differences between soil taken from plots of sugar cane and from plots of Guinea grass, at least until 20 months following planting. In experiments at the  Goat and Rabbit Research Centre and on farms (Nguyen Thi Mui et al 1996; 1997a,b) it was shown conclusively that growing of sugar cane led to significant improvements in soil fertility and in yield of sugar cane, when the dead leaves were returned to the soil. The fact that the biotest at 20 months indicated significantly better fertility in the sugar cane plots compared with the guinea grass on the same contour, plus the finding that yields of sugar cane were higher in the second than in the first 10 months, all point to increased soil fertility with time as a result of growing sugar cane.

 

In contrast, the tendency with the Trichanthera  trees was in the reverse direction with an apparent negative effect on soil fertility as compared with the companion Guinea grass. There were no apparent differences in fertility between soil taken from the Trichanthera  plots compared with those from plots of Guinea grass grown on the same contour. Trichanthera  is not a legume but there are reports that Mycorrhiza  species are found in association with its root system (Rosales 1997). On-farm observations in Colombia (Murgueitio E, personal communication) and in Vietnam (Dinh Van Binh, personal communication) indicate that soil fertility under this tree is maintained even when the foliage is harvested repeatedly as livestock feed.

 

Biomass production

We have not been able to find any report in the literature which compares effects on biomass production and soil fertility of different cropping systems on sloping land. The Sloping Land Technology system developed in Philippines has had a favourable impact in many locations as a strategy for controlling erosion and improving soil fertility, but precise comparisons with other land management systems have apparently not been made.

 

The strategy in the mixed cropping system was to combine specialised “energy” crops such as sugar cane with tree species, the primary role of which is to produce protein. It was hypothesised  that the feeding value for ruminants of the products from the mixed  system would be superior to that from guinea grass. The greater dry matter and protein production from guinea grass compared with the mixed system in the first 10 months could be attributed to the slower establishment of the tree and shrub species which do not come into full production until at least 5 to 8 months after planting. In fact, production increased in the second period of 10 months for all three component crops in the mixed system, especially for Flemingia and sugar cane and to a lesser extent for Trichanthera . There is reason to believe that effects on soil fertility and on biomass yield in future years will increasingly favour the mixed cropping system. The other factor that it has not been possible to quantify is the value of the nitrogenous fraction which is likely to be superior in the mixed system compared with the Guinea grass. It is known that leaves from trees and shrubs are richer in tannins than are the leaves of grass. This usually confers on the tree and shrub species distinct benefits as potential source of bypass (escape) protein (Leng 1998).

 

Conclusions

The first attempt to evaluate contrasting systems of perennial grass (Guinea grass) and mixed crops of sugarcane and multi-purpose trees (Trichanthera  gigantea and Flemingia macrophylla) in terms of biomass yield which can used as animal feed indicates contour planting of Flemingia (contour 1), Sugar cane (contour 2) and Trichanthera  (contour 3) has the following advantages compared with a pure grass:

·        Greater increase in soil carbon and improving nutrient status of the soil after 20 months

·        Higher yield of digestible dry matter

 

References

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