Effect of cattle manure and biodigester effluent levels on growth and composition of water spinach

Nguyen Van Hiep and T R Preston*

Nong Lam University, Thu Duc, Viet Nam
nguyenvanhiep5@yahoo.com
^*Finca Ecológica, TOSOLY, UTA (Colombia)
AA #48, Socorro, Santander, Colombia
trpreston@mekarn.org

Abstract

An experiment was conducted at the Livestock Research Center, Laos from 16^th of September to 14^th of  October, 2005. The objective was to evaluate the effect of different levels of effluent from a biodigester charged with pig manure and the interaction with low and high cattle manure level mixed with the soil.  A randomized complete block design with two factors was used. The factors were: cattle manure at levels of 20 and 100 tones/ha and biodigester effluent at levels of 0, 10, 20, 30 and 40 kg/ha. There were 3 replications of each treatment arranged within 3 blocks.

In a 28 day growth period, yield of fresh leaves, stems and entire plant biomass of water spinach was increased by 109, 155 and 137% for the 100 compared with 20 tonnes/ha cow manure application. Comparable figures for DM yields were: 100, 135 and 114%. There were linear responses in biomass yield to increasing levels of biodigester effluent.

In terms of response to added N this was about 1.25 kg DM per 1 kg additional N from cow manure, compared with an additional 7.5 kg DM biomass per 1 kg added N from biodgester effluent.

Key words: biodigester effluent, biomass, cattle manure, fertilizer, water spinach

Introduction

Water spinach (Ipomoea aquatica ) has a high biomass yield. It is used traditionally in tropical regions for consumption by people and animals. Using water spinach as a protein source for Ba Xuyen and Large White sows has given good results in feed intake and digestibility (Le Thi Men et al 1999). Water spinach has a short growth period and is resistant to common insect pests. It can grow both in soil and in water and it is very easy to grow by farmers. Vegetables require many nutrient elements for good growth and production, but N, P and K are three elements of most concern. Leafy vegetables such as water spinach, are especially heavy users of nitrogen. The traditional practice is to use urea as fertilizer but price of urea is expensive in the recent years. In an experiment reported by Le Thi Luyen and Preston (2004), the maximum water spinach biomass yield was obtained with 30 kg N of urea/ha.

Effluent from biodigesters charged with pig manure is high in N sources and is much cheaper than urea when produced on the farm.  Biodigester effluent at 75 kg N/ha supported the same fresh biomass yield of water spinach as urea at the same N application rate (Kean Sophea and Preston 2001).

Observations in Colombia (Rodríguez 2005, personal communication) indicate that water spinach has a high need of organic matter in the soil in order to grow well.

Hypothesis

The response of water spinach to fertilization with biodigester effluent will be greater when it is grown on beds with high content of cattle manure.

Materials and methods

Location

The experiment was conducted at the Livestock Research Center, Naxay Thong district, Vientiane province, Laos,  from September 16 to October 14 2005.

Design and treatments

The study was a 2*5 factorial arrangement, in a randomized complete block design (CRBD).  The first factor was cattle manure with 2 levels (20 and 100 tonnes/ha) which was mixed in the soil before seeding the water spinach. The second factor was N from biodigester effluent with 5 levels (0, 10, 20, 30 and 40 kg/ha) which was applied 20% after first week, 40% after second week and 40% after third week. There were 3 replications of each treatment arranged within 3 blocks. The plot size was 1m² with a space of 50 cm between plots (Table 1).

Land preparation

The soil was cultivated by hoe, raising beds 15 cm high and separating the plots. The cattle manure was applied to each soil plot before seeding.

Planting and management

The water spinach seed rate was 60 g/m²; the seed was planted in rows at spacing 5 cm and 2-3 cm depth. The low biodigester effluent  treatments were supplemented with water to balance with the highest biodigester effluent volume treatment. Water was also irrigated in the same volume on all treatments every day.

The biodigester was a "plug-flow" tubular polyethylene model, similar to that described by San Thy et al (2003). The diameter of the tube was 0.9m and the length 8m, giving a liquid volume of 3.8 m³ (75% of the total volume). It was charged with washings from a building housing growing pigs fed mainly on rice byproducts. There was no control of the loading rates nor of the ratio of excreta to water in the influent.

Measurements

The biodigester effluent was analyzed for N content to calculate effluent volumes for each treatment. Soil samples at the beginning were collected to analyze for DM, OM and N. Germination and color of plants were observed every day. Plant height (from the bottom to top) and leaf length of water spinach were measured every 3 days. The biomass was harvested at 28 days and immediately separated into leaves and stems to analysis for DM content. The leaves and stem samples were ground with a coffee grinder and analyzed for DM content by microwave radiation (Undersander et al 1993). OM and N were analyzed according to AOAC (1990).

Statistical analysis

The data were analyzed by ANOVA using the software of Minitab version 13.31 (MTAB 2000). Sources of variation were: level of manure, effluent N, interaction manure*effluent N, blocks and error.
 

Results

The level of organic matter in the soil (Table 2) was similar to that reported by Tran Thi Bich Ngoc and Preston (2006) for a "good" soil (11.5%); however, the N level was less than was reported by these authors for the "poorest" soil in their experiment (0.15% in DM). The N content of the biodigester effluent was 85 mg/litre. This value is much lower than was reported by San Thy et al (2003) for a similar type of biodigester (900 to 1400 mg/litre), probably because of too high a ratio of water to manure in the influent to the biodigester

The germination rate of the water spinach, observed from 5^th to 10^th days after seeding,  showed no significant difference between cattle manure levels as well as among nitrogen levels from biodigester effluent (Table 3). The overall germination rate was not very high (48.3%),  maybe because of low seed quality.

The colour of the water spinach was more intense in the high than in the low manure treatments (Photo 1) with no apparent differences due to effluent level.

Photo 1: Colour intensity of the water spinach according to level of cattle manure
(HM or LM = 20 or 100 tonnes/ha) and level of biodigester effluent (0 to 40 kg N/ha)

There were overall increases of the order of 50% in height of the plants and length of the leaves when the level of cow manure was increased from 20 to 100 tonnes/ha (Table 4 and Figures 1 and 2). There was a tendency (P=0.068) for plant height to be increased with increase of effluent level, with the effect being more apparent on the low cattle manure treatment.

Biomass yield, of leaves and stems, was almost twice as high in the plots with 100 tonnes/ha cow manure compared with those with 20 tonnes/ha (Table 5 and Figures 3 and 4). By contrast the yields increased by only some 20% as the effluent level was raised from 0 to 40 kg N/ha.

The DM content of leaves was higher than of stems, and was higher for water spinach grown with lower levels of cow manure (Table 6 and Figures 5 and 6).

Total yield of dry biomass was twice as high for the 100 compared with the 20 tonne/ha level of cow manure (Figure 7).  There were also linear trends in biomass yield as the biodigester effluent level was increased (Figure 8). The implication is that the water spinach would have continued to increase in yield with N  levels beyond 40 kg N/ha. In fact, Kean Sophea and Preston (2001) registered linear increases in water spinach yield with up to 140 kg N/ha from biodigester effluent.

Crude protein content was higher in leaves than in stems and tended to be higher in water spinach grown on the higher level of cow manure (Table 7). Crude protein levels tended to be higher with increasing effluent levels but this was only apparent on the high cow manure treatments.

Discussion

The cow manure used in the experiment contained 1.007% N in DM. Thus the application of 20 and 100 tonnes of fresh cow manure per ha was equivalent to additions of 124 and 620 kg/ha of N. It was therefore to be expected that the response to increasing the cow manure application from 20 to 100 tonnes/ha would be much superior to the application of 40 kg N /ha from biodigester effluent. In terms of response to added N this was much higher (7.5 kg biomass DM/kg N) for the effluent than for the cow manure (about 1.7 kg biomass DM per kg N). Similar findings were reported by Le Ha Chau (1998a,b);  cassava grown for foliage and duckweed grown in ponds showed greater yield responses to N in biodigester effluent than to the N in the original manure used to charge the biodigesters. The superiority of the biodigester effluent can be partially explained by the conversion of organic to inorganic N during the process of anaerobic digestion. It is reported that ammonia N accounts for some 60 to 70% of the total N in the effluent compared with only about 15 to 20% in the original manure) (Pedraza et al 2002; San Thy et al 2003).
 

Conclusions

• Increasing the application to the soil of cow manure from 20 to 100 tonnes/ha doubled the biomass yield of water spinach from 0.8 to 1.6 tonnes DM/ha in 28 days of growth; however, in terms of response to added N this was only about 1.7 kg DM per 1 kg additional N.

• At both levels of cow manure there were linear responses to application of biodigester effluent over the range 0 to 40 kg N/ha, equivalent to an additional 7.5 kg DM biomass per 1 kg added N.

Acknowledgements

The author is grateful to sidaSAREC for financing this research through the MEKARN project. Thanks are given to MSc. Chhay Ty, MSc Lamphuey Kaensombath and all staff of the Livestock Research Center, NARFI, Vientiane, Laos, for advice and assistance in the organization of the experiment.

References

AOAC 1990 Official Methods of Analysis. Association of Official Analytical Chemists. 15^th edition (K Helrick editor) Arlington pp 1230

Kean Sophea and Preston T R 2001 Comparison of biodigester effluent and urea as fertilizer for water spinach vegetable. Livestock Research for Rural Development. (13) 6: http://cipav.org.co/lrrd/lrrd13/6/Kean136.htm

Le Ha Chau 1998a Biodigester effluent versus manure from pigs or cattle as fertilizer for production of cassava foliage (Manihot esculenta).  Livestock Research for Rural Development. Vol. 10, No 3. Retrieved, from http://www.cipav.org.co/lrrd/lrrd10/3/chau1.htm

Le Ha Chau 1998b Biodigester effluent versus manure from pigs or cattle as fertilizer for duckweed (Lemna spp.). Livestock Research for Rural Development. Vol. 10, No 3. Retrieved, from http://www.cipav.org.co/lrrd/lrrd10/3/chau2.htm

Le Thi Men, Ogle B and Vo Van Son 1999 Evaluation of water spinach as a protein source for Ba Xuyen and Large White sows and fattening crossbred pigs. MSc thesis, Swedish University of Agricultural Sciences, Uppsala.

Ly Thi Luyen and Preston T R 2004 Effect of level of urea fertilizer on biomass production of water spinach (Ipomoea aquatica) grown in soil and in water. Livestock Research for Rural Development. Vol.16, Art. #81. Retrieved, from http://www.cipav.org.co/lrrd/lrrd16/10/luye16081.htm