Effect of harvest interval on biomass yield and composition of mulberry (Morus alba) managed for feed and fuel
 

Huy Sokchea, Chhay Ty, Khieu Borin, T R Preston and Mech Phalla

Center for Livestock and Agriculture Development (CelAgrid)

PO box 2423, Phnom Penh 3, Cambodia
huysokchea@celagrid.org
* UTA-TOSOLY, AA #48, Socorro, Santander, Colombia
 

Abstract 

Mulberry foliage was harvested at intervals of 6, 8, 10 or 12 weeks and separated into leaves and stems, simulating the use of the former as a protein supplement for goats/pigs and the latter as feedstock for gasification. The mulberry was planted in November 2005 with the first harvest 6 months later.  The biomass was harvested by cutting the stems 40cm above soil level. Fertilization was 700 kg N/ha/year with effluent from a biodigester charged with pig manure and applied every week. Annual DM yields of leaves were not affected by harvest frequency but yield of stems and total biomass increased as the harvest interval was increased from 6 to 12 weeks. Leaves accounted for 65% of the biomass DM at the 6 week interval decreasing to 55% at 12 weeks. Content of crude protein in DM of  leaves and stems decreased linearly with increased harvest interval (from 22.6 to 17.5% for leaves and from 8.5 to 5.5% for stems).  It is concluded that harvesting mulberry foliage at intervals of 12 weeks optimizes total production of  biomass without detriment to leaf yield.

Key words: Crude protein, energy, gasification, leaf:stem ratio

Introduction

Mulberry (Morus alba) cultivation offers a great opportunity, at the present time, because of its high biomass yield, the suitability of the leaves as feed for a wide range of livestock and the tolerance of the plant to varied ecological zones (Benavides No date). In addition to the value of the leaves as feed, mulberry has the potential to serve in the energy sector to add value in integrated farming systems of feed/food and energy. Recently, mulberry stems and branches were tested successfully in a gasifier to produce electricity with high efficiency of 0.85 KWh/kg DM of stems (Phalla and Preston 2005).

In Malaysia, Saddul et al (2004) studied the harvest interval of mulberry over the range of 3 to 9 weeks. The data showed that the leaf yield was similar (about 5 tonnes DM/ha/year) at all intervals, but that yield of stem increased from 1 to 7 tonnes DM/ha/year as the interval was extended from 3 to 9 weeks.

There would thus appear to be advantages in having an extended harvest interval when the aim is to use the mulberry biomass both as animal feed (the leaves) and feedstock (the stems) for gasification.

The purpose of this study was to:

• Evaluate the biomass yield and leaf:stem ratio of mulberry submitted to different harvest intervals.

• Estimate the economic outcome of the co-generation of mulberry in terms of feed and electricity

Materials and Methods

Location

The experiment was conducted at the Center for Livestock and Agriculture Development (CelAgrid), which is about 25 km from Phnom Penh city towards the South. The air temperature is in the range of 35 to 38 °C with rainfall from ??.  The experiment was done over two years starting from July 2005 including seeding and site preparation.

Treatments and design

The four treatments were harvest intervals of 6, 8, 10 and 12 weeks.

·   H6:      (harvest interval 6 weeks)

·   H8:      (harvest interval 8 weeks)

·   H10  :  (harvest interval 10 weeks)

·   H12  :  (harvest interval 12 weeks)

The design was a complete randomized block layout (RCBD) with each of the 4 blocks as the replicates (Table 1). Each plot was 5 x 5 m.

Planting

Mulberry stems of 25-30 cm length were germinated in plastic bags (capacity 2 liters) containing a mixture of 30 % composted cow manure and 70% of soil. After 2 months in the nursery the germinated seedlings were transplanted (1 November 2005) in a former rice field with density of 40,000 plants/ha (0.5m*0.5m spacing). Effluent (670 mg/litre total N; 401 mg/litre NH₄-N) from biodigesters charged with pig manure was applied at weekly intervals in amounts equivalent to 700 kg/ha of nitrogen per year. The plots were irrigated in the dry season using a sprinkler system.

Harvesting

The plants were harvested by cutting the stems 40 cm above the ground and separating the biomass into leaves and stems. The first harvest was 6 months after planting with subsequent harvests depending on the treatment (6, 8, 10 and 12 weeks). For calculation of effect of harvest interval on biomass yields and composition, the data from the first harvest were discarded.

Gasification

Samples of the harvested stems from each treatment were sun-dried to >80% DM and used as feedstock in a downdraft gasifier (Mech Phalla and Preston 2005). Weighed quantities of stems were placed in the gasifier which was linked to a generator submitted to a fixed load of 9 KW from lighting and laboratory equipment.

Measurements

The first harvest was in June 2006, 6 months after planting. During the subsequent 12 months the  mulberry foliage in treatments H6, H8, H10 and H12 weeks was harvested 8, 6, 5 and 4 times, respectively. The overall intervals from the first to the final harvest were: 395, 395, 409 and 365 days, for treatments H6, H8, H10 and H12, respectively. Yields were corrected to 365 days prior to analysis.

At each harvest, the yields of leaves and stems were recorded and samples taken for determination of DM by micro-wave radiation (Undersander et al 1993) and N (AOAC 1990).

The time to consume all the biomass in the gasifier was recorded. The quantities of residual char were weighed after each test.

Statistical analysis

The data were analysed using the General Linear Model in the ANOVA software of Minitab (2000). Sources of variation were treatment, block and error. Yield parameters were correlated with harvest interval using the regression program in the same Minitab software.

Result and discussion

General

Yield of leaves on fresh and DM basis was not affected by harvest interval (Table 2; Figure 1). In contrast, stem DM yield increased with a curvilinear trend according to harvest interval (Figure 3) and was twice as high with 12 week compared with the 4 week harvest interval.  As a result the proportions of leaf in the total biomass decreased with harvest interval.  Total biomass yield followed the trends for stem yield.

The mean crude protein content of leaves and stems decreased linearly with increased harvest interval (Table 3; Figure 4).

Gasification

There were no differences in the operating characteristics of the gasifier when the stems for the different treatments were used as feedstock (Table 4).