Effect of additives on ensiling of taro (Colocasia esculenta (L.) Schott) leaves and Stylo CIAT 184 (Stylosanthes guianensis (Aubl.) Sw. var. guianensis) forage

Abstract

This study examined the effects of locally available carbohydrate-rich feed additives in the ensiling of cultivated Stylo CIAT 184 (Stylosanthes guianensis (Aubl) Sw. Var. guianensis) forage and taro (Colocasia esculenta (L.) Schott) leaves.  Cassava root meal, sugar cane molasses and taro root meal were used as silage additives, incorporated at levels of 0, 25, 50 and 75 g kg^-1 pre-wilted forage and leaves, with 5 g common salt (NaCl) added to all treatments. The experiment was arranged according to a completely randomised factorial design, with additive source and level of additive as factors. Silage samples were collected at 0, 7, 14, 21, 56 and 84 days of ensiling and were analysed for pH, dry matter (DM), crude protein (CP), ammonia nitrogen (NH₃-N) and ash. Neutral detergent fibre (NDF) was analysed in samples from days 21 and 84.

Incorporation of cassava or taro root meal as fermentation additives had no effect on the pattern of pH change in ensiled Stylo forage or ensiled taro leaves. Molasses as silage additive accelerated the fall in pH in both Stylo and taro leaf silage, with the effects more pronounced for the latter. Breakdown of crude protein to ammonia was greater in Taro silage reaching 10% of the total N fraction after 84 days, compared with 6% for the Stylo silage.

Keywords: ammonia, cassava root meal, molasses, silage additive, taro root meal, pigs

Introduction

Green plant materials are important ingredients in the diet of pigs in smallholder farming systems in Lao PDR (Chittavong et al 2012; Phengsavanh et al 2011). In these farming systems, pig feeding is based on maize, cassava roots and agricultural by-products, together with vegetables and weeds that grow wild in the forests, along ponds and river banks, and in cropping areas (Phengsavanh et al 2010). However, most of the locally available and cultivated plants are only abundant in the rainy season, while there are usually feed shortages in the dry season. Therefore, in order to ensure sufficient feed supply to the pigs throughout the year, strategies are needed that will allow safe preservation of the plant material available in excess in the rainy season. In this context, ensiling is an attractive, cheap and simple option that can be applied at farm level and that  makes it possible to safely preserve and store feed from one season to another.  

The ensiling technique is used to preserve high moisture content materials through controlled lactic acid fermentation under anaerobic conditions. However, in order to produce stable and hygienic high-moisture silage of high nutritional value, the pH has to be reduced rapidly at the onset of the ensiling process and maintained at low levels during storage (McDonald et al 1991). This may be difficult to achieve without including additives in the material to be ensiled in order to facilitate the fermentation process. For example, plant material with a high crude protein (CP) content has a buffering capacity that can delay, or even prevent, a sufficient reduction in the pH by neutralising the acids produced during fermentation. Moreover, if the plant material is also low in soluble carbohydrates and has a high moisture content, this poses further challenges to successful ensiling (McDonald et al 1991).

Sugar cane molasses is a commonly used silage additive that has a high content of water-soluble carbohydrates and has been successfully used in ensiling different plant materials (McDonald et al 1981; McDonald et al 1911). Cassava root meal is a locally available starch-rich product with high dry matter (DM) content that has been successfully used as a silage additive for ensiling of cassava leaves in Vietnam (Loc et al 2000). Taro root meal is another locally available starch-rich product with high DM content that should have the potential for use as a silage additive for different plant materials in a similar way to cassava root meal.   

Taro (Colocasia esculenta spp.) is cultivated for human consumption of the corm and is abundant in the wild in most parts of Lao PDR. Its green parts (leaves and stalks) are commonly used as pig feed by smallholders, and are boiled with maize, cassava root and rice by-products prior to feeding. The CP content in the leaves is high, ranging from 160 to 260 g kg DM^-1, and the leaves can be harvested at 4-week intervals without negative effects on DM or corm yield (Kaensombath and Frankow-Lindberg 2012b).  

CIAT Stylo 184 (Stylosanthes guianensis (Aubl.) Sw. var. guianensis) can produce large quantities of good quality forage under tropical conditions and has been introduced into many countries in Southeast Asia, including Lao PDR (Guodao et al 1997). Initially, it was introduced as a feed for ruminants, but has later also been promoted as a feed for pigs (Horne 1997). During the rainy season, cultivated Stylo has the potential to produce large quantities of forage biomass that can be used to supply CP in the traditional pig diet. However, to obtain the quality needed with respect to CP and fibre content, the biomass should be harvested at 4- to 6-week intervals (Kaensombath and Frankow-Lindberg 2012a).

The aim of the present study was to compare cassava root meal, sugar cane molasses and taro root meal as silage additives for preservation of pre-wilted Stylo forage and taro leaves, and to establish suitable inclusion levels of the respective silage additive. The starting hypothesis was that sugar cane molasses would give a more rapid reduction in pH than cassava root meal and taro root meal, and that lower inclusion levels of sugar cane molasses would be needed to maintain stable silage over time.

Materials and methods

Experimental design

Stylo forage and taro leaves were ensiled for 84 days with three sources of readily available carbohydrate (molasses, and root meals of cassava and taro) at each of four levels (0, 25, 50 and 75 g kg^-1 pre-wilted forage or leaves). The design for each forage was a completely randomised 3*4 factorial arrangement with three replicates per treatment. The silages were sampled on day 0, 7, 14, 21, 56 and 84 of ensiling for determination of biochemical traits and for analysis of chemical composition.   

Ensiling of the forages

The studies were carried out at the Faculty of Agriculture, National University of Laos, Vientiane, Lao PDR. The trial with Stylo forage was performed from 8 August to 31 October 2008 and  with taro leaves from 12 August to 4 November 2008. Average air temperature during the study was 27.6 ºC and the relative humidity ranged from 59 to 92%.   

Stylo forage (1^st cut of second season) was collected at 28 days after defoliation and taro leaves were collected at 73 days after planting. The forage and leaves were chopped into small pieces (1 cm length) and then spread out for wilting on a concrete floor under shade for 4 h for Stylo forage and overnight for taro leaves.  Common salt (NaCl) was added at 5 g kg^-1 pre-wilted material in all treatments. Plant material, additive and salt were mixed thoroughly for each treatment and 200 g samples were sealed in double plastic bags (capacity 1000 g) and placed in a plastic container for storage.

The cassava roots and taro roots (1^st season plantation) were collected from fields near the campus, sun-dried and milled before being used in silage making. Sugar cane molasses (76 Brix) was purchased from a sugar factory in Vientiane.
 

Sampling and data recording 

On each sampling occasion, one plastic bag per treatment was broken open and sampled.   The pH was determined immediately using a pH electrode (Orion 230 A+) and the physical characteristics (e.g. smell, colour) were recorded. Thereafter, the samples were oven-dried at 60 ºC for 24 h, and then milled through a 1-mm mesh screen before chemical analysis. The contents of CP, ammonia nitrogen (NH₃-N) and ash were determined in the samples at 7, 14, 21, 56 and 84 days of ensiling; NDF was analysed at 21 and 84 days of ensiling. 
 

Chemical analysis

Pre-wilted material and silage samples were analysed for DM using microwave radiation (Undersander et al 1993). AOAC methods (AOAC 1990) were used to measure CP (984.13), ash (942.05) and NH₃-N. NDF was estimated according to Van Soest et al (1991).
 

Statistical analysis

The data were subjected to analysis of variance (ANOVA) according to a factorial design using the General Linear Model (GLM) procedure of Minitab software version 16.2.1 (Minitab 2010). Tukey’s pair-wise comparison was used to confirm differences between additives and levels at P<0.05.

Results

The taro leaves had higher concentrations of CP and ash and less NDF than the stylo forage (Table 1).

After 7 days of ensiling, both Stylo and taro silage had a distinct silage aroma, which became stronger at 14 days of ensiling onwards. The silage aroma appeared to be stronger with molasses as additive than with cassava or taro root meal.       

Effects on pH

Both Stylo forage and taro leaves were ensiled satisfactorily without additives; however, there was a marked difference in the pattern of change, with pH falling to 5.3 after 7 days in ensiled taro leaves whereas with Stylo the fall in pH to 5.4 took place steadily over a period of 56 days (Figure 1). When molasses was applied at levels of 50 g kg-1 the time for the ensiled Stylo to reach pH 5.4 was reduced to 21 days (Figure 2).  Similar trends in the pattern of change in pH, between Stylo forage and taro leaves, were observed for all additives when these were added at 50 g kg^-1 (Tables 2 and 3; Figures 3 and 3).;  pH values were lower when molasses was the additive. In the taro silage, the fall in pH with molasses was more pronounced compared with other additives, with a value of  4.4 after 14 days which remained at that level through the remaining 64 days (Table 3; Figure 4).  Figures 3 and 4 also indicate there were no benefits in rate of reduction of the pH in Stylo silage or taro leaf silage from addition of root meals of cassava and taro compared with no additive, as final pH at 84 days was the sam e on all treatments; by comparison the greater fall in pH with molasses was evident in both Stylo and taro leaf silages.

Effects on ammonia-N

Ammonia-N levels in both silages were constant over the first 21 days of ensiling and then increased from 21 to 56 days (Figures 5 and 6). Levels were some 50% lower in stylo silage than in taro leaf silage.  This is consistent with the report of Ajayi and Babayemi (2008) that  tannin levels in Stylo guianensis  exceeded 100 g kg^-1 of DM, which would result in a reduced solubility of the stylo protein and hence less degradation to ammonia.. There were no consistent differences among the additives in effects on ammonia levels.

Effects on DM, crude protein, ash and NDF

As was to be expected, the DM content of the silages was increased by adding molasses and cassava and taro root meals, all of which were much higher in DM than the stylo forage and taro leaves, but there were no major effects on levels of crude protein and ash (Tables 2 and 3). NDF levels were increased in Stylo silage and decreased in Taro leaf silage as the ensiling period was extended from 21 to 84 days.

Figure 1. Pattern of change in pH in ensiling of Stylo forage and taro leaves without additive	Figure 2. Pattern of change in pH in ensiling of Stylo forage and taro leaves with 50 g kg-1 of molasses added to the wilted material

Figure 3. Trends in pH in ensiled Stylo forage with no additive or with molasses or cassava or taro root meal at 50 g kg-1	Figure 4. Trends in pH in ensiled taro leaves with no additive or with molasses or cassava ot taro root meal at 50 g kg-1

Figure 5. Effect of ensiling duration and source of carbohydrate additive (50 g kg-1) on ammonia-N in Stylo silage	Figure 6 . Effect of ensiling duration and source of carbohydrate additive (50 g kg-1) on ammonia-N in taro leaf silage

Discussion

The successful ensiling of taro leaves with molasses confirms earlier reports for this procedure (Chittavong et al 2008; Rodriguez and Preston 2009). The apparent absence in the literature of reports on ensiling of stylo forage may be because use of this plant has mainly been as ruminant feed in grazing situations.

In temperate countries much emphasis is given to reducing the moisture content of forages prior to ensiling and to the need for additives to facilitate the fermentation (see: McDonald et al 1981). However, these restrictions may not apply to tropical plants, many of which have a relatively high inherent content of soluble sugars.  In the experiments reported by Rodríguez and Preston (2009), the taro leaves were found to contain 19.5% soluble sugars in the DM (measured by refractometer).  The fact that in our experiment, the taro leaves (but not Stylo forage) were also ensiled satisfactorily without molasses confirms the hypothesis that the leaves/foliage of certain tropical plants, and taro in particular, do not need addition of fermentable carbohydrate for satisfactory ensiling (Rodriguez and Preston 2009). Dao Thi My Tien et al (2010) confirmed that taro plants (in this case the combined leaf and petiole) could be ensiled satisfactorily without additives (pH fell to 4.5 in 24 h). They also showed that the pseudo-stem of banana (another tropical plant) could be ensiled satisfactorily without additives, and ascribed this to the appreciable concentration of sugars that were present (30% in DM). .

The need for wilting taro leaves and stylo foliage before ensiling does not appear to have been tested, the rationale usually being that it would be desirable to reduce the moisture content of the leaves/foliage prior to ensiling. Wilting of taro leaves was not necessary in the experiment reported by Rodriguez and Preston (2007), where the leaves were "macerated" in a high-speed (3500 rpm) forage chopper which would facilitate disruption of the cells and microbial access to the plant cell contents. By contrast, the traditional method in Vietnam is to cut the leaves into small pieces by hand using a sharp knife. Further research is needed to assess if wilting of taro leaves is an essential first step in the ensiling process employing manual chopping, as inevitably some nutrients will be lost or modified (eg: water soluble carbohydrates will be fermented) in the process and it is an additional, relatively labour-intensive task

Ensiling without a carbohydrate-rich additive resulted in higher pH values for Stylo than for taro leaves, and a longer time for an adequately low pH to be achieved. which indicates that the fermentation process was not efficient. This can be explained by the low content of water-soluble carbohydrates in Stylo forage  (Bradbury and Holloway 1988; Liu et al 2011).

After ensiling for 84 days, levels of ammonia-N reached 6% of total N in Stylo and 10% in taro leaves. The nutritional implications of these differences are hard to predict as the two forages differ considerably in other factors of nutritional significance (eg: tannins in the Stylo [Aijayi and Babayemi 2008] and oxalate in the taro [Du Thanh Hamg et al 2011]).

The increase in NDF content with duration of ensiling in the Stylo silage could be due to greater plant respiration and microbial fermentation leading to increases in temperature.  Increasing temperature in the fermentation process has been shown to increase the NDF content of alfalfa silage (Nelson and Bozich 1996). The reduction in NDF content with duration of ensiling of taro leaves is similar to findings reported by Chittavong et al (2008). This increased solubilisation of the fibre fraction with increased duration of ensiling of the taro leaves, possibly could be due to the more acid conditions in the ensiled taro. A decrease in NDF with increasing silage acidity was reported by McDonald et al (1991). 

Implications

By introducing the ensiling technique to smallholder pig farmers, the surplus feed produced during the rainy season could be preserved and used to overcome feed shortages during the dry season. This would markedly improve the nutrition of pigs under smallholder-farming conditions. It could also improve the economic returns by reducing feed costs, as protein from Stylo forage silage and taro leaf silage could be used to replace soybean protein in the pig diet.

Conclusions

• Incorporation of cassava or taro root meal as fermentation additives had no effect on the pattern of pH change in ensiled Stylo forage or ensiled taro leaves.

• Molasses as silage additive accelerated the fall in pH in both Stylo and taro leaf silage, with the effects more pronounced for the latter.

• Breakdown of crude protein to ammonia was greater in Taro silage reaching 10% of the total N fraction after 84 days, compared with 6% for the Stylo silage

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Received 21 February 2013; Accepted 24 March 2013; Published 2 April 2013

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