RESEARCH WORK
Evaluation of mixed silages of Saccharum officinarum and Gliricidia sepium using additives
R. Suárez1, J. Mejía1, M. González1, D.E. García2* y D.A. Perdomo3
2Departamento de Ciencias Agrarias, NURR, ULA
*E-mail: dagamar8@hotmail.com
3Instituto Nacional de Investigaciones Agrícolas (INIA), estado Trujillo, Venezuela
ABSTRACT
A trial was conducted in Trujillo, Venezuela, in order to evaluate the effect of the addition of molasses and urea in silages of burnt sugarcane tops and Gliricidia sepium, using macrobags of capacity 1,18 m3 and vacuum, by means of a 4 (type of silage) x 3 (fermentation time: 20, 40 and 60 days) factorial arrangement and five replications. The treatments were T1 (sugarcane tops, 75% plus G. sepium, 25%); T2 (T1 plus urea, 0,5%); T3 (T1 plus molasses, 4%) and T4 (T3 plus urea, 0,5%). The following indicators were determined: pH, DM, CP, ammonia, soluble nitrogen, CF, nitrogen-free extract, ether extract, Ca, P and ash. There was no silage type by fermentation time interaction. With the addition of urea (T2) the highest values of pH (5,03), CP (8,27%), NH3/Nt (18,13%) and SN/Nt (38,31%) were observed. With the addition of molasses (T3) a higher percentage of DM (35,34) and NFE (50,93) was detected, and with the two additives (T4) a lower content of EE (1,02%) was obtained. The CF, mineral and ash values did not show differences among treatments. After 40 days of fermentation a higher content of DM (44,80%) and NFE (49,72%) was observed. However, after 60 days a higher NH3/Nt (11,17%) and SN/Nt (33,61%) ratio was obtained. By pondering the most relevant variables, it was concluded that the best quality silages were obtained when adding only molasses (4%) or when it was combined with urea (0,5%); while the quality of the preserved material was lower 60 days after being elaborated.
Key words: Additives, Gliricidia sepium, Saccharum officinarum, silage.
INTRODUCTION
In Venezuela, as in other tropical countries, climatic variations and low quality of the pastures commonly used in animal production constitute two of the factors that limit the development of national livestock production. For such reason, feedstuff preservation as silage can be an alternative for the utilization of forage production and harvest residues, at times of higher availability.
In this sense, legumes show better nutritional value than the species from other botanical families, favoring mixed conservation with grasses (Cabral, 2007). Among tree legumes, Gliricidia sepium is one of the most promising species for animal feeding under tropical conditions. It is described as a multipurpose tree, but it can show variations in its acceptability when it is supplied fresh, because its chemical composition can vary according to age, part of the plant and provenance (Cabral, 2007).
In spite of the high protein levels, there are few studies conducted in the country with G. sepium as silage (Cabral et al., 2007). Considering that this legume shows high natural distribution in the Trujillo state, Venezuela, and that its foliage is not generally utilized by farmers as supplementary protein source for ruminants, this plant is underutilized in most animal production systems in the Venezuelan Andean Zone.
On the other hand, the Trujillo state is essentially agricultural and one of its main activities is sugarcane (Saccharum officinarum) production. The harvest process of this grass in the region implies burning the plantation before the harvest labors, which is done mainly manually. This procedure generates a large amount of plant material that is discarded and not utilized. Taking into consideration the advantages of this source regarding its feasibility to be ensiled (Siqueira et al., 2009), burnt sugarcane tops constitute an adequate material for its conservation with other forages that enrich the silage integrally.
Likewise, the use of additives is known to enhance forage conservation, by benefitting a predominance of lactic bacteria during the fermentation process. In this regard, molasses represents one of the most frequently used carbohydrate sources in silages (Titterton and Bareeba, 2000). Additionally, the use of urea also improves quality, by decreasing the yeast and fungal populations, and generally reduces the losses of dry matter and soluble carbohydrates. In addition, a better chemical composition has been obtained in silages enriched with this nitrogen source as compared to the ones which have not been treated (Cabral, 2007).
Considering that mixed silages (grasses-legumes) have advantages, based on the utilization of the protein and nutritional values of legumes and the better fermentative characteristics of grasses (Ojeda et al., 2006), the objective of this trial was to evaluate the effect of the addition of molasses and urea in silages of burnt sugarcane tops and G. sepium.
MATERIALS AND METHODS
Trial location
The experiment was conducted in areas of the Underutilized Resources Research Unit (UNIRS) (9º25'11" N and 70º28'22" W) located in the "Rafael Rangel" University Nucleus of the University of Los Andes (ULA), Trujillo state, Venezuela, at 419 masl, in a zone of tropical dry forest with average temperature of 27,4ºC, annual rainfall of 1 690 mm and relative humidity of 60%.
Provenance of the plant material
The forage material (S. officinarum and G. sepium) was obtained from plantations of the Experimental and Agricultural Production Station «Rafael Rangel» (EEPARR) of the University of Los Andes, La Paz parish, Pampán municipality, Trujillo state, Venezuela, located at 9º35'00" N and 70º27'00" W, at 300 masl, in a zone of tropical dry forest with relatively uniform temperature throughout the year (22-27ºC) and cumulative mean annual rainfall of 1 480 mm.
Cutting, collection, transportation and chopping of sugarcane tops
The whole tops (variety: Puerto Rico 61632) previously burned (pre-harvest procedure in the Trujillo state) were collected. The crop had been planted at a density of 800-1 000 cuttings/ha. The sugarcane used for obtaining the tops had optimum harvest age (11 months).
The biomass was collected directly from the soil (1 500 kg), using a harvester used to fill sugarcane trucks, which facilitated the loading and collection labors.
The collected material was transported to the UNIRS laboratory. The next day it was chopped by a chopper-mill, into 3-5 cm chops; although this chopping is not considered optimum for silage elaboration, the machine used for chopping is the same as the one used by farmers in Trujillo.
G. sepium cutting, collection, transportation and chopping
The G. sepium foliage was obtained from the pruning of trees used as living fences. It was manually harvested from eight trees, by machetes, in order to obtain 500 kg of forage (leaves and leaflets). The harvested material was carried to the UNIRS facilities and it was homogeneously extended on a cement floor for pre-drying (nine hours-sunlight). The foliage was chopped to a size between 3 and 5 cm.
G. sepium, at the moment of foliage collection, was in vegetative state and it was eight years old. The regrowth age of the biomass was 90 days.
The chemical-nutritional composition of the two forage components used in the elaboration of the silages is shown in table 1.
Component weighing
The whole tops, G. sepium and molasses were weighed by means of a scale with 20 kg capacity. The urea (Petroquímica de Venezuela S.A.) was weighed in an AND electronic scale (model EK-120A) with capacity of 120 ± 0,01 g.
Silage elaboration
Four types of mixed silages were formulated (T1, T2, T3 and T4).
• T1: 15 kg of sugarcane tops (75%) plus 5 kg G. sepium (25%). First, the components were manually pre-mixed; afterwards the mix was homogenized using a Magnum type Kohler mixer, until obtaining a homogeneous mix.
• T2: T1 and addition of 100 g of urea diluted in 100 mL of water, which represented 0,5% of the mix total. It was prepared in a similar way as T1, adding the non-protein nitrogen source into the formulated mix, before stopping homogenization.
• T3: T1 and addition of 800 g molasses diluted in 1,6 L water, which represented 4% of the mix total. Its preparation was made using the procedure described for T2.
• T4: T1 and addition of 5 kg G. sepium (25%) and 100 g urea (0,5%) plus 800 g molasses (4%). The mixing and homogenization of the forages was made as in the previous treatments. The molasses and urea were diluted in 1,6 L of water until obtaining uniform dilution, and it was added to the plant material at the moment it was being mixed in the mixer.
Experimental units. Black Implasval® polyethylene bags of 20 micra; 1 m diameter and 1,5 m long (volume: 1,18 m3), putting 20 kg of forage material in each one, according to the treatment.
Air extraction in the silo-bags. The vacuum system was used, in order to extract the air contained in the bags at the moment of ensiling. A two-piston compressor with ½ horsepower engine was adapted. In the suction nozzle a 2-m long hose with ¾ inch diameter was adapted. In the hose mouth a filter was placed to prevent the entrance of plant material to the compressor. First, the free end of the hose was introduced in the silo-bag, until the first third, at the moment the filling took place. When the material was introduced, the free end of the bag was coiled around the hose, and at the same time pressure was exerted on it, in order to minimize the air escape at the moment of its extraction; then the compressor was activated, which worked for three minutes. At the end of this process the hose was withdrawn without loosening the coiling. A rubber band was placed on the coiled end, it was folded over itself and a second rubber band was placed, leaving the opening of the bags sealed.
Sampling. Three samplings were randomly made post-elaboration, at 20-day intervals (20, 40 and 60 days). For each fermentation time, five bags of every silage type were sampled, extracting a representative portion from the center.
Sample treatment. The collected samples (300 g) were dried in a forced-air oven for 72 h at 40ºC, in order to avoid the possible loss of volatile compounds. To grind the samples the chopper-mill was used, but a sieve was placed to obtain an adequate size (1 mm). The material was introduced in airtight bags and they were duly identified for their transportation to the Laboratory of Chemical Analysis of Convaca® (Concentrados Valera C.A), located in Valera City, Trujillo state, Venezuela.
Measurements. The pH was measured in the fresh sample, with an Orion glass hydronium ion-selective electrode. Crude protein (CP) (fresh sample), crude fiber (CF), nitrogen-free extract (NFE), ether extract (EE), calcium (Ca), phosphorus (P) and ash were determined by the traditional methods (AOAC, 1995). The DM was determined in a forced-air oven, for 48 hours at 70ºC, in the newly-collected material. Soluble nitrogen (SN) and ammonia (NH3) were determined from the extraction of silage juice by means of hydraulic press, according to the methodology proposed by Dulphy and Demarquilly (1981).
Ponderation of nutritional indicators. In order to evaluate hierarchically the silages due to the quantity of studied indicators and their possible interactions, a ponderation of the most important indicators was made, to discern the superiority of a treatment over another (Ojeda and Montejo, 2001).
The adopted methodology was using the superscripts of the means as defining element of the differences among treatments, as basis to assign the score. In the cases in which there were two superscripts for a value, the mean of the established ponderations was taken. The system used for the indicators is shown in table 2. In the case of the pH/DM ratio the indications given by Ojeda and Montejo (2001) for the scoring table (0-10) that relates these two variables were followed.
Experimental design and statistical analysis. A factorial arrangement (4 x 3) and five repetitions were used. The evaluated factors were the silage types (T1, T2, T3 and T4) and the opening times of the bags (20, 40 and 60 days post-elaboration). The information was processed using the statistical pack SPSS 10.0 (Visauta, 1998), through a general lineal model, with a significance degree of P<0,05. Those variables that showed significant differences were subject to Duncan's test (Duncan, 1955). The correlations were made with the same statistical pack, using Pearson's correlation coefficient.
RESULTS AND DISCUSSION
Tables 3 and 4 show the effect of silage type and fermentation time on the measured variables. No interaction of silage type by opening time of the silo-bags (P>0,05) was observed for any of the variables.
The pH was high, as compared to the traditional grass silages, because it oscillated between 4,53 and 5,03, while values lower than 4,2 would be desirable. This is considered to have been caused by the presence of tree foliage (G. sepium) in the silage, because legumes are difficult to ensile due to their high content of ash and protein. Nevertheless, their usage is justified by the increase of the protein fraction, in addition to the fact that these pH values did not hinder the fermentation process, according to the evaluated variables.
With the addition of urea at 0,50% in the silage of sugarcane tops and G. sepium, the highest pH was obtained (5,03). However, in the remaining treatments the values varied between 4,13 and 4,53, without significant differences between the acidity of the silages. Similar results were obtained by Santos et al. (2006), who evaluated sugarcane silages with different additives at two pruning ages. These results also corroborate the reports made by many authors regarding the fact that the inclusion of tree foliage hinders the ensilage process, as compared to the one obtained with grass only, because the tree biomass shows a very different fermentation pattern from that of these species.
The values obtained in this study are similar to the ones reported by Siqueira et al. (2009), who proved that with the inclusion of burnt sugarcane tops in the initial stage of fermentation in silos, the pH showed a moderate range between 3,9 and 6,4; while these authors stated that with the addition of urea at 1,5% the highest pH was observed in all the treatments with additives (4,1-5,8), results which coincide with the ones in this study.
In this sense, urea decomposition in an acidified medium produces carbon dioxide and ammonia ions, which act as buffers in the pH decrease (Siqueira et al., 2009). Yet, the medium-term invariant performance of pH indicates that the fermentative process, ruled by the inclusion of tree foliage, is an intrinsic characteristic of this type of preserved material.
Likewise, the pH values obtained with the treatments are within the range reported by Vallejo (1995) in silages of non-legume plants such as Morus alba (4,0-4,3), Guazuma ulmifolia (3,8-5,2), Trichanthera gigantea (5,2-7,19) and Malvaviscus arboroeus (4,2-5,6) subject to different conservation strategies, which indicates that the high pH values are characteristic of silages in which tree foliage of any species has been added, independently from their taxonomic location.
The highest DM values were observed in T1 and T3. Nevertheless, with the addition of urea and the joint incorporation of urea and molasses the DM content was lower (P<0,01). However, in all the cases it was higher than 25%, established as adequate in tropical silages when there is no effluent production (Clavero and Razz, 2008). In addition, the results are higher than the ones reported by Bravo et al. (2006) in silages opened after 30 days and elaborated with sugarcane without pre-wilting, in the presence and absence of additives.
The CP contents were significantly higher in T2 and T4, in which the non-protein source was added. Similar results were reported by Oliveira et al. (2007) in silages of grasses enriched with urea, and Cabral (2007) in sugarcane and G. sepium silages to which organic nitrogen was added.
If it is considered that in the last decade higher importance has been allotted to the CP content as fundamental nutritional element in a preserved feedstuff, within the evaluation system of tropical forages, the four treatments constitute good choices for feeding ruminants where these two forage sources are available. However, the NH3/Nt and SN/Nt ratios were higher in the treatment in which only urea was included.
The highest ammonia concentration in T2 is justified by the inclusion of the nitrogen source. In this sense, 9% ammonia with regards to Nt in silages has been established as adequate and not causing nutritional problems in ruminants. Yet, the higher percentage in T2 and T4 is within the values reported by Oliveira et al. (2007) for silages of Brachiaria sp., Panicum sp. and Pennisetum sp., enriched with 5% urea, which proves that the inclusion of urea in grasses generates a variable NH3 production regarding the fermentation conditions and the characteristics of each species.
The CF values did not show significant differences among treatments. The values varied between 33,20 and 36,06%, which are considered high, due to the fact that the highest silage proportion was constituted by burnt sugarcane tops, which shows a high cell wall fraction (Cabral, 2007). Although at present the CF determination is almost not used to characterize feedstuffs, because it has been proven that the fractioning of the cell wall shows higher correlation with the indicators of nutritional value and animal response (Van Soest et al., 1991), it constitutes an indicator which describes, approximately, the fibrous characteristics of nutritional components and forages.
On the other hand, higher NFE were observed in T1 and T4. In this sense, it has been proven that NFE is correlated to other nutritional fractions, such as CF and EE. Hence the importance of considering this variable, because in many cases it has been associated to the energy potential of feedstuffs (Chinea et al., 1999), and constitutes a routine determination within their evaluation process.
On the other hand, EE, Ca and P contents, as well as ash, did not show significant differences among treatments (P>0,05). The Ca and P values have also been included as relevant variables, considering that feedstuff conservation as silage in the tropics is aimed at achieving that this source of nutrients is used preferably in the elaboration of the rations in the dry season, when the roughage availability is scarce. The values of these minerals were similar, and in some cases higher, as the ones reported by Minson (1992) for a wide group of tropical grasses destined to feeding ruminants.
Regarding the effect of fermentation time, it was observed that after 20 days there was a higher CF content. Nevertheless, after 60 days the DM content was lower and there was a higher proportion of NH3/Nt and SN/Nt, which proves the fermentation of the protein fraction and urea decomposition. In this sense, although a higher SN/Nt ratio is good in terms of conservation, values lower than 40% are considered adequate and their increase in time describes that the CF solubilization process continues to occur.
These results indicate that a nitrogen loss possibly occurred in the silage because of volatilization as NH3, aspect which is also manifested in its higher content regarding Nt and the higher amount of SN throughout the fermentation process. Such performance has been described, in general, by Cabral (2007) in silages elaborated under tropical conditions; perhaps for this reason there was a variation in the content of DM and other associated fractions.
The fact that the pH did not decrease during the time the trial lasted may be the cause of the losses and deterioration, in terms of the measured variables, which was observed 60 days after elaboration.
In the intermediate opening time (40 days) the highest DM and NFE values were observed, which ratifies the statements by Chinea et al. (1999) regarding the fact that the optimum utilization time of tropical silages is limited, as compared to the ones elaborated in the subtropics, if it is considered that the NFE content generically represents highly digestible carbohydrates (Van Soest et al., 1991). Hence the need of planning rational forage conservations for the critical stages, strictly regarding the needs of production units, due to the spontaneous aerobic deterioration that occurs in these feedstuffs.
Table 5 shows the results related to treatment hierarchization and the estimation of the silage nutritional value losses in time.
Among all the silages the one with the highest score was T3, followed by T4, T1 and finally T2, which indicates that the usage of combined additives, or just molasses, in silages of burnt sugarcane tops and G. sepium is positive, because it integrally improves the quality of the preserved feedstuff. However, the score obtained by T2 was relatively far from the potential maximum value of the ponderation (18 points), for which this silage did not show a good performance in most variables.
When analyzing the effect of the fermentation time, after 20 and 40 days the highest scores were observed. After 60 days the value abruptly decreased, which describes a lower integral nutritional quality of the silage. This performance has been also described by Cabral (2007), who stated that under tropical conditions fermentation in the aerobic and anaerobic stages occurs more rapidly than in other regions, conditioned by high temperatures, types of microorganisms present and climatic changes, among other factors. This is in addition to the singular fermentative characteristics of silages with tree foliage and the pH stabilization over 4, which facilitates the appearance of silage-damaging microorganisms, such as saccharolytic and proteolytic Clostridium, as well as some yeasts that promote medium-term aerobic deterioration of silages (Ojeda et al., 2006).
It should also be stated that silage elaboration in macrobags of 1,18 m3 capacity, disregarding other smaller ones (1-2 kg), commonly used at laboratory scale, was conducted in order to simulate, approximately, real elaboration conditions of silages in the Trujillo farms, where in practice the aerobic deterioration of silages is observed when they are made from large substratum quantities. In general, this performance is not observed in the studies made under controlled conditions, which are away from the reality that is lived in the farms of livestock-producing farmers (Patto et al., 2004).
Likewise, in the Trujillo state there is no bunker-silage culture, but it is done in tanks, in small recipients and bags; hence the importance of validating this technology for its large-scale usage by farmers.
The correlations among the measured variables in the silages are shown in table 6. No significant relationship was found between pH and DM. However, the pH values were significantly related to CF, NH3/Nt ratio and calcium values (P<0,05).
The DM content was negatively related to the NH3/Nt ratio (P<0,01) and to SN/Nt (P<0,01). The CF concentration showed a negative link to Ca levels (P<0,01); while the NH3/Nt ratio was positively related to SN/Nt (P<0,01). The CF values showed a negative relationship to NFE (P<0,01) and a positive one to EE (P<0,01).
On the other hand, the NFE also showed a consistent relationship with EE (P<0,05). However, the P and ash contents did not show relationship with any of the measured chemical variables.
Through the correlation analysis it was proven, in the case of the silages of sugarcane tops with G. sepium and additives, that high pH values are closely related to protein contents in the silage and the ammonia proportion generated, which is possibly capable of neutralizing the acidity produced by volatile fatty acids, especially lactic acid; in this regard, Clavero and Razz (2008) referred to the neutralization that occurs in tropical silages with the appearance of metabolites which have basic properties.
On the other hand, the high CP values, originated by the addition of urea, were also observed to affect the fermentative process which generated the pH decrease, taking into consideration that the nitrogen concentration only increased regarding the addition of non protein nitrogen.
On the other hand, with the Ca increases a positive effect was detected on the pH decrease. This performance has not been documented in studies with silages, which should be the topic of future research. Maybe the relationship between acidity and this metallic element (Ca) is superfluous and the real link occurs with the buffering capacity of silages, if it is taken into account that the latter is a variable of high influence on the medium-term pH performance. Although calcium is considered a passive element within silages, it is also characterized as a soft cation with stressed basic characteristics in biological media; this is an important reason for considering the possibility of an underlying relationship between the buffering level and the concentration of such element under the experiment conditions.
It was also established that the decrease of DM contents which occurs in these silages, is significantly correlated to the ammonia production and the solubilization of protein nitrogen as well as of the nitrogen from urea fermentation, because the NH3 and SN contents showed an important link between themselves.
The correlation between the CP values with NFE and EE, although little documented, describes at least the practical relevance which can mean silage characterization only regarding one of these variables. In this regard, the statistical relationships between NFE and EE and other nutritional indicators often do not have biological significance, among other reasons because the bromatological variables from routine fractioning do not show the complex process of biochemical transformations that occurs in silages.
The little relation between ash and other variables, as well as the insignificant influence of these fractions on the fermentation of the evaluated silages, except Ca, shows that they are intrinsic chemical characteristics of the components used for elaborating the preserved material and that the proportion of minerals, at least in these cases, does not directly affect fermentative processes.
CONCLUSIONS
Burnt sugarcane tops and G. sepium foliage are potentially valuable resources for the elaboration of silage with or without additives, which nutritional composition is adequate for supplementing ruminants. The inclusion of 4% molasses, or its combination with urea, produces the best results.
Independently from the use of additives, under the conditions the trial was conducted the silage was found as having good characteristics during the post-elaboration period. However, after 60 days of fermentation it showed a lower integral nutritional quality.
RECOMMENDATIONS
The vacuum silage technique in 1,18 m3-polyethylene bags represents an alternative for studying the conservation of underutilized resources as silage, because it constitutes a similar technology as the one used in commercial lines in Trujillo state, Venezuela.
