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Paper 9.0: Additives to improve the silage making process with tropical forages - Paulo R.F. Mühlbach


Paulo R.F. Mühlbach


Departamento de Zootecnia, Faculdade de Agronomia
Universidade Federal do Rio Grande do Sul

Porto Alegre, Brazil.

E-mail: muhlbach@orion.ufrgs.br

INTRODUCTION

This contribution focuses on the ensilage of forages produced in tropical and subtropical climates. The low DM and WSC content of tropical (C4) grasses results in poor fermentation of freshly cut material. Wilting could be beneficial, but during adverse climatic conditions wilting would need to be prolonged, which might lead to poor fermentation due to proteolysis by endogenous enzymes, which is reflected by a lower “true protein” proportion in the forage and, consequently, a higher ammonia-N proportion in the silage (Table 1). Use of certain additives may be an alternative to wilting, particularly with thick-stemmed, erect fodder crop grasses (Pennisetum, Panicum, etc.) that produce a large mass of plant material, where pre-conditioning and handling is difficult to mechanize and labour-consuming. Tropical forage grasses (Cynodon, Brachiaria, Digitaria, Setaria, Chloris, etc.) can be wilted more easily but, when wilted excessively it affects compression in the silo and thus fermentation quality (Catchpoole and Henzell, 1971). Even under controlled wilting conditions, additives are being recommended to improve fermentation and nutritive value of conventional as well as round bale silages (Bates, et al., 1989; Staples, 1995).

In farm situations, silage making often faces drawbacks which compromise the basic principles of silage making, especially where technology is limiting, such as with small-scale producers in the tropics and subtropics (Bayer and Waters-Bayer, 1998). Additives can never be a substitute for good ensiling management. For example, additives will not make up for the negative effects on fermentation quality of tropical forages caused by practices such as the use of low quality, oxygen-permeable plastic covers, or extended storage under temperatures in excess of 30°C (Tjandraatmadja et al., 1991).

It should also be emphasized that the efficacy of any additive will ultimately be assessed by animal performance and by DM recovery from the silo, which are parameters not commonly determined. Most of the experiments are restricted to measurements of traditional fermentation patterns under controlled laboratory conditions, where even untreated silages made from thick-stemmed Pennisetum species may show acceptable preservation (Woodard et al., 1991; Spitaleri et al., 1995). In contrast, bad fermentation products, such as biogenic amines that cause intake depression in ruminants (Phuntsok et al., 1998) are not detected by conventional silage analysis. It has been suggested that the current parameters used to predict silage fermentation and quality may need some re-evaluation (Jones, 1995).

BIOLOGICAL ADDITIVES

Inoculants and enzyme preparations are regarded as natural products that are safe to handle, non-corrosive to machinery, and do not cause environmental problems. Consequently, their use has expanded remarkably in the last decades. Perhaps no other area of silage management has received as much attention among both researchers and livestock producers as bacterial inoculants (Bolsen, 1999). There are many commercial products with variable efficacy available. However, dosage and method of application are decisive for effectiveness.

Inoculants

Based on a survey of inoculant studies, Muck (1993) concluded that inoculants are most successful in alfalfa and temperate grass silages and that with corn [maize] silage their success has been limited. However, Bolsen (1999) emphatically recommended that bacterial inoculants should be applied to every load of forage ensiled, based on results from over 200 laboratory-scale studies and from 28 farm-scale trials where this type of additive consistently improved fermentation efficiency, DM recovery, food efficiency, and liveweight gain per ton of crop ensiled in corn [maize] and forage sorghum silages.

Table 1. Effects of wilting on fermentation of tropical fodders


Forage

Silage

DM
(%)

WSC
(% of fresh material)

True protein
(% of CP)

NH3-N
(% of total N)

pH

Total acids
(% of DM)

Butyric acid
(% of total acids)

Lactic acid
(% of total acids)

Elephant grass (120 days growth)(1)

Unwilted

19.7

2.17

88.0a

9.4b

4.4

5.9

2.4

66.6

Wilted for 50 h

26.6

3.00

62.0b

14.8a

4.5

5.1

4.1

41.7

Millet (45 days regrowth)(1)

Unwilted

16.0

2.70

82.1a

6.2b

4.0

7.4a

0.9

78.6

Wilted for 48 h

31.0

4.24

58.5b

10.3a

4.2

4.5b

2.0

60.6

78% Millet+ 22% Cowpea(2)

Unwilted

20.8

1.40

78.6a

8.9

3.64b

7.5a

0.3

72.7

Wilted for 26 h

46.2

3.19

71.6b

7.5

3.85a

6.5b

0.3

71.3

60% Elephant grass+40% Cassava tops(3)

Fresh

20.5

1.62

84.7a

9.5b

3.8

8.9a

2.1

83.4

Wilted for 20 h

26.5

2.81

76.1b

11.7a

3.9

7.3b

2.2

79.5

Notes: a, b indicate means within column differing at P< 0,05.

Sources: (1) Pinheiro et al., 1986. (2) Figueiredo and Mühlbach, 1984. (3) Zanotelli and Mühlbach, 1989.

Enzymes

The addition of enzyme preparations either alone or combined with inoculants is proposed as a strategy to increase available substrate to improve lactic acid fermentation in silage or to increase nutritive value of forage, or both. With silages of temperate forages produced in subtropical conditions, an inoculant/enzyme mixture (Sill-All®) improved fermentation quality of unwilted forage oats (Avena strigosa) (Berto and Mühlbach, 1997) and reduced NDF contents of both unwilted and wilted alfalfa (Rangrab et al., 1996). The complex interactions occurring with the addition of both products is not completely understood and earlier results indicate varying degrees of success (Jaster, 1994). The first products were based on complexes containing ill-defined amounts of various enzymes from crude fermentations of fungi and results were inconsistent due to variable application rates, plant species, plant maturity and DM content of the materials (Muck, 1993).

Similarly, results have also been conflicting in experiments where enzymes were added to corn [maize] silage with no clear effects on fermentation characteristics, despite a decrease of ADF, NDF, and hemicellulose contents (Sheperd and Kung, 1996). Positive results with cellulase in combination with organic acid have been reported with the ensilage of temperate grass forages (Nadeau et al. 1996).

More recently, newer enzyme preparations have prompted a renewed interest in the potential of these products also as feed additives for ruminants, enhancing forage digestion and milk production (Yang et al., 1999; Schingoethe et al., 1999).

Results with tropical forages

If different types of additive suit different crops (Wilkinson, 1998) one should not expect that the effects achieved with biological additives on the ensilage of temperate forages will also be realized with tropical species, where fibre contents are generally much higher and more lignified. It stands to reason to consider that the constraints imposed by the structural, anatomical and chemical characteristics that are peculiar to tropical forages and which impair nutritive value, i.e. intake and fermentation in the rumen (Wilson, 1994, 1997), might also affect harvesting, wilting, chopping and compressing of the original material in the silo, as well as influence the direction of fermentation.

Whilst literature on the use of biological additives for temperate forages is relatively abundant, the information for tropical species is scarce. The data reviewed with this kind of additive is presented according to the type of tropical forage tested.

Sorghum spp.

Forage sorghum (Sorghum bicolor L.) is a fodder crop with a sweet juicy pith having types with finer, more numerous and more leafy stems (Bogdan, 1977). Of the tropical forages it is one of the best suited for ensiling.

When harvested at the soft dough stage (29% DM) it can have about 14% WSC and 50% NDF in DM, and after a 30-day fermentation period the silage can still present 10% WSC in DM. To such forage, a mixture of Lactobacillus plantarum and Streptococcus faecium (Pioneer brand® 1129) was added at 1.1 × 105 CFU per gram of fresh forage in a microsilo trial. Inoculation reduced silage pH but did not affect concentration or in vitro digestibility of NDF or ADF, neither did it prevent aerobic deterioration (Sanderson, 1993).

Froetschel et al. (1995) ensiled either untreated or inoculated forage sorghum harvested at the milk stage (61.5% NDF) in 900-kg capacity concrete tower silos. Silage lactic, acetic and total volatile fatty acids were increased 9.2 to 15.3%, and DM recovery was increased 7.1% with inoculation. The level of response was considered cost effective in a 1.7:l return on investment based on average prices for silage and inoculant. In a feeding trial with steers, inoculation did not influence digestibility of DM or fibre components of silages and silage-based rations.

A sorghum-Sudan grass hybrid was harvested at 90 days of growth (26% DM) in Puerto Rico and ensiled in laboratory silos, either untreated or inoculated with 106 CFU of Lactobacillus plantarum alone or mixed with a multi-enzyme complex containing arabinase, cellulase, ß-glucanase, hemicellulase and xylanase, applied at 0.1% of fresh material. The addition of inoculant either alone or in mixture with the enzyme complex improved silage quality as shown by lower pH and a greater LAB population (Rodriguez et al., 1994a). However, additives did not reduce aerobic deterioration of forage sorghum ensiled in a tropical environment (Rodriguez et al. 1994b).

Similar results with the ensilage of Sorghum bicolor were obtained more recently by Cai et al. (1999), where selected strains of either Lactobacillus casei FG 1 or Lactobacillus plantarum FG 10 isolated from corn [maize] and Panicum maximum were used at 105 CFU per gram of fresh matter. Both inoculants effectively improved fermentation, decreasing contents of volatile fatty acids and ammonia N and reducing gas production and DM loss as compared to the control silage. Again, the LAB-treated sorghum silages that contained relatively high concentrations of residual WSC and lactic acid suffered a faster aerobic deterioration than the control silage.

Johnson grass (Sorghum halepense) is a rhizomatous perennial, aggressive forage grass that can be established from seed for pasture and fodder (Mannetje and Jones, 1992). This forage was harvested at 45 (22.6% DM) and 110 days of regrowth (43.8% DM), chopped into 2.5-cm pieces, and ensiled in laboratory silos either untreated or treated with a mixture of Lactobacillus plantarum at a rate of 106 CFU per gram of fresh material plus 0.1% of a multi-enzyme complex with arabinase, cellulase, ß-glucanase, hemicellulase and xylanase (Rodriguez et al., 1998). For both stages of regrowth, Johnson grass treated with microbial inoculant plus enzymes had lower pH and higher LAB populations and higher lactic acid contents than untreated silage. Silage additives also decreased butyric acid content in grass ensiled at 45 days regrowth, and reduced ethanol content in the more mature forage. However, the authors also concluded that the resulting silage did not meet the criteria for good quality silage, suggesting more research to evaluate other sources of addditives, as well as the rates of additives used.

Pennisetum spp.

Elephant or Napier grass (Pennisetum purpureum) has thick, erect stems 2-6 m tall, with 30-120-cm long leaves and has been introduced to practically all tropical countries, being widely grown for fodder, and less often for grazing (Mannetje and Jones, 1992). Van Onselen and López (1988) reported on a trial from 1981 with the use of a commercial enzymatic product (sucrase and cellulase) added to elephant grass from a 105-day regrowth (19.4% DM) at a rate of 0.1% of fresh forage plus 0.9% corn [maize] meal. When compared to a control treatment with 7% corn [maize] meal the enzymatic product showed higher pH and ammonia-N values and a lower lactic acid content, resulting in a silage of bad quality.

A 60-day regrowth of elephant grass (14% DM, 70% FDN in DM) was ensiled in 200-litre plastic containers, testing the effects of two commercial products with bacteria and enzymes (Bio-Silo® and Bio-Silo P.U. soluble®, from Katec Kaiowa Ltda., Brazil). The product Bio-Silo was added at 0.1% of forage fresh matter plus 0.9% of corn [maize] meal, while Bio-Silo P.U. was diluted in water and also added at 0.1%, according to the manufacturer’s recommendations. No effects of additive use were detected, neither on silage composition and pH and ammonia-N values nor on nutrient intake and digestibility coefficients measured with sheep (Henrique and Bose, 1992).

Tamada et al., (1999) conducted experiments in different latitudes in Japan with different harvest dates of Napier grass and two silage storage temperatures to test the effects of a mixture of cellulases (Acremonium cellulolyticus and Trichoderma viride) alone or mixed with a commercial inoculum (Lactobacillus casei, 108 CFU/kg wilted forage) and of a preparation of fermented green juice extracted from macerated Napier grass alone or mixed with the cellulases, as compared to a control. All treatments were applied to wilted Napier grass (averaging 22.7% DM and 4.6% WSC in DM) ensiled in 0.9-litre bottles; in two experiments, 40 g of glucose/kg wilted forage was included to each treatment with additive. Improved fermentations with lower pH values and ammonia contents and increased lactic acid over control were obtained only when sufficient fermentable substrate was secured by adding the cellulases or glucose.

Kikuyu grass (Pennisetum clandestinum) is a creeping perennial with strong, thick stolons, requires fertile soil, can be associated with white clover, but is not well adapted to high temperatures (Mannetje and Jones, 1992). De Figueiredo and Marais (1994) used wilted Kikuyu grass (30% DM, 3.2% WSC in DM) treated with inoculant alone (Lactobacillus acidophilus + Lactobacillus bulgaricus at 5 × 104/g grass ensiled) or in combination with two different enzyme preparations (either from Trichoderma reesei or from Aspergillus spp.). The best fermentation - with lower pH and ammonia-N and higher lactic acid - in micro-silos of polythene bags was obtained with the combination of inoculant plus the enzyme from T. reesei. In a second experiment, the authors used the same forage unwilted (19.2% DM, 3.7% WSC) alone or with an inoculant (Lactobacillus plantarum strain MTD/1) alone and in combination with molasses meal (5 or 10% on a DM basis) at ensiling. The only significant effect as compared to untreated silage was a pH decrease with the inoculant combined to the 10% molasses level.

Pearl millet (Pennisetum americanum). A late summer crop of pearl millet grain hybrid (HGM-100) at the soft dough stage of grain maturity (18.9% DM, 60.2% NDF in DM) was wilted, treated with inoculant (Pioneer 1174®) and stored in a concrete stave silo. The resulting silage was poorly preserved, with a predominantly acetic fermentation (4.23% acetic acid in DM) and the need to add a low level of readily available carbohydrates was indicated (Utley et al., 1995).

Other genera

Bermudagrass (Cynodon dactylon) is a stoloniferous and rhizomatous perennial, growing in the tropical, subtropical and warm temperate regions, with cultivars that tolerate frosts (Mannetje and Jones, 1992). Wilted bermudagrass conserved as round bale silage is being used in the southeastern USA as an alternative to hay making, and some of the first tests with a combination of enzymes containing cellulase and an inoculant showed potential to improve fermentation and DM recovery (Bates et al., 1989).

A thorough study by Umaña et al. (1991) was conducted with bermudagrass cv. Tifton 81, harvested at the late jointing stage of growth and ensiled either unwilted (32.4% DM, 2.85% WSC in DM) or wilted (44.1% DM, 4.14% WSC in DM). Both materials were chopped and either left untreated or dried sugar cane molasses at 5% of forage DM added; inoculated with a mixture of Lactobacillus plantarum and Streptococcus faecium (1174 Pioneer ® at 3 × 105 CFU/g DM); prepared with a combination of the inoculant treatment plus the dried sugar cane molasses, all treatments being packed by hand in 19-litre plastic containers. According to the authors, all unwilted silages went through a less than satisfactory fermentation, whilst the application of molasses and inoculant to wilted bermudagrass had an additive effect and produced stable silages having the lowest pH, lowest concentration of ammonia, and greatest lactic acid concentration and in vitro organic matter digestibility. Hence, according to the Cooperative Extension Service from the University of Florida, adding a bacterial inoculant and molasses to wilted bermudagrass is more beneficial than adding just molasses or inoculant alone (Staples, 1995).

Rhodes grass (Chloris gayana) is a stoloniferous, creeping or occasionally tufted perennial that thrives under a wide range of tropical and subtropical temperatures (Mannetje and Jones, 1992). Ridla and Ushida (1998) used a first growth of Rhodes grass harvested at the heading stage (21.8% DM, 5% WSC and 66.4% NDF in DM) ensiled in 2-litre vinyl bottle silos. An inoculant with Lactobacillus casei was added at 105 CFU/g fresh sample, either alone or combined with increasing levels of cellulases (‘A’ Acremonium cellulolyticus and/or ‘M’ Trichoderma viride). The combined treatment with inoculant plus enzymes showed lower pH values and higher lactic acid contents with increasing amount of cellulase addition. All combined treatments reduced NDF, ADF and in vitro DM digestibility of silages compared with the untreated silage, probably meaning that enzymes hydrolyzed especially the more digestible components of plant cell wall. The combinations inoculant plus cellulase ‘A’ were the most effective. A parallel test with fermentation temperatures suggested that samples incubated at 40°C resulted in better silages than those at 20° or 30°C. It was also concluded that the absence of effect in the inoculant treatment was due to the low WSC available in Rhodes grass. These same treatments were also applied to Italian ryegrass harvested at the heading stage (21.7% DM, 7.2% WSC and 59.2% NDF in DM) and the results on NDF and ADF disappearances of the inoculant and enzyme mixtures as compared to those with Rhodes grass suggest that the cell wall components in Rhodes grass silages were more resistant to degradation by the cellulases (Ridla and Ushida, 1999b). In general, within the various treatments for fermentation products and chemical composition, ryegrass produced better silages than Rhodes grass (Ridla and Ushida, 1999a).

Weeping lovegrass (Eragrostis curvula) is a densely tufted perennial, stems slender to robust, 30-120 cm high, drought-resistant (Bogdan, 1977). A six-week growth of this forage (37.8% DM, <2% WSC, 79.2% NDF) was ensiled unwilted, but treated with an inoculant/enzyme mixture (Sill-All® with L. plantarum, S. faecium and Pediococcus acidilactici at 106 CFU/g fresh material) resulting in a silage with lower pH, ammonia-N, butyric and acetic acid content and a higher lactic acid content compared to the control silage (Meeske, 1998).

FEED INGREDIENTS AND BY-PRODUCTS AS ADDITIVES

The incorporation of easily fermentable feed ingredients such as sugar or molasses to low-DM, sugar-limited tropical forages is a way to improve silage fermentation. Feed-grade products such as grains in general and processed by-products such as corn [maize] or sorghum meal, rice bran, cassava meal, citrus pulp, etc., can also be used as additives, partly to provide fermentable substrate, but also to direct the course of fermentation by absorbing excessive moisture. To optimize their effectiveness by avoiding effluent losses, they have to be used in relatively high rates (aiming for a DM content >25% of the mixture) and adequately mixed with the chopped forage, which demands extra labour and/or appropriate equipment. This type of additive may be of seasonal and local supply, and cost effectiveness assessment should also consider the improvement achieved in nutritive value.

Molasses

Cane molasses (75% DM) has been widely used, added up to 10% w/w to provide fast fermentable carbohydrate for the ensilage of tropical herbages. Due to its viscosity, it is difficult to apply and should be diluted, preferably with a small volume of warm water to minimize seepage losses. When applied to tropical grasses, molasses should be used in relatively high concentrations (4 to 5%). With crops of very low DM content, a considerable proportion of the additive may be lost in the effluent during the first days of ensilage (Henderson, 1993).

However, according to Woolford (1984), the provision of extraneous sugar alone is not sufficient to permit the LAB to compete with other components of the silage microflora and thus ensure preservation. So, under high moisture conditions, molasses can also induce a clostridial spoilage, especially with forages contaminated with soil.

Sugar cane molasses added at the rate of 3% (w/w, fresh basis) to Napier grass (12.9% DM, 6.6% WSC) produced silages of reasonably good fermentation quality, reducing, however, the nutrient recovery from the silo, as compared to formic-acid-treated silage (Boin, 1975). The same molasses dose also resulted in increased in vitro DM digestibility coefficients for Napier grass ensiled at 51, 96 and 121 days of vegetative growth (Silveira et al., 1973).

Dwarf elephant grass (cv. Mott) cut at 72 days regrowth (14.4% DM, 7.1% WSC) with a high buffering capacity, was treated with 4% molasses and ensiled in 4-kg polythene bags, with the resulting silage having lower pH and ammonia-N than the control silage (Tosi et al., 1995).

Four levels (0, 4, 8 and 12%) of dried molasses (97% DM) were applied to chopped bermudagrass (32.4% DM, 70.2% NDF) pre-treated with 1174 Pioneer® silage inoculant (1.7 l/t of forage) and packed in 19-litre plastic containers. The increasing molasses levels lowered pH, ADF, and NDF percentages and increased in vitro DM digestibilities in bermudagrass silages (Nayigihugu et al., 1995).

Guinea grass (Panicum maximum) at 4- and 8-weeks old (18.6% DM, 26.5% DM, respectively) was ensiled untreated or with 4% molasses in 400-g laboratory silos. The pH varied from 4.4 to 5.4 and from 4.0 to 4.7, and ammonia-N ranged from 23.5 to 35.3 and from 15 to 39, respectively, for untreated and molasses-treated silages (Esperance et al., 1985).

Tjandraatmadja et al. (1994) tested the effects of 4% and 8% molasses added at the ensilage of Panicum maximum cv. Hamil, pangola grass (Digitaria decumbens) and setaria (Setaria sphacelata cv. Kazungula) harvested at 4, 8 and 12 weeks of growth. The results from a laboratory trial with 500-g vacuum-sealed silo bags kept in a dark, temperature-controlled room led to the conclusion that 4% (w/w) molasses should be sufficient to achieve effective preservation. Pangola grass, which had a highly significant different chemical composition prior to ensiling, with lower NDF and lignin content, presented a dominant homofermentative LAB population in silage, which was fairly well preserved even without molasses.

Starch sources

It is controversial to what extent starch is an available substrate for LAB (Woolford, 1984). Jones (1988) recovered 100 and 90% of starch from barley and oats, respectively, added at the ensilage of ryegrass, attributing an improved fermentation to the substrate available from 3 to 4% of soluble carbohydrates or from fractions such as ß-glucan contained in the cereals, and not to a hydrolysis of starch.

The effects of adding molasses (5% w/w) or ground maize (5% and 10% w/w) to star grass (Cynodon nlemfluensis) alone or mixed with four levels (0, 15, 30, 45% w/w) of legume (Desmodium uncinatum) were studied in a laboratory trial by Sibanda et al. (1997). In general, both additives improved fermentation up to the level of 30% of legume inclusion, but addition of molasses resulted in lower levels of volatile N and higher lactic acid content compared to the control and both ground maize treatments.

A first growth of Napier grass was hand-harvested under rainy conditions (8.6% DM, 67.6% NDF), chopped to 3 cm, treated with 4% molasses and/or 15% de-fatted rice bran (2.0% crude fat) on a fresh grass basis and ensiled in plastic bags. DM contents of silages were 13.4%, 20.1% and 22.5%, and spoilage losses were 5.6%, 0.3% and 3.0% for treatments with molasses, rice bran and their mixture, respectively. Treatment with plain rice bran had the highest content of acetic (6.7% of DM) and propionic (1.4% of DM) acids and ammonia-N, but the lowest content of lactic acid. The authors (Yokota et al., 1998) concluded that the combination molasses and rice bran could improve the fermentation quality and enhance the utilization of the silage by goats, more than each additive as a single treatment.

Cassava (Manihot esculenta) tuber meal (72.1% WSC) and coconut (Cocos nucifera) oil meal (17.6% WSC) were both added (5% wet basis) to Guinea - A (Panicum maximum) with 17.7% DM and 6.3% WSC and to NB-21 (Pennisetum purpureum × Pennisetum americanum) with 16.3% DM and 9.9% WSC forages, chopped (1.5 cm) and ensiled in 2-kg laboratory silos. Both additives improved fermentation compared to untreated silages of both forages, with greater effects in silages with cassava tuber meal (Panditharatne et al., 1986).

Elephant grass was harvested at 75 days growth (19.4% DM, 72% NDF) and ensiled in 300-kg asbestos cement containers, either unwilted or wilted (29.6% DM), both materials with or without 8% ground sorghum grain (w/w). Wilting was achieved by exposing crushed forage stems three hours in windrow after harvesting with a New Holland mower-conditioner. Sorghum addition to both wilted and unwilted silage increased DM contents, reduced ethanol and acetic acid contents and increased intake of digestible energy as measured in sheep (Alberto et al., 1993). Silages of elephant grass cv. Guaçu were obtained, adding 0, 8, 16 or 24% (w/w) either of ground ear corn [maize] with husks, wheat bran or “sacharin” (urea-treated sugar cane, with 12.6% CP, 17.5% crude fibre in DM) to unwilted forage (12.4% DM, 10.4% WSC) harvested with a precision chopper (3 mm chop length) and packed into 200-litre plastic containers with a layer of ground hay at the bottom to absorb effluent (Andrade and Lavezzo, 1998a). Ground ear corn [maize] was more effective in increasing DM content and restricting lactic acid production while reducing ammonia-N, which reached 31.3% and 36.2% for the “sacharin” and wheat bran treatments, respectively (Andrade and Lavezzo, 1998b).

The fermentation pattern of wilted elephant grass cv. Taiwan-A146 silage (8 hour wilting, 26.6% DM, 6.74% WSC) did not differ from silages made of unwilted grass (23.5% DM, 7.2% WSC) prepared with a cassava starch by-product added at 2, 4, 8 or 12% (w/w). According to the authors (Ferrari et al., 1999), the relatively low lactic acid levels demonstrate that the substrate was not available to LAB.

Citrus pulp

Fresh citrus peels have been added with levels up to 50% to the ensilage of Napier grass, improving fermentation quality as measured by low pH values and low butyric acid content and adequate lactic acid production (Faria et al., 1972). Citrus peel may contain 50% WSC in DM but the low DM content (14 - 21%) and intensive initial fermentation lead to high seepage losses, causing a serious pollution problem (Ashbell, 1992).

Dried citrus pulp added at the time of ensilage to low-DM forages may increase its weight by 145% by absorbing excessive moisture, thus preserving nutrients which otherwise would be lost by effluent and uncontrolled fermentation (Vilela, 1998). The DM, WSC and fermentation acids content of elephant grass silage was increased whilst pH was reduced with the use 0, 5, 10, 15 or 20% of dried citrus pulp (Faria et al., 1972). Levels up to 30% of dried ground citrus pulp were added to a 75-day regrowth cut of elephant grass, resulting in silages with a corresponding linear increase of DM content (y = 0.49x + 24.0), a pH in a range from 3.49 to 3.68 and a linear decrease of ammonia-N (Evangelista et al., 1996).

FORMIC ACID AND/OR FORMALDEHYDE TREATMENTS

Commercial formic acid (85%) has been extensively used for the ensilage of unwilted temperate grasses, but is gradually being substituted by biological additives, certainly because it is dangerous in handling and application and corrosive to equipment. Information about the use of such additives on tropical forages is limited to research data and no literature was found reporting farm-scale adoption.

Earlier studies by Boin (1975) on the ensiling of young, high-protein, low-WSC and -DM elephant grass have shown that a 0.8% rate of formic acid is needed for a reasonably good silage fermentation, while Vilela (1984) found no effectiveness based on silage composition when applying formic acid at various rates to unwilted or wilted elephant grass. In contrast, 0.5% formic acid treated elephant grass had not only an improved fermentation but also higher intake and digestibility compared to the untreated control (Silveira et al., 1980).

King grass (Pennisetum purpureum × P. typhoides) silage treated with formic acid (3.5 l/t) showed better fermentation quality than benzoic acid treated and untreated silages (Ojeda and Cáceres, 1984). In a review by Ojeda (1993) on the use of mineral or organic acids as well as of anti-microbials, it is concluded that for the ensilage of tropical forages, the kind of additive and application rate need to be determined specifically according to the type of forage.

Formalin (35-40% formaldehyde solution) has also been used as a silage preservative, especially aiming at reduced protein degradation in the silo and thus increasing undegradable protein in the rumen of silage-fed animals. Formaldehyde restricts considerably fermentation of silage: 0.8% formalin (w/w fresh basis) almost sterilized an ensiled mass of elephant grass and reduced digestibility of silage (Boin, 1975). A dosage of 0.5% formalin (w/w) applied to a mixture of elephant grass with cassava tops (20.3% DM, 8.5% WSC) reduced ammonia-N and increased precipitable protein in silage, however without suppressing a clostridial fermentation (Zanotelli and Mühlbach, 1989). Studies with a 70% formalin plus 26% formic acid plus 4% water mixture applied 0.2% (w/w) to elephant grass (13% DM), aiming at a rate of 4 g formaldehyde/100 g CP in the forage, resulted in poor fermentation quality and impairment of the nutritive value of silages produced (Lavezzo et al., 1984). Accurate formaldehyde rates necessary to improve fermentation in the silo as well as to obtain a protein protection effect in the rumen are difficult to achieve, especially under farm-scale conditions (Mühlbach and Kaufmann, 1979).

OTHER ADDITIVES

Salt

The addition of 1% sodium chloride to a mixture of wilted elephant grass and cassava tops (28% DM, 9.5% WSC) was not effective in improving fermentation of silage compared to the unwilted control (Zanotelli and Mühlbach, 1989).

Non-protein nitrogen additives

Non-protein nitrogen (NPN) additives, especially urea, when added to high-DM, low buffering forages (maize or sorghum grain) increase CP content and are claimed to improve aerobic stability of silage at feed out. In a review by Lavezzo (1993) on the use of urea as a silage additive for elephant grass, it was concluded that with low-DM forage and in the absence of additives rich in WSC, such a product should not be recommended when aiming at an improvement of fermentation. Generally, pH value, ammonia-N and acetic and butyric acid contents are increased. Singh et al. (1996) registered the highest pH values and ammonia-N levels, associated with higher anaerobic proteolytic bacterial populations, in Sorghum bicolor silages (34% DM) made with 0.5% urea. Other NPN sources, such as ammonium sulphate and biuret, either alone or in combination with urea, calcium carbonate or starch sources, have also been tested for their effects on silage fermentation, digestibility and intake. The results, as reviewed by Vilela (1984), do not favour their use as silage additives. According to Bolsen (1999), NPN always acts as a buffer during fermentation, requiring extra lactic acid to be produced to lower the pH enough for preservation, thus increasing DM loss.

Poultry litter

This waste product cannot be considered as a typical ensilage additive but has been mixed with easily fermentable forages as a means to increase CP content and to eliminate potential pathogens in litter through fermentation (Al-Rokayan et al., 1998; Rasool et al., 1998; Fontenot and Jurubescu, 1980). It can also be used to increase DM content of the ensilage of elephant grass (Lavezzo, 1993). It may present high protein together with a high ash content, which increases buffering capacity and may negatively affect fermentation. Almeida et al. (1986) ensiled elephant grass (20.3% DM, 7.9% WSC) together with 15% sugar cane and 5% broiler litter producing a silage of good fermentation quality; however the mixture with solely 10% litter produced silages with very high butyric acid content (2.36% of DM) and ammonia-N.

CONCLUSIONS AND RECOMMENDATIONS

- Biological additives, when applied to higher quality tropical forages that are more suitable to ensilage, such as forage sorghum, improve fermentation and reduce in-silo losses. However, silages are more liable to aerobic deterioration, demanding good feed-out management, particularly with large silos.

- With good quality (early growth stage) tropical pasture forages, which lend themselves to fast wilting, both wilting and biological additives (inoculant alone or in combination with an effectively proven enzyme mixture) can be recommended. Products so far tested, particularly with the ensilage of the thick-stemmed Pennisetum species, do not show consistent positive results regarding fermentation characteristics of silages. More field-scale research is needed to test additive effect on nutrient recovery with silage stored in small, well-sealed plastic silos, as might be realizable with small-scale producers. Biological additives are generally available as powders or granules, which need to be applied mixed with water to allow proper mixing with the forage. Under small-scale conditions, sprinkling homogeneously with a watering can could be an alternative to spraying with a metered liquid sprayer.

- High quality feeds and by-products are so far the best option found as additives for the hard-to-ensile forages such as the thick-stemmed Pennisetum and Panicum species. They may be relatively expensive, but cost-effectiveness should always consider the improvement in nutritive value of the ensiled forage. Molasses would be more adequate for wilted or higher DM (>25%) materials, while starch sources could be used alone, and also combined with molasses for the ensilage of low-DM, unwilted forages. Locally available and cost-effective absorbents such as dried citrus pulp can also be a good alternative.

- Additives with restricted use, such as formic acid, can improve fermentation but most probably will be neither cost effective nor realizable under small-scale conditions. More tests would be needed with other acids to determine dosage according to the type of forage. Formalin could be cheaper, but results with the ensilage of tropical forages have been inconsistent. NPN products are not the choice additives for low-DM, low-WSC forages; they could be used with wilted forage, preferably in combination with a readily fermentable substrate such as molasses.

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