Silage additives

Animal Feed Science and Technology, 45 (1993) 35-56

35

0377-8401/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

Silage additives Nancy Henderson1 Scottish Agricultural College, WestMains Road, Edinburgh EH9 3JG, UK

Abstract The biochemical and microbiological factors involved in the ensilage process are reviewed, and the effects of wilting and of the various categories of silage additives, stimulants, inhibitors, nutrients and absorbents on these factors are discussed. Some examples are given of the effects of additives on dry matter loss, especially through effluent production, and on voluntary intake and animal production. It is concluded that given appropriate conditions relating to weather, substrate availability and good management, a well-preserved silage may be prepared with relative ease. However, when conditions are less favourable, the use of additives will aid fermentation.

Introduction

It is estimated that in Western Europe 60% of the forage conserved for winter feed, equivalent to approximately 77 Mt of dry matter (DM), is in the form of silage (Wilkinson and Stark, 1987). In western parts of the UK the proportion is even higher. In Northern Ireland, for example, grass silage accounts for approximately 85% of conserved forage (Mayne and Steen, 1990 ). The conservation of a crop as silage depends upon the natural fermentation under anaerobic conditions of the sugars in the crop to acids, mainly lactic and acetic, by the lactic acid bacteria. Although silage is likely to represent 50-60% of the winter feed for ruminants, the silage fermentation process is largely left to chance. Most of the soluble carbohydrates present in the fresh forage are fermented to reduced products, which themselves may be endproducts of ruminal fermentation, and most of the nitrogenous compounds are rendered highly degradable. Silage energy and nitrogen sources are therefore not synchronized for optimal utilization of ammonia by rumen organisms. Silage additives have been developed over the years to take some of the risk out of the ensilage process and to improve the nutritive value of silages. Ideally, a silage additive should be safe to handle, reduce DM losses, improve the hygienic quality of the silage, limit secondary fermentation and improve aerobic stability, increase the nutritive value by increasing the efficiency of utilization of the silage and give the farmer a return greater than the cost of the additive (Merensalmi and Virkki, 1991 ). ~Present address: c/o Dr. F. D'Mello, Scottish Agricultural College, West Mains Road, Edinburgh, EH9 3JG, UK.

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N. Henderson / Animal Feed Science and Technology 45 (1993) 35-56

Silage additives may be chemical or biological, and can be categorized as stimulants, inhibitors, nutrients or absorbents (McDonald et al., 1991 ). The list of commercial products available to farmers in the UK is long and diverse. Approximately 140 named products were on the market in 1991 (Weddell et al., 1991 ), but to understand how their application to crops affects the ensilage process it is necessary to have some knowledge of the reactions involved in the process. The ensilage process

Biochemistry Biochemical processes occurring after the plant is cut and during conservation result from the continuing metabolism of plant cells, from the enzymes of the dead tissue and from the micro-organisms present on the plant (Table 1 ) (Henderson, 1991 ).

Respiration Plants obtain energy and reducing power not only from light reactions of photosynthesis but also from the degradation or respiration of products of photosynthetic carbon dioxide fixation (Duffus and Duffus, 1984). The overall reaction for respiration is generally represented as the complete oxidation of a molecule of glucose: C 6 H 1 2 0 6 "+'602 --~ 6 C O 2

+ 6H2 O + energy

In the cut plant virtually all this energy is converted into heat. In the field the heat is dissipated into the atmosphere but in the silo much of the heat generated is retained. Because the acceleration of respiration with temperature is exponential there is a progressive increase with time until the oxygen supply Table 1 Factors influencing the conservation of crops Enzymes

Micro-organisms

Respiratory Proteolytic Polysaccharide-degrading

Lactic acid bacteria Enterobacteria Clostridia Fungi Yeasts Moulds Bacillus Listeria Acetic acid bacteria Propionic acid bacteria

N. Henderson/Animal Feed Science and Technology 45 (1993) 35-56

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is exhausted or the enzymes are inactivated by the acid conditions, or, in extreme conditions, by temperatures of approximately 70 oC.

Proteolysis In the fresh herbage 75-90% of the total nitrogen is present as protein, but this may fall to 60-65% after ammonia or nitrate fertilization. The important factors which influence the extent of the degradation of proteins by plant enzymes are DM content, the presence of oxygen, pH and temperature. During a moist wilt with no change in the DM content there is an increase in the amide fraction, and prolonged wilting in moist conditions increases the level of low molecular weight nitrogen compounds, including ammonia (Spoelstra and Hindle, 1989). When oxygen is used up in the silo the rate of proteolysis is much more rapid and the degradation is extensive. Plant proteases are most active between pH 6 and 7 but the activity does continue at a much reduced rate at values below pH 4 (Heron et al., 1989). The more rapid the drop in pH in the silo, the less extensive is the breakdown of protein. Proteolysis and amino acid degradation are also less extensive as the DM content of herbage increases, and they are inhibited by dry or wet heat treatment (Mandel et al., 1989; Charmley and Veira, 1990).

Polysaccharide-degrading enzymes Polysaccharides are condensation polymers based on monosaccharides joined together by glycosidic linkages (Duffus and Duffus, 1984). Cellulose is the most abundant naturally occurring organic compound and is the main structural component of the plant cell wall. When grass is ensiled, lignin remains unchanged and only a small decrease, less than 5%, occurs in the cellulose fraction (Morrison, 1979). Hemicellulosesare defined as a class of polysaccharides associated with cellulose and are soluble in alkali. Growing primary walls in grasses are composed of approximately 65% water, with 30% of the dry weight of the unlignified cell wall as fl-( 1-~3), (1-~4)-glucan and 30% as arabinoxylan (Fry, 1988 ). During ensiling, losses of hemicellulose are not uniform (Morrison, 1979) and, depending on the stage of growth and DM of the grass, as much as 40% of the hemicellulose fraction may disappear (Gonzalez-Yanez, 1990 ). Campbell et al. (1990) concluded that pectin in lucerne is not metabolized during ensilage but hemiceUulosecan be to a variable extent. During the ensilage of low-DM crops the amount of acid produced is frequently in excess of the water-soluble carbohydrates (WSC) in the crop. Proteins, amino acids and organic acids all contribute to the production of fermentation acids but the hemicelluloses are the major source of additional substrate. Ohyama and Masaki (1977) and others have demonstrated that much of the additional sugar produced in low-DM silages is glucose with some

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N. Henderson /Animal Feed Science and Technology 45 (1993) 35-56

arabinose and xylose. Previously, the glucose was thought to come from cellulose but now it is thought more likely that it comes from the glucan in the primary cell walls (Fry, 1988 ). In higher-DM silages the activity of the polysaccharide-degrading enzymes is inhibited.

Microbiology Lactic acid bacteria The role of microflora in forage conservation was discussed in detail by Woolford (1984 ) and updated recently by Pahlow (1991 ). The majority of the bacteria on a crop require oxygen to survive and die off rapidly in the early stages of ensilage. Of the microflora which remain, the lactic acid bacteria (LAB), enterobacteria, yeasts and clostridia are the most important. In the past, it was thought that the numbers of LAB on the standing crop could be very low. Recent research, however, has established that they are present, but in a dormant state, and that the harvesting procedures which lacerate the crop and release cell contents from the ruptured plant tissue result in the recovery of the LAB, which can be enumerated by standard methods (Pahlow, 1991 ). In general, the numbers of the epiphytic LAB on grass increase during the summer months and may be as high as 107 colony forming units (cfu) g-~ grass. These include the efficient homofermentative LAB which convert glucose and fructose to lactic acid only and the heterofermentative LAB which convert sugars to a range of products, not all of which assist in lowering the pH. When there is no more available carbohydrate in the silage, LAB can use lactic acid as substrate, with the production of acetic acid (Lindgren et al., 1990). Although O'Kiely et al. (1986) concluded from a series of experiments that there is a critical level of 30 g WSC 1- ~of grass juice extract below which silages are generally poorly preserved, predicting silage pH from the sugar content of the ensiled herbage is not possible (Mo and Fyrileiv, 1979 ). Analyses have shown that WSC at the time of ensiling may account for only 63.3 _+25 % of the lactic acid in silage (Wilson, 1986 ). Enterobacteria Enterobacteria compete with LAB for available carbohydrates during the initial stages of ensilage and some can produce ammonia (Seale, 1986 ). The metabolic activity of the enterobacteria is readily inhibited during the conservation process either by anaerobiosis or by acidification. The toxic substances contained in the many Gram-negative bacteria are stable, however, and will remain largely unaffected over extended periods (Lindgren, 1991 ). The final concentration of these endotoxins will be closely related to the maximum population reached by the enterobacteria. As yet, there is no proof, but it is possible that the endotoxins may have a detrimental effect on the palatability and therefore on the nutritive value of the silage.

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Yeasts All yeasts grow well in the presence of oxygen, and their significant role in the aerobic deterioration of silage is well established (Woolford, 1990). In silage a subgroup develops which can compete successfully for fermentable carbohydrate anaerobically. These are superseded by a subgroup able to utilize organic acids. These organisms usually represent up to 100% of the yeast flora in farm silages after extended storage. Silages with high yeast counts, in excess of 105 cfu g-1, are likely to be unstable on exposure to air (Woolford, 1990).

Clostridia Both saccharolytic clostridia and proteolytic clostridia are present in silages as contaminants derived from soil particles. Clostridia are particularly sensitive to water availability, and in very wet crops even the achievement ofa pH value as low as 4.0 may not inhibit their growth. Clostridia play a major role in the anaerobic spoilage of silage, and saccharolytic clostridial spores cause problems in hard cheese production (Pahlow, 1991 ).

Moulds Most moulds are dependent on oxygen for their growth and propagation but even minute quantities of oxygen are sufficient to maintain the metabolism of certain members of the group (Pahlow, 1991 ). Mycotoxins are found not only in spoiled silage but also some distance from areas of visible moulding (Oldenburg, 1991 ).

Silage additives Wilting Before the various categories of silage additives are reviewed, mention should be made of wilting, regarded by many as an alternative to additive use. In conditions in which a rapid wilt to 250-300 g DM kg-1 is possible, this will be beneficial as it will reduce effluent production without having a significant effect on the nutritive value of the silage. In a collaboration programme carried out in European research institutes, field losses and in-silo DM losses averaged 18.6% and 17.1% for unwilted additive-treated silages and wilted silages, respectively. Additives had little effect on losses when used on wilted herbage but they did improve the nutritive value of the silage (Zimmer and Wilkins, 1984). Under good weather conditions the DM increases and the sugars are concentrated in the DM, but under poor weather conditions the DM content may increase very little, if at all, and if the wilting period is extended over several days soluble carbohydrates will be lost, protein-N contents may be reduced

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Table 2 Composition of'ineffective wilt' silages Additive

pH

Ammonia-N (gkg -t total N)

WSC (gkg -~ DM)

Lactic acid (gkg -~ DM)

None ADD-F (formic acid) Silaform (formic acid-formalin) Sylade (sulphuric acid-formalin) Kylage (calcium formate-sodium nitrite)

3.82

153

0

150

4.02

117

67

96

3.81

143

37

102

3.97

144

14

115

3.95

152

1

158

and deamination of amino acids may increase. If this occurs the silage is likely to have a high ammonia-N content even with the application of an effective additive (N. Henderson and P. McDonald, unpublished data (Table 2 ) ). It is generally accepted that a well-preserved silage should have an ammonia-N content less than 80 g kg- 1total nitrogen (TN). Carbohydrate sources

Carbohydrate-rich materials such as sugar, molasses, whey, citrus pulp and potatoes are added to silage crops to increase the supply of substrate for the LAB. Molasses is the carbohydrate source used most frequently, and is of particular benefit when applied to crops low in soluble carbohydrates such as legumes and tropical grasses, although to obtain maximum benefit it must be used in relatively high concentrations (about 40-50 g kg -1 ). If the treated crop has a very low DM content, a considerable proportion of the added carbohydrate may be lost in the effluent during the first few days of ensilage. The effect of this and any other category of silage additive on DM loss between the field and the feed trough will depend upon their effect on respiration, effluent flow, inhibition or stimulation of the various micro-organisms and their activities, and on the aerobic stability of the silage. Acid-based additives

For years, acid-based additives were the most widely researched and used in Europe and North America. Since the introduction of the AIV process (Virtanen, 1933) in which mineral acids were applied to the crop to lower the pH to 3.5 and, more recently, with the development of efficient additive applicators there has been an increase in the use of mineral and organic acids, their salts and acids mixed with formalin. Until recently, products such as

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ADD-F (formic acid) (BP Nutrition UK Ltd., Northwich, UK) and Sylade (sulphuric acid-formalin) (ICI Ltd., Cleveland, UK) dominated the UK market. By lowering the pH of the herbage, acids inhibit the activities of the respiratory and proteolytic enzymes. Whether acid additives act as stimulants or inhibitors of LAB depends upon the concentration of the active ingredient or ingredients in the commercial product and upon the rate at which the product is applied to the crop. Acid salts are less effective than the equivalent acid and therefore they must be applied at a higher rate to obtain a similar effect.

Mineral acids Mineral acids lower the pH of the herbage, inhibiting the activity of undesirable bacteria such as enterobacteria and clostridia and stimulating the LAB to use the available substrate and lower the pH further. In crops in which substrate is in short supply this can be beneficial. Sulphuric acid is cheaper than organic acids but its use as a silage additive has been called into question (Mayne and Steen, 1990). Poor responses in animal performance obtained with sulphuric acid may be related to detrimental effects on liver copper status as observed by O'Kiely et al. ( 1989 ).

Organic acids Organic acids, in particular formic acid, have an antibacterial action, as a result of both a hydrogen ion concentration effect and a selective bactericidal action of the undissociated acid (Woolford, 1984). When formic acid or sulphuric acid is added to grass to lower the pH to a similar level the composition of the silages may be very different (Table 3) (Carpintero et al., 1979). In this example, formic acid restricts the activity of the LAB, thus conserving WSC in the silage. This should increase the silage energy available for microbial growth in the rumen. Yeasts have been found to be particularly tolerant of formic acid, and high counts of these organisms have been noted in silages treated with this additive applied at the recommended rate (Henderson et al., 1972). Under anaerobic Table 3 Composition of acid-treated silages Additive

Application Grass Silage Ammonia-N WSC Acetic acid Lactic acid rate pH pH (gkg -~ TN) (gkg -~ DM) (g kg-~ DM) (g kg-~ DM) (lt -1 )

Untreated Formic acid (85%) 4 Sulphuric acid (50%) 3

5.85

3.87

95

12

28.8

122

4.05

3.88

12

211

4.5

66

4.00

3.64

42

64

20.4

102

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conditions yeasts obtain energy from the fermentation of sugars with the production of ethanol and loss of DM. As enterobacteria are known to generate formic acid, this acid probably has less effect on reducing their growth than the growth of LAB, and it is therefore the rapid proliferation of LAB rather than the decrease in pH associated with the application of formic acid which is important in reducing numbers of enterobacteria (Chamberlain and Quig, 1987). Intermediate levels of application of formic acid ( 3-41 t - 1) may inhibit the LAB to a greater extent than the enterobacteria and thus have a deleterious effect on the fermentation. At higher levels of application both LAB and enterobacteria are inhibited. Formic acid is frequently used as a positive control treatment in experiments designed to evaluate other silage additives. The rate at which the formic acid is applied may therefore have a significant effect on the conclusions drawn from the results of these experiments. When Mayne and Steen (1990) investigated the data from 21 recent studies in which the effects of using formic acid on fermentation and animal performance were considered, they concluded that the results supported the hypothesis of Parker and Crawshaw (1982) that in situations where treatment with formic acid resulted in an improvement in silage fermentation, positive effects on digestibility and intake of silage were obtained, and that these effects were reflected in enhanced animal performance. However, acid additives can increase effluent production on young grass by up to a third depending on the level applied (McAllan et al., 1991 ). When formic acid is applied at a high level ( 5 1 t - 1 or more) much of the WSC is retained in the silage, and the acid content and buffering capacity are much lower than those of an untreated silage from the same sward (Henderson et al., 1990a). Some of the data from a trial conducted at Edinburgh are shown in Table 4 (Henderson et al., 1989). In this experiment, application of formic acid was sufficient to lower the pH of the grass immediately to 4.0. The beneficial effects of this are seen in the improved intakes and performances of the steers given the treated silage supplemented with 1.5 kg of brewers' dark grains per day compared with those of the steers on the untreated silage with the same supplement.

Formaldehyde-acids Interest in formaldehyde as a silage additive arose from its bacteriostatic properties and because it was known to protect plant proteins from degradation in the silage and in the rumen. When applied at high levels, formalin depresses DM digestibility and intake, whereas at low levels of application it tends to encourage growth of clostridia. For this reason, commercial products contain mixtures of acids and formalin. The effects of formic acid and a formic acid-formalin additive on the composition and nutritive value of ryegrass silages are shown in Table 5 (Hinks et al., 1980). In addition to preserving

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Table 4 Composition and nutritive value of silage treated with a high level of formic acid (5 1t - 1 ) Treatment

Untreated

Formic acid

DM (g kg -a) pH Ammonia-N (g kg- ~TN)

224 3.70 69

233 3.91 35

18 137 718 11.4

105 51 719 11,7

SED

Components of DM (g kg- l) WSC Lactic acid Digestible organic matter (DOM) Metabolizable energy (ME) (MJ kg -~ DM)

Intake and liveweight gain (LWG) of steers Silage DM intake (kg day- ~) Total DM intake (kg day-~ ) LWG (kg day -1)

6.00 7.33 0.658

6.85 8.18 0.804

0.145 0.0431

Table 5 Composition and nutritive value of wilted silages treated with either formic acid or a formic acidformalin mixture Treatment

Formic acid (3.14 kgt - t )

Formic acid plus formalin (2.86 kg acid t -~ plus 1.44 kg formalin t -~ )

DM pH Protein-N (g kg -~ TN) Ammonia-N (g kg- ~TN)

272 4.09 383 70

280 4.08 508 50

57 92 10.4

99 74 11.1

SEM

Components of DM (g kg- 1) WSC Lactic acid ME (MJ kg -~ DM)

In takes and L WG of steers Silage DM intake (kg head- ~) Total DM intake (kg head -~ ) LWG (kg day- t ) (adjusted to constant ME intake)

764 895 0.829

765 892 0.933

32.4 33.0 0.0516

WSC, additives based on acids, especially those containing formalin, inhibit proteolysis by lowering the pH and through the binding of formaldehyde to the nitrogenous components (McDonald et al., 1991 ). This, in turn, reduces the deamination of the amino acids and the production of ammonia. Although formalin-acid mixtures are very effective on grass, their use has been banned in some countries. The potential health risks of exposure to

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formaldehyde are now recognized, and effort should be put into the containment of formaldehyde vapour where it is used.

Acid salts Despite the positive effects of acids, especially organic acids, their corrosive action against machinery and their health risk towards man if precautions are not taken in handling them have focused attention on alternatives such as acid salts. The inhibitory effect of nitrite on clostridia in silages has been studied in detail (Spoelstra, 1985 ), and nitrite-containingadditives are marketed in most countries in Western Europe, although up to now results with these additives have been variable. In an experiment in which grass was ensiled after a long ineffective wilt, a commercial additive containing sodium nitrite and calcium formate was less effective than acid additives when added at the recommended rate (Table 2 ). To overcome the problems associated with formic acid, a complex acid salt, ammonium tetraformate, was developed for commercial use (Drysdale and Berry, 1980). More recently, a product containing ammonium salts of formic acid and propionic acid with caprylic acid, but mainly a complex salt of formic acid, has been introduced to the UK market (Maxgrass; BP Nutrition UK). This has a recommended rate of application of 6 1 t - 1 and will inhibit the activity of the micro-organisms, as seen from the results of a laboratory trial in which ryegrass was treated with this additive (Table 6) (McGinn et al., 1990). Much of the ammonia in the treated silage was applied to the grass in the additive and is not a product of the deamination of amino acids in the silage.

Biological additives Biological additives are safe to handle. They either provide additional substrate for the indigenous population of micro-organisms or increase the popTable 6 The composition of ryegrass silage treated with a range of commercial additives Treatment

Rate of application

pH

WSC (g kg -~ D M )

Ammonia-N (gkg -~ TN)

Lactic acid (gkg -~ D M )

None Maxgrass Molasses ADD-F ADD-F Biomax Silaform SED

0 6.01 t -~ 9.01 t -~ 2.51 t-~ 5.01 t -~ 106 LAB g-~ 2.51 t-~

4.05 4.33 4.02 3.92 4.11 3.96 3.94 0.026

29 184 42 43 173 34 93 9.3

90 52 (15) a 73 29 16 80 34 2.8

153 3 121 99 9 141 92 8.4

aValue corrected for ammonia in additive.

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ulation ofhomofermentative LAB. In some products, the LAB are added with substrate or with enzymes to provide additional substrate.

Bacterial inoculants The concept of applying strains of homofermentative LAB to herbage to improve the fermentation is not new. Throughout this century, scientists have experimented with the addition of bacteria with or without sugar. Whittenbury ( 1961 ) defined the criteria which a potential organism should satisfy for use in silage, but it was not until freeze-drying and encapsulation techniques were developed that the commercial exploitation of cultures of LAB as additives for silage was possible. Initially, many products did not contain sufficient live organisms to outnumber the indigenous population and dominate the fermentation, but products have improved, and, provided the correct storage conditions are maintained and the directions for use are followed, most will give the intended inoculation rate of 105-106 cfu g- ~ herbage. Several products contain fewer LAB but are supplied with a broth in which the farmer grows up the LAB. If this is done at the correct temperature, around 25 °C, and with attention to detail, a study on farms in the Edinburgh area has shown that the correct inoculation rate will be achieved. To be successful, the LAB in the inoculant must outnumber the epiphytic population of LAB. Pahlow ( 1991 ) found that an inoculation factor (IF) of two, i.e. a two-fold increase in LAB, was the minimum required to achieve a positive effect on fermentation quality, but Satter et al. (1987) ranked their production trials according to the IF and concluded that a positive response in milk production had only been obtained with an IF of 10 or more. When the results of the Eurobac Conference (Lindgren and Pettersson, 1990) were compiled, it was found that in laboratory studies successful inoculation increased the lactic acid/acetic acid ratio by both increasing the lactic acid and decreasing the acetic acid contents, lowered pH and ammonia-N concentrations and decreased DM losses by 20-30 g kg-~ DM. The increased rate of fermentation with inoculation results in the suppression of proteolysis and deamination of herbage protein (Heron et al., 1987 ) and in a more efficient use of WSC, with more sugar retained in the silage (Gordon, 1989 ). The data from one trial in which a mixture of Lactobacillusplantarum and Pediococcus pentosaceus or L. plantarum only were applied to perennial ryegrass are shown in Table 7 (Henderson et al., 1990c). Pediococci or streptococci are included in many commercial products, as they are active within the pH range 5.0-6.5, but other products contain only L. plantarum, which is known to satisfy most of the criteria suggested by Whittenbury. Although in this trial the compositions of the untreated silage and silage treated with the lower rate of LAB were similar, a time course study showed that the pH fell more rapidly in the treated silage. Despite differences in the chemical compositions of the two silages

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Table 7 Composition and nutritive value of silages treated with formic acid or LAB Treatment

Untreated Formic acid L. plantarum (31t -~) (105cfug -1)

L. plantarum plus SED P. pentosaceus (106cfug - l )

DM (gkg -~) pH Ammonia-N (g kg- l TN )

168 4.55 130

182 4.44 109

163 4.40 131

181 4.09 88

Components of D M (g k g - t ) WSC Lactic acid Acetic acid Ethanol DOM ME (MJ kg -1 DM) DM loss (%)

0 59 46 13 710 11.4 17.8

11 51 35 33 726 11.9 18.3

0 71 45 9 723 11.5 15.3

20 84 30 7 737 12.5 13.6

Intakes and L WG of lambs Silage DM intake (g day- l ) Total DM intake (g day-1 ) LWG (g day- 1)

681 857 71

692 868 94

753 929 124

792 968 129

34.3 41.8 14.8

inoculated with LAB, their nutritive values were similar and significantly better than those of the untreated or formic acid treated silages. Since 1986, many trials have been carried out with bacterial inoculants under farm conditions, and a number of these have been reviewed by Mayne and Steen (1990). In these studies, inoculant treatment did not improve the fermentation characteristics of the silages significantly. Despite this, responses in both silage digestibility and animal performance were obtained in a number of studies. In nine studies in which liveweight gains were recorded, a mean increase of 17.9% was obtained compared with untreated silages. A much smaller response of 3.7% was obtained in milk energy output (mean of seven studies), equivalent to an increase in milk energy output of 2.5 MJ day- 1. Both Spoelstra ( 1991 ) and Mayne and Steen (1990) have concluded that the content of WSC in the herbage has limited use in predicting the response to inoculation. Even in situations where there is little improvement in fermentation characteristics measured, the use of a bacterial inoculant can result in an improvement in animal performance (Thomas et al., 1991 ). The data in Table 7 (Henderson et al., 1990c) support the conclusion of Spoelstra ( 1991 ) that it is the first stage of silage fermentation and the effects of additives during this stage which require further investigation to explain how bacterial inoculants are influencing intake and animal performance, but some way of measuring this in the silage must be found.

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Cell wall degrading enzymes The use of cellulolytic and hemicellulolytic enzymes as silage additives has been considered from two points of view; first, as a means of increasing the content of WSC as substrate for the LAB, and, second, as a method of improving the digestibility of the organic matter of the crop (McDonald et al., 1991 ). Most commercial enzymes are crude preparations containing many enzyme activities. As much of the hemiceUulose (up to 40%) is degraded during silage fermentation (Gonzalez-Yanez, 1990), it is the action on cellulose, or on the acid detergent fibre (ADF) fraction, which produces most of the additional substrate (Henderson et al., 1991 ). Enzyme preparations, like plant cell wall degrading enzymes, are most active in immature, low-DM silages and less active in wilted and mature silages (Spoelstra, 1991 ), and they are active over a wide temperature range (2050 ° C). Laboratory studies with enzyme-treated silages have shown that lactic and acetic acid concentrations are higher than those in untreated control silages, and ammonia-N concentrations and pH are lower (Rauramaa et al., 1987). When poorly fermentable grass is ensiled, the application of enzymes does not prevent a butyric acid fermentation. Enzymes applied at commercial dosages do not appear to liberate sufficient additional sugar during the onset of silage fermentation (Honig and Pahlow, 1990 ). Although cell wall degrading enzymes do increase the WSC content and lower the fibre content during ensilage, in none of the animal trials reviewed by Spoelstra ( 1991 ) was there a significant improvement in digestibility with treatment (Jacobs and McAllan, 1990). Van Vuuren et al. (1991) demonstrated that when dairy cows "consumed enzyme-treated silage there was a higher rate of fermentation in the rumen and an increased outflow of organic matter from the rumen, but these had no effect on the intake of organic matter. Improved milk production was reported by Chamberlain and Robertson (1989) when feeding a low-concentrate diet but not when feeding the same enzyme-treated silage in a high-concentrate diet. Many commercial inoculants contain some cell wall degrading enzymes but, as the optimum pH of the enzymes is 4-5, it is unlikely that they produce sugar at a sufficiently early stage to be effective or that they are there in sufficient quantities to be effective at a later stage in the fermentation. The data from a trial carried out by Gonzalez-Yanez et al. (1990) demonstrate that it is the bacterial inoculants which are effective in such products, rather than the enzymes (Table 8 ). In this trial, the commercial enzyme was effective in producing additional substrate for the LAB, principally from the ADF or cellulose-lignin fraction. This resulted in a significant decrease in the digestibility of the organic matter and no significant improvement in animal performance. The silage treated with the commercial inoculant and enzymes was used most efficiently by the lambs for liveweight gain. McAllan et al. (1991) studied the effect of various additives on effluent

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Table 8 Composition and nutritive value of enzyme-treated silages offered to lambs Treatment

Untreated

Cellulasehemicellulase enzyme

Bacterial inoculantcellulasehemicellulase

DM pH

180 4.20 87

202 3.72 61

169 4.00 83

DOM

3 70 62 471 286 185 691

31 110 32 425 246 179 656

6 99 48 491 304 187 676

Intake and L WG of lambs Silage intake (g DM day- ~) Liveweight gain (g day- l )

785 72.0

770 81.8

811 95.6

Ammonia-N (g kg- ~TN)

SED

Components of DM (g k g - l) WSC

Lactic acid Acetic acid Neutral detergent fibre Acid detergent fibre Hemicellulose

8.6

36.0 14.38

production and found that enzyme additives had the greatest effect, increasing effluent flow compared with untreated grass, for both young and mature grasses. Research is being carried out using genetic engineering to create lactobacilli with the ability to degrade plant cell walls (Hahts and Javorsky, 1991). Aerobic deterioration inhibitors As yeasts play an important role in the aerobic deterioration of grass silages, potential deterioration inhibitors must act against yeasts (Woolford, 1990). Yeasts develop during wilting and when oxygen infiltrates the silage during the storage period. Complete exclusion of air from a farm silo is practically impossible, but aerobic deterioration of silage can be minimized if the silo is filled rapidly, sealed efficiently and good management is practised at feed-out. Some additives will improve aerobic stability by delaying the onset of deterioration, but they will not prevent it. Acids Propionic acid inhibits most but not all of the organisms responsible for silage deterioration, but only when applied to crops in relatively high concentrations. Similarly high levels of formic acid may delay the onset of deterioration. In a recent trial (Henderson et al., 1990b), perennial ryegrass was

N. Henderson/AnimalFeedScience and Technology45(1993) 35-56

49

treated with Maxgrass at a rate of 6 1 t- ~ and ensiled in bunker silos of 100 t capacity. The chemical and microbiological composition of core samples taken from the untreated and treated silages before and after exposure to air in polystyrene containers for 4 days is shown in Table 9. Lactate assimilating yeasts increased in numbers in both silages but those in the untreated silage had a higher capacity for lactic acid assimilation and, in the short term, the treated silage was more stable than the untreated silage.

Bacterial inoculants When Pahlow (1982) ensiled prewilted grass with an inoculum of LAB the inoculant restricted the development of yeasts and rendered the silage more stable than an untreated silage. However, reductions in stability with inoculation have been reported frequently (Spoelstra, 1991 ). Inoculation can only improve aerobic stability if yeasts are kept below the threshold value of l0 s g-~ silage and the air supply remains low, but it is not a reliable measure against aerobic deterioration (Honig, 1990). Bacterial inoculants-chemicals In an attempt to prevent inoculation with LAB producing silages with high contents of lactic acid and very low contents of volatile fatty acids and thereTable 9 Chemical and microbiological composition of Maxgrass-treated silage before and after 4 days' exposure to air Treatment

DM (g kg - I ) pH

Ammonia-N (g kg - 1 T N ) Components of DM (g kg- l) WSC

Lactic acid Acetic acid

Untreated

Maxgrass (6 1 t - ~)

Before

After

Before

After

190 3.83 89

169 8.24 101

200 3.97 111

195 4.55 113

177 51 18

115 23 58

0 132 32

11 0 6

Micro-organisms (cfu g - 1) LAB

Enterobacteria Total yeasts Lactate assimilating yeasts Moulds Temperature rise above ambient in 4 days

7.1 X 10 8 2.2)< 105 5.1 X 105 1.1 X 107 2.5 × 102

8.0X 10 9

1.0× 106 1.4X 10 9

2.1 X 101° < 10

15.5 ° C

1.5X 3.3× 5.6× 5.5 × 2.3 ×

l0 II 104 10 s 106

3.9× 1012 8.9× 102 9 . 5 × 101°

9.5 × 10~° 4.1X 102

10 4

0 °C

50

N. Henderson/AnimalFeed Science and Technology 45 (1993) 35-56

fore prone to aerobic deterioration, the inoculum of LAB is now being combined with a chemical such as calcium formate or sodium formate (Set~il~iet al., 1990; Weissbach et al., 1991 ). Although this approach is still at the experimental stage, it appears that these salts develop an antimicrobial effect with increasing acidity in the silage. The treated silages contain less lactic acid, fewer clostridial spores and are more stable than corresponding untreated silages. Nutrients Nutrient additives are defined as substances which, when added to ensiled material, contribute significantly to the nutritional needs of animals consuming the silage. These include molasses, cereals and whey, which also act as fermentation stimulants (McDonald et al., 1991 ). The crude protein content of crops such as maize which are nutritionally deficient in nitrogen can be increased by the application of urea or ammonia. Maize is also a poor source of calcium, and improvements in animal performance have been noted on silages treated with limestone and urea compared with silages treated with urea only. Absorbents The shift from hay to silage over the past 40 years and the realization that direct-cut silage treated with an effective additive is used more efficiently by stock than wilted silage (Zimmer and Wilkins, 1984) has meant that silage effluent pollution is now a major environmental problem, and in some years it is the predominant source of agricultural pollution (Offer et al., 1991 ). Absorbents will alleviate the problem but in wet climates they will not eliminate it, and farmers must ensure that their silos do not leak and that their effluent tanks have sufficient capacity for the weight of grass the silos will hold. A heavy fine or even imprisonment may result if effluent finds its way into a nearby water course. For environmental reasons, a rapid wilt to 250-300 g DM kg- 1 should be the technique adopted in the future. However, heavy swaths and poor climatic conditions frequently render this impossible in northern Europe. Where there is a risk of pollution, additives, such as enzymes or formic acid, which increase effluent flow or alter the pattern of effluent flow should be avoided, and the use of absorbents should be considered. McAllan et al. ( 1991 ) demonstrated that enzyme treatment and formic acid increased effluent flow but bacterial inoculants had no effect compared with untreated grass. In a trial conducted by Kennedy and Carson (1991), Maxgrass-treated silage produced 30% more effluent than untreated silage in the first 5 days. Although some absorbents may have only a minimal effect on total effluent

N. Henderson/Animal Feed Science and Technology 45 (l 993) 35-56

51

production, they may change the pattern of effluent flow and ease the shortterm problem of effluent storage and disposal (O'Kiely, 1990a). Of the absorbents tested, fibrous by-products such as sugar beet pulp (O'Kiely, 1990b; Ferris and Mayne, 1990) or distillers' dried grains appear most promising. Chopped straw is an effective absorbent but it increases the silo capacity required and lowers the ME value of the combined product. Alkali-treated straw cubes are available commercially as an alternative to chopped straw. Under farm conditions, effluent retention rates rarely exceed 1 1kg- ~added absorbent, and for low-DM grass the absorbent may compose as much as 38% of the food on a DM basis for complete effluent retention. This would be uneconomic. In the majority of animal trials, effluent production is reduced with absorbents (Kennedy, 1988; Jones and Jones, 1988) but the effluent can have a higher DM content than the effluent from untreated silage (Kennedy, 1988). When Ferris and Mayne (1990) examined the effect of inclusion of sugar beet pulp with grass at ensiling, the inclusion of increasing levels reduced effluent output from 242 1 t-~ grass ensiled in the untreated to 26 1 t-~ at the highest level of inclusion. Although dairy cows consumed more of the ensiled blends, there was a trend for yields of milk, milk fat and milk protein to be greater when sugar beet pulp was offered as a supplement to control silage rather than as an ensiled blend. Economic appraisal of additive use

Silage additives are an insurance policy against the production of poorly preserved, inedible silage and this is how they are regarded by many farmers, especially dairy farmers. Additives cost from 40p to treat a tonne of herbage with sulphuric acid up to £ 12.20 to treat a tonne of low-DM herbage with an absorbent (Weddell et al., 1991 ). Whether or not an additive has been costeffective is impossible to gauge without an untreated control and detailed information on DM recovery and animal performance. If additives do improve animal performance without significantly affecting DM losses or DM intakes they will be cost-effective if the return exceeds the cost of the additive. Of the fermentation inhibitors, Maxgrass is probably the most expensive. With this type of treatment more silage is consumed by stock, and the cost of the extra silage and additive must be included in the calculation. Savings can then be made by reducing the intake of concentrate. In a recent trial at Edinburgh (C.A. Morgan et al., unpublished data) Maxgrasstreated silage was compared with a control silage as a basal forage for pregnant ewes. Supplementary concentrate in late pregnancy was adjusted to maintain equal ME intake and fl-hydroxybutyrate blood levels when the silages were offered ad libitum. The application of Maxgrass saved 300 g of concentrate per ewe per day in the last 38 days of pregnancy with no signifi-

52

N. Henderson /Animal Feed Science and Technology 45 (1993) 35-56

cant differences at lambing in ewe weight, condition score or lambing performance. Using costs of £ 160 t - 1 for the concentrate, £20 t - 1 for the control silage and £24 t - 1 for the Maxgrass-treated silage, the financial implications are shown in Table 10. The DM losses from the silages were similar (control 18.5%, Maxgrass 17.8%). Absorbents are the most expensive of the silage additives, but inclusion of molassed sugar beet pulp or distillers' by-products has been shown to improve silage DM intake and animal performance when compared with untreated silage. This strategy has the advantage that nutrients that would be lost in the effluent are fed to stock and, in some trials (Dulphy and Demarquilly, 1976; Jones and Jones, 1988), but not all (O'Kiely, 1990a; Steen, 1991 ), improved performance has been observed when the absorbent is ensiled with the grass rather than offered at the same level with untreated silage. Although the absorbent must be purchased earlier in the year, this may prove to be costeffective. Conclusions

Given the right conditions, good weather, sufficient substrate for the LAB and good management, it is possible to make a well-fermented silage. The crop should be mown when it has reached its driest point in the day, normally in the afternoon, and wilted for no more than 24 h. When conditions are less than ideal, there is a wide range of effective products which will aid the fermentation. In future research, the emphasis should be on the development of additives which will reduce losses during storage, improve the efficiency of utilization of silages, i.e. improve animal performance without increasing silage DM intake, and on matching concentrates to silage type. Table 10 Evaluation of Maxgrass-treated silage for pregnant ewes Treatment

Untreated

Maxgrass (61t -1)

Intake per ewe in 38 days (kg)

Silage Concentrates

154 22.8

176 11.4

Cost per ewe in 38 days (p)

Silage Concentrates Total Saving per ewe

308 365 673

422 182 604 69p

N. Henderson/Animal Feed Science and Technology 45 (1993) 35-56

53

References Campbell, C., Taylor, K., Matsouka, S., Marshall, S. and Buchanan-Smith, J.G., 1990. Inoculants and enzymes as additives for lucerne silage with measurements of changes in structural carbohydrates and pectin during the ensiling period. Proc. 9th Silage Conf., Newcastle upon Tyne, September 1990, pp. 14-15. Carpintero, C.M., Henderson, A.R. and McDonald, P., 1979. The effect of some pre-treatments on proteolysis during the ensiling of herbage. Grass Forage Sci., 34:311-315. Chamberlain, D.G. and Quig, J., 1987. The effects of the rate of addition of formic acid and sulphuric acid on the ensilage of perennial ryegrass in laboratory silos. J. Sci. Food Agric., 38:217-228. Chamberlain, D.G. and Robertson, S., 1989. The effects of various enzyme mixtures as silage additives on food intake and milk production of dairy cows. Br. Grassl. Soc., Occas. Symp., 23: 187-189. Charmley, E. and Veira, D.M., 1990. Inhibition of proteolysis at harvest using heat in alfalfa silages: effect on silage composition and digestion by sheep. J. Anim. Sci., 68: 758-766. Drysdale, A.D. and Berry, D., 1980. The development of a new silage additive. Br. Grassl. Soc. Occas. Symp., 11: 262-270. Duffus, C.M. and Duffus, J.H., 1984. Carbohydrate Metabolism in Plants. Longman, London, pp. 1-21. Dulphy, J.P. and DemarquiUy, C., 1976. Incorporation of dry beet pulp in silage: utilisation by dairy cows. Bull. Tech. Centre Rech. Zootech. Vet. Theix, 22: 45-52. Ferris, C. and Mayne, C.S., 1990. Effect on milk production of feeding silage and four levels of sugar beet pulp, either as a mixed ration or as an ensiled blend. Proc. 9th Silage Conf., Newcastle upon Tyne, September 1990, pp. 76-77. Fry, S.C., 1988. The Growing Plant Cell-Wall: Chemical and Metabolic Analysis. Longman, London, pp. 1-7. Gonzalez-Yanez, M., 1990. Polysaccharide-degrading enzymes as additives for silage. M.Phil. Thesis, University of Edinburgh. Gonzalez-Yanez, M., McGinn, R., Anderson, D.H., Henderson, A.R. and Phillips, P., 1990. The effect of biological additives on the composition and nutritive value of silage. Anim. Prod., 50: 586. Gordon, F.J., 1989. An evaluation through lactating cattle of bacterial inoculant as an additive for grass silage. Grass Forage Sci., 44: 169-179. Hahis, P. and Javorsky, P., 1991. Genetic constructions for creation of cellulase producing lactobacilli. Proc. 5th Int. Symp. Forage Preservation, Nitra, Czechoslovakia, September 1991, pp. 86-91. Henderson, A.R., 1991. Biochemistry in forage conservation. In: G. Pahlow and H. Honig (Editors), Proc. European Grassland Federation Conf., Forage Conservation towards 2000, Braunschweig, January 1991, pp. 37-47. Henderson, A.R., McDonald, P. and Woolford, M.K., 1972. Chemical changes and losses during the ensilage of wilted grass treated with formic acid. J. Sci. Food Agric., 23: 1079-1087. Henderson, A.R., Anderson, D.H., Neilson, D., Hunter, E.A. and Phillips, P., 1989. The effect of a high rate of application of formic acid during ensilage of ryegrass on silage dry matter intake of sheep and cattle. Anim. Prod., 48: 663-664. Henderson, A.R., Anderson, D.H., Scott, N.A. and Hunter, E.A., 1990a. A comparison of the nutritive value of silages treated with either a bacterial inoculant/enzyme or a high level of formic acid. Proc. 9th Silage Conf., University of Newcastle Upon Tyne, Newcastle upon Tyne, September 1990, pp. 70-71. Henderson, A.R., Stanway, A.P. and McGinn, R., 1990b. Aerobic stability of Maxgrass-treated silages. Br. Grassl. Soc. Occas. Symp., 25: 224-227.

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Henderson, A.R., Seale, D.R., Anderson, D.H. and Heron, S.J.E., 1990c. The effect of formic acid and bacterial inoculants on the fermentation and nutritive value of perennial ryegrass silages. In: S. Lindgren and K.L. Pettersson (Editors), Proc. Eurobac Conf., Uppsala, August 1986. Swedish University of Agricultural Sciences, Uppsala, pp. 93-98. Henderson, A.R., McGinn, R., Stanway, A.P. and Morgan, C.A., 1991. A technique designed to evaluate commercial polysaccharide degrading enzymes as additives for grass silage. Proc. 5th Int. Symp. Forage Preservation, Nitra, Czechoslovakia, September 1991, pp. 92-95. Heron, S.J.E., Henderson, A.R. and Cunningham, M., 1987. The effects of inoculation with enterobacteria and proteolytic clostridia on ensiling sterile and non-sterile ryegrass. Proc. 8th Silage Conf., Hurley, UK, September 1987. AFRC Institute for Grassland and Animal Production, Hurley, pp. 5-6. Heron, S.J.E., Edwards, R.A. and Phillips, P., 1989. The effect ofpH on the activity of ryegrass proteases. J. Sci. Food Agric., 46: 267-277. Hinks, C.E., Henderson, A.R., Gilchrist-Shirlaw, D.W., Parkinson, H. and Prescott, J.H.D., 1980. The utilisation of lucerne and ryegrass silages and the effects of patterns of barley supplementation on the growth and carcass composition of fattening steers. Br. Grassl. Soc. Occas. Symp., 11: 413-423. Honig, H., 1990. The effect of inoculation under slight air influence. In: S. Lindgren and K.L. Pettersson (Editors), Proc. Eurobac Conf., Uppsala, August 1986. Swedish University of Agricultural Sciences, Uppsala, pp. 68-73. Honig, H. and Pahlow, G., 1990. The effect of an enzyme preparation on the fermentation of grass silage. Proc. 9th Silage Conf., University of Newcastle upon Tyne, Newcastle upon Tyne, September 1990, pp. 18-19. Jacobs, J.L. and McAllan, A.B., 1990. The effect of enzyme treatment on silage composition and in sacco degradability of ADF and NDF. Proc. 9th Silage Conf., University of Newcastle upon Tyne, Newcastle upon Tyne, September 1990, pp. 86-88. Jones, R. and Jones, D.I.H., 1988. Effect of absorbents on effluent production and silage quality. In: B.A. Stark and J.M. Wilkinson (Editors), Silage Effluent. Chalcombe Publications, Marlow, UK, pp. 47-48. Kennedy, S.J., 1988. An absorbing experiment. In: B.A Stark and J.M. Wilkinson (Editors), Silage Effluent. Chalcombe Publications, Marlow, UK, pp. 52-53. Kennedy, S.J. and Carson, T., 1991. The effect of Maxgrass silage additive and level of concentrate supplementation on intake and performance of finishing beef cattle. In: G. Pahlow and H. Honig (Editors), Proc. European Grassland Federation Conf., Forage Conservation towards 2000, Braunschweig, January 1991, pp. 396-400. Lindgren, S., 1991. Hygienic problems in conserved forage. In: G. Pahlow and H. Honig (Editors), Proc. European Grassland Federation Conf., Forage Conservation towards 2000, Braunschweig, January 1991, pp. 177-190. Lindgren, S. and Pettersson, K.L. (Editors), 1990. Proc. Eurobac Conference. August 1986, Uppsala. Swedish University of Agricultural Sciences, Uppsala. Lindgren, S.E., Axelsson, L.T. and McFeeters, R.F., 1990. Anaerobic L-lactate degradation by Lactobacillus plantarum. FEMS Microbiol. Lett., 66:209-214. Mandel, I.B., Mowart, B.N., Bilanski, W.K. and Rai, S.N., 1989. Effect of heat treatment of alfalfa prior to ensiling on nitrogen solubility and in vitro ammonia production. J. Dairy Sci., 72(8): 2046-2054. Mayne, C.S. and Steen, R.W.J., 1990. Recent research on silage additives for milk and beef production. 63rd Annual Report 1989-1990, Agricultural Research Institute of Northern Ireland, pp. 31-42. McAllan, A.B., Jacobs, J.L. and Merry, R.J., 1991. Factors influencing the amount and pattern of silage effluent production. In: G. Pahlow and H. Honig (Editors), European Grassland

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