Grass silage forms the basal forage for the vast majority of dairy and beef cattle in Ireland and the United Kingdom. During the ensiling process major changes occur in the chemical composition of herbage. Two major changes are firstly, the conversion of water soluble carbohydrate primarily to lactic and volatile fatty acids and secondly, an increase in the rapidly soluble component of crude protein due to proteolysis and deamination processes.
Traditionally, it has been considered that ensilage results in a reduction in intake and animal performance, as in practice cows grazing grass outdoors have higher intakes and consequently higher milk yields than those indoors receiving silage. However, this is not a valid comparison. Recent studies (Keady et al., 1996; Keady and Murphy, 1998) clearly show that when cows are offered herbage either as fresh grass, zero-grazed, or grass conserved as silage using a good ensiling technique, ensiling does not affect intake but decreases milk yield and protein concentration (Table 1). The reduction in animal performance with ensilage is due to the conversion of water soluble carbohydrates to lactic and volatile fatty acids, which are poor energy sources for rumen microbes. The fermentation process also results in a reduction in microbial protein synthesis.
In the production of grass silage the main aim is to produce a product that will support high levels of animal performance, as the producer is paid for performance, be it milk or beef. The aim of this paper is to highlight some of the main factors which influence animal performance and consequently profit from grass silage.
Factors affecting silage intake
Many studies have been undertaken to determine the factors influencing silage intake. Initial studies (Wilkins et al., 1978; Rook and Gill, 1990) have attempted to produce multifactor relationships to predict the intake of silage. However, the accuracy of these relationships is limited due to the fact they are based on data obtained from a number of studies and are confounded by factors such as breed of animal, previous nutritional history, physiological state, length of feeding period, etc. Recently a major study (Steen et al., 1995) to determine the factors affecting silage intake has been completed at this Institute. One hundred and thirty-six grass silages produced on farms across Northern Ireland were offered ad libitum as the sole source of nutrition to growing beef cattle.
The chemical compositions and intakes of the 136 silages (Table 2), differed dramatically. For example the intake of the best silage was more than 2.5 times that of the poorest silage.
Table 1. The effects of ensilage on herbage composition and animal performance.
Keady et al., 1996; Keady and Murphy, 1998.
Table 2. Chemical composition of the silages offered in the intake study.
Steen et al., 1995.
Relationships between individual constituents within the silages and silage intake have been developed to provide an overview of the extent to which intake is related to, or determined by, the content of different chemical constituents within the silages. The results of this analysis indicate that intake was poorly related to some chemical constituents such as pH, buffering capacity and lactic, acetic and butyric acids, which previously were considered to have a major effect on silage intake. Factors of moderate importance included dry matter (DM) and ammonia nitrogen (NH3-N) concentrations. The key factors identified as affecting silage intake were the protein and fibre fractions, and the rate and extent of digestion of these components within the animal.
The most interesting outcome of this major study was the fact that near infrared reflectance spectroscopy (NIRS), both on dried and fresh samples, provided the most accurate prediction of silage intake (Park et al., 1997).
Subsequently a silage Feeding Information System has been developed commercially using NIRS. This system provides information on potential levels of animal performance which may be obtained from dairy and suckler cows, growing and finishing beef cattle, purchased feeder lambs, and ewes when offered the silage supplemented with varying levels of concentrate.
In order to produce high quality silage it is important to have swards with low contents of non-sown species as these are more difficult to ensile and produce lower herbage yields and silage of lower feeding value relative to perennial ryegrass swards.
For first, second and third cut silages the recommended levels of nitrogen (N) fertiliser application are 125, 100 and 90 kg/ha, respectively. If undiluted slurry has been applied in cool damp weather (e.g. during February or March), then 6–10 kg of N should be allowed per 4,546 litres (1,000 gallons) applied per hectare. However if slurry is applied in hot dry weather (e.g. late May or July) then only 1–3 kg of N should be allowed per 4,546 litres. Excess application of N can have adverse effects on herbage ensilability at harvest (Keady and O’Kiely, 1996; Keady et al., 1998b), rate of fermentation post-harvest (Keady and O’Kiely, 1996) and on the subsequent silage intake and milk yield of lactating dairy cows (Table 3) (Keady et al., 1995). These results highlight the dangers of applying excess fertiliser N and also full allowance should be made for the N content of slurry, if applied.
The quantities of phosphate and potash which should be applied to the silage area depends on soil fertility, which should be determined by soil analysis every 4 to 5 years. Slurry is a valuable source of phosphate and potash. Four thousand five hundred and forty-six litres (1,000 gallons) of undiluted dairy cattle slurry contains the equivalent of 4.5 and 24 kg phosphate and potash, respectively.
Depending on the results of soil analysis, up to 120, 100 and 100 kg potash/ha may be required for first, second and third cut silages, respectively. In a recent study, Keady and O’Kiely (1998) have shown that applications of excessive quantities of potash to silage ground (soil index = 2) has no effect on herbage ensilability, silage fermentation or silage feeding value (Table 4). However, applications of excess potash obviously represents a risk of nutrient loss and zero capital return.
Table 3. Effects of fertiliser nitrogen on silage crude protein concentration and performance of dairy cows.
Keady et al., 1995. *Formic acid applied at levels to ensure similar silage fermentation.
Table 4. Effect of fertiliser potash on herbage composition, silage fermentation and feeding value.
Keady and O’Kiely, 1998.
Silage digestibility, intake and performance
Digestibility is one of the most important factors influencing DM intake and animal performance from silage. The effects of digestibility on performance of dairy cows was reviewed by Gordon (1989c) who concluded that a 10 g/kg increase in digestible organic matter in silage DM (D-value) resulted in an increase in silage DM intake and milk yield of lactating dairy cattle of 0.16 and 0.37 kg/cow/day, respectively. Also, increasing silage digestibility is one of the major factors affecting milk protein content. From a review of studies undertaken at the Institute it is concluded that for each 10 g/kg increase in silage D-value, milk protein content is improved by 0.16 g. Similarly, Steen (1987) concluded that a 10 g/kg increase in D-value resulted in an increase in carcass gain of beef cattle of 33 g/day when silage was offered as the sole diet and 28 g/day when concentrates constituted proportionally 0.20 to 0.37 of total DM intake.
Date of harvest
Date of harvest is the most important factor affecting digestibility. Digestibility of herbage harvested between 10 May and 7 June declines linearly by 3.6 units per week harvest is delayed (Keady et al., 1998b). Similarly, the rate of decline in herbage digestibility from the primary regrowth is similar to that of the primary growth. For example, Gordon (1980) and Keady et al. (1997) reported declines of 3.4 and 3.5 units D-value per week delay in harvesting the primary regrowth between weeks 5 and 10 of growth. Consequently, to produce a similar milk yield or carcass gain with grass harvested 1 week later, 1.5 and 1.2 kg additional concentrate must be fed daily to lactating dairy and finishing beef cattle, respectively.
Lodging or flattening of the grass crop before harvest accelerates the rate of decline in grass digestibility. In severely lodged crops digestibility may decline by as much as nine units per week (O’Kiely et al., 1987).
Silage produced from old permanent pastures normally has a lower digestibility relative to silage produced from perennial ryegrass swards. Also, it is generally assumed that the optimum time to harvest grass is when the sward reaches 50% ear emergence. However, recent studies (Steen, 1992) indicate that in order to obtain silages with similar digestibility from early and late heading varieties of perennial ryegrass, they must be harvested within 7–8 days of each other. Delaying harvest until the late varieties reach 50% ear emergence would result in a reduction in D-value of 8 units relative to the early heading varieties consequently reducing silage intake and animal performance.
Relative to well-preserved silages, poorly preserved silages with low lactic acid and high NH3-N contents normally have lower digestibility. The decline in digestibility due to a deterioration in silage fermentation may be as high as 5 to 6 units of D-value.
There has been renewed interest in pre-wilting of grass prior to ensiling, given the development of sophisticated conditioning and tedding equipment and the desire to reduce effluent output from an environmental point of view.
Detailed studies undertaken at this Institute (Wright, 1997) examining factors affecting the speed of wilting clearly indicate that the most important weather factor is the duration and intensity of sunshine and the most important management factor is the depth of the swath, i.e. the lower the depth the greater the drying rate.
Table 5. Effects of wilting on animal performance.
Patterson et al., 1996, 1998.
A total of eleven recent studies have been undertaken at this Institute (Patterson et al., 1996, 1998) to evaluate the effects of rapid wilting of herbage on subsequent dairy cow performance (Table 5). These studies indicate that when herbage DM was increased from 160 to 320 g/kg at ensiling, rapid wilting (approximately 30 hours) dramatically increased intake by 17% and slightly improved milk yield by 2.4%. Also, rapid wilting increased milk fat and protein contents, subsequently increasing fat plus protein yield by 6%. However, although wilting increases silo life and reduces effluent output it reduces milk output/per hectare.
TYPES AND PURPOSES OF SILAGE ADDITIVES
Until recently, the principal objective in applying a silage additive was to improve silage fermentation under difficult ensiling conditions. This was achieved by applying acid or sugar-based additives. However, more recent research has shown that the use of effective inoculants can substantially improve animal performance without necessarily altering the fermentation quality of the silage at the time of feeding.
Animal performance is the most important measure of the efficacy of a silage additive, as producers are paid for animal product and not for the preservation quality of silage as measured by conventional laboratory analysis.
When applying additives it is important to apply them at the correct rate, taking account of changes in the moisture content of the grass being ensiled.
For example, if the dry matter of the herbage is increased from 180 to 250 g/kg, the fresh weight of grass will be reduced from 29.5 to 21 t/ha consequently reducing additive requirement by 40% per ha.
Recently UKASTA (The United Kingdom Agricultural Supply Trade Association) published its new forage additive approval scheme. This is a voluntary scheme in which companies submit dossiers of trial results which are subsequently submitted to a team of independent scientists from the Department of Agriculture for Northern Ireland, the Agricultural Development and Advisory Service, and the Scottish Agricultural College. The additives are then given approval in different categories depending on information available.
The categories in which approval can be obtained include: (C): silage quality, (B): feed intake and efficiency of nutrient use and (A): performance of beef and dairy cattle. Each additive is listed under one of the following headings: inoculant and inoculant and enzyme additives; absorbent additives; acids and salts; enzyme only additives or sugar additives. The scheme is an aid to selecting the correct silage additive for particular farm circumstances; however, the user needs to have good background information on the mode of action of the various classes of additives to obtain the maximum benefit. More information needs to be added to the list including the number of trials undertaken with each product, the proportion of these which produced positive results, the ensiling conditions and what types of conditions are likely to lead to the maximum benefit from the use of different products.
Formic acid is an organic acid and is the most tried and tested additive available on the market. Upon application, formic acid directly acidifies the crop restricting bacterial activity, consequently conserving greater quantities of water soluble carbohydrates and reducing proteolysis.
The effects of formic acid on silage intake and animal performance as measured by fat and protein yield of dairy cows and carcass gain of beef cattle are presented in Tables 6 and 7. Data in Table 6 are from 16 comparisons in which formic acid treatment had little effect on silage fermentation. These data clearly indicate that formic acid treatment is unlikely to result in an economic response in animal performance of beef and dairy cattle when applied to herbage which would preserve well without additive treatment.
In contrast, the data in Table 7 are from 18 comparisons in which formic acid treatment improved silage fermentation relative to poorly preserved untreated silages. These data demonstrate that formic acid treatment under difficult ensiling conditions improves silage fermentation and subsequent animal performance of beef and dairy cattle by 7 and 17%, respectively.
It can be concluded that formic acid treatment will produce an economic response where the untreated silage would be of a poor fermentation quality. However, under conditions where the silage would preserve well in the absence of an additive, formic acid is unlikely to result in an economic response in terms of increased animal performance. Also, formic acid treatment is likely to increase silage intake regardless of ensiling conditions. These differences in response to animal performance highlight the need to develop a system of grass analysis which will enable the magnitude of the response which can be achieved due to additive treatment to be measured based on the chemical composition of the standing grass crop.
The use of sulphuric acid as an additive became popular in the 1970s because of its ability to improve silage fermentation at a lower cost relative to formic acid treatment. Sulphuric acid acidifies the crop directly upon application, but does not have the same antimicrobial activity as formic acid.
The effects of sulphuric acid on silage fermentation, intake and animal performance of beef and dairy cattle from 11 comparisons are presented in Table 8. There is a substantial volume of evidence to indicate that sulphuric acid has the same positive effects on silage fermentation as achieved from the application of formic acid. However, in terms of animal performance, as measured by fat and protein yield of dairy cows and carcass gain of beef cattle, the data in Table 8 clearly show treatment with sulphuric acid tends to decrease performance of beef and dairy cattle by 4 and 2%, respectively, while tending to increase silage intake by 3.3%.
Furthermore, there is no experimental evidence in the published literature to indicate that relative to an untreated silage, treatment with sulphuric acid increased animal performance as measured by carcass gain of beef cattle or fat and protein yield of lactating dairy cows.
Table 6. Effects of formic acid on silage intake and animal performance (carcass gain‡, fat plus protein yield†). (Studies with small improvements in fermentation).
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Table 7. Effects of formic acid on silage intake and animal performance (carcass gain‡, fat plus protein yield†). (Studies with large improvements in fermentation).
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* Personal communication.
Molasses is normally applied at 9–18 litres (2–4 gallons) per tonne of herbage ensiled. The mode of action of molasses as a silage additive is to supply sugar to the crop providing additional substrate for the indigenous microbial population. The sugar concentration of herbage is increased by 4 g/kg (0.4%) fresh weight for each 9 litres (2 gallons) applied per tonne at ensiling. However, from a comprehensive review of the literature, Keady (1996) concluded that a sizeable proportion of the sugars from applied molasses is either lost in effluent, as gas, or converted into undesirable fermentation acids (e.g. acetic and butyric).
The effects of molasses on silage fermentation, animal performance and intake are presented in Table 9. Comparisons in which formic acid was used as a positive control are also included. From the average of 11 studies in which molasses was applied at 15.8 l/t, molasses improved silage fermentation as measured by reductions in pH and NH3-N concentrations.
Molasses treatment resulted in a slight improvement in animal performance relative to the poorly preserved untreated silages, while at the same time formic acid treatment significantly increased performance by 17%. From this major review Keady (1996) concluded that the response obtained from molasses (applied at 15.8 l/t) was only 29% that of formic acid (applied at 3.03 l/t). The absence of a significant response to molasses treatment in animal performance occurred in spite of the fact that it increased silage intake by 10%. Furthermore Keady (1996) concluded from the mean of six studies that molasses treatment had no effect on silage digestibility.
Cellulase and hemicellulase are the main fibrolytic enzymes used as silage additives. The mode of action of enzymes when applied as silage additives is to degrade the fibrous fractions of the herbage consequently providing extra substrate for the indigenous microbial population. The effects of enzyme additives on silage fermentation, animal performance and intake assessed from 11 comparisons are presented in Table 10. These data clearly show that enzyme additives improved silage fermentation, but did not affect animal performance, tending to reduce fat and protein yield and carcass gain by 1 and 5%, respectively.
Table 8. Effects of sulphuric acid on silage intake and animal performance (carcass gain‡, fat plus protein yield†).
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Table 9. The effects of molasses and formic acid on silage fermentation, digestibility and animal performance.*
* Adapted from Keady, 1996.
Sugar beet pulp is normally applied at the rate of 50 kg/t of silage ensiled. The use of beet pulp as an additive became popular in the mid 1980s because of its ability to retain effluent within the silo. For each kg of beet pulp ensiled, 1 to 2 kg of effluent is retained within the silo. The quantity of effluent retained depends on the DM of the herbage at ensiling, the quantity of pulp applied and how evenly the pulp is distributed in the silo. The effect of beet pulp inclusion on silage fermentation, animal performance and intake from five comparisons is presented in Table 11. These data clearly indicate that even though beet pulp inclusion has the ability to improve silage fermentation as measured by reductions in pH and NH3-N concentration, it had no beneficial effect on animal performance of lactating dairy cows or finishing beef cattle.
An inoculant additive may be defined as any product which applies bacteria to grass at ensiling. Inoculants vary considerably in biological composition as some contain a single strain of Lactobacillus plantarum while others contain numerous strains of bacteria in combination with enzymes, clostridiaphages and nutrients. When applying an inoculant it is important that the bacteria are active, otherwise application will be uneconomic.
Consequently, it is important that inoculant-based products are stored under proper conditions and are applied prior to their expiration date. The effects of inoculant treatment on silage intake and on fat and protein yield of dairy cows and carcass gain of beef cattle when the untreated silage was well preserved are presented in Table 12. These data, on average, show that inoculant treatment increased silage intake by 5% and performance of beef and dairy cattle by 13 and 5%, respectively. The data in Table 12 are from studies which evaluated different inoculant products. While the average improvement in animal performance was 7%, it ranged from -2 to +26%. This body of data clearly shows the importance of ensuring that a particular product has the ability to improve animal performance under similar conditions in which its use is proposed.
Within the industry there is a popular misconception that inoculant treatment will not improve animal performance under conditions in which untreated silage would be poorly preserved. Data from nine comparisons of inoculant treatment effects on silage fermentation, animal performance and intake when the untreated silages were poorly preserved are presented in Table 13. This data set demonstrates that even when the untreated silages were poorly preserved, inoculant treatment on average increased performance as measured by fat and protein yield of dairy cows and liveweight gain or carcass gain of beef cattle by 3, 25 and 14%, respectively in the absence of any major effect on silage intake. The data presented in Tables 12 and 13 show that use of effective inoculants can produce beneficial effects on animal performance across a wide range of ensiling conditions.
Table 10. The effects of enzyme additives on silage intake and animal performance (carcass gain‡, fat plus protein yield†)
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Table 11. The effects of inclusion of sugar beet pulp (SBP) on silage fermentation and animal performance (carcass gain‡, fat plus protein yield†).
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Table 12. The effects of inoculant treatment on silage fermentation and animal performance (carcass gain‡, fat plus protein yield†). (Studies with well fermented control silages).
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Mode of action of inoculants
Numerous studies (Gordon, 1989a; Mayne, 1990; Keady and Steen, 1994, 1995) have shown that inoculant treatment results in a more rapid fermentation as measured by fall in pH immediately post-ensiling relative to untreated herbage. This more rapid fermentation due to inoculant treatment suppresses proteolysis and deamination processes of herbage protein (Heron et al., 1987) and results in higher retention of soluble components.
From a review of 40 experiments in which the organic matter digestibility of inoculated and untreated silages were compared, Keady (1991) concluded that inoculant treatment increased organic matter digestibility by 2%. In a more recent series of five comparisons at this Institute, Keady et al. (1994) and Keady and Steen (1994, 1995) concluded that the use of an effective inoculant containing a single strain of L. plantarum substantially increased silage digestibility, particularly of difficult-to-ensile herbage primarily due to dramatic improvements in the digestibility of the fibre fractions. The same authors concluded that the improvement in animal performance following treatment with an effective inoculant can be attributed to the retention of greater proportions of the more soluble components of the plant due to a more rapid and efficient fermentation process within the silo. This subsequently results in increased silage digestibility and a reduction in the extent of protein break down.
Type of concentrate supplementation
Considerable progress has been made recently at our Institute in predicting silage intake when offered as the sole diet to beef cattle. However, in most farm situations silages are usually supplemented with varying levels and types of concentrates when offered to high producing dairy and beef cattle.
Silages differ widely in terms of DM, fermentation, digestibility and intake characteristics; consequently there is considerable discussion regarding the relative merits of feeding different types of concentrates, depending on silage composition, to maximise animal performance.
Two studies (Keady et al., 1997, 1998a) have been completed recently at the Institute where nine silages differing considerably in chemical composition (Table 14) were offered to dairy cows in early lactation. These silages were supplemented with 10 kg of concentrates per day offered in four equal feeds through out-of-parlour feeders.
Eleven different concentrates were formulated to contain similar crude protein and metabolisable energy concentrations but differing levels of starch and digestible undegradable protein (DUP) concentration. The low starch concentrates were formulated predominantly from sugar beet pulp, citrus pulp and soyabean meal while the high starch (56% cereals) concentrates were formulated predominantly from wheat, barley and soyabean meal. The effects of level of starch in the concentrate on silage intake of the nine contrasting silages are presented in Table 15. Regardless of silage composition, which varied dramatically in fermentation, DM and digestibility characteristics, increasing the level of starch in the concentrate did not alter silage intake.
Table 13. The effects of inoculant treatment on silage fermentation and animal performance (carcass gain‡, liveweight gain*, fat plus protein yield†).
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Table 14. Chemical composition of silages offered in silage type by concentrate type studies.
Keady et al. 1997, 1998a.
Table 15. Effects of silage and concentrate types on silage intake (kg DM/day).
After Keady et al., 1997; 1998a.
* For silages 1–4: low, medium and high= 22, 148 and 273 g starch/kg DM. For silages 5–9: low, medium and high=50, 209 and 384 g starch/kg DM.
Furthermore, these studies concluded that regardless of silage type, increasing the level of starch in the concentrate did not alter silage intake or milk yield, but increased milk protein concentration (Table 16) and consequently would be expected to increase farm revenue in a milk quota situation.
The protein source in the low and high DUP concentrates were soyabean and SoyPass (soyabean protected using lignosulphonate), respectively. From these studies it can be concluded that increasing the level of DUP did not alter silage intake or animal performance at this relatively high level of concentrate feeding. These results suggest that there is no scientific basis for formulating different concentrate types to complement silages which differ in fermentation, digestibility or intake characteristics.
Table 16. Effect of concentrate starch content on silage intake and animal performance.
Keady et al., 1997, 1998a.
In many parts of Ireland and the United Kingdom grass silage is, and will remain, the basic forage for the majority of dairy and beef cattle during the winter period. For the production of high levels of animal performance, high digestibility and high intake silage is required.
Silage intake is strongly related to the content of protein and fibre components and digestibility. However, silage fermentation characteristics, e.g. pH and lactic, acetic and butyric acids etc., have little or no effect on intake, in contrast to previous popular conception.
The key factors in producing silage are:
(a) Apply the correct level of plant nutrients to produce quality swards.
(b) Harvest high digestibility herbage at a leafy stage of growth.
(c) Ensile the herbage cleanly and rapidly, consolidate well and seal quickly.
(d) Use an effective additive to supplement and not to substitute for good management.
(e) Additives should be chosen on their proven ability to increase animal performance as many additives improve fermentation but do not increase animal performance.
(f) If wilting grass, a rapid wilt (i.e. less than 30 hours) using tedding and conditioning is necessary. Wilting reduces animal product output per hectare.
(g) There is no scientific evidence for formulating different concentrate types to complement silages which differ in fermentation, digestibility or intake characteristics.
Author: TIMW.J. KEADY
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