22 November 1996


First priority when deciding which additive to use is to ensure the product improves animal performance. Sue Rider reports

HIGH quality silage depends on harvesting high digestibility grass, avoiding soil and slurry contamination, and ensiling as rapidly as possible.

So says Tim Keady of the Agricultural Research Institute of Northern Ireland, Hillsborough. Additives can be used to aid good ensiling management, but they are not a substitute for it, he stresses. When an additive is needed, his advice on product choice is clear: "The first priority is to ensure that the product improves animal performance relative to untreated silages under conditions similar to your own," says Dr Keady.

He then suggests checking that product claims are supported by trials. Other silage additive selection criteria include the effects of the additive on fermentation and effluent output, additive price, weather at harvesting, risks of soil or slurry contamination at harvesting, and type of stock to which the silages are to be offered.

Inoculants are the additives to use, says Dr Keady, unless ensiling conditions are tricky in which cases he recommends formic acid.

"Improvements in animal performance have been obtained with effective inoculant additives under a range of conditions, with responses obtained using formic acid under difficult ensiling conditions."

Although molasses, sulphuric acid, enzymes, and beet pulp improve fermentation, there is little evidence to suggest these products improve animal performance, says Dr Keady.

He divides the silage additives on the market into two broad categories:

&#8226 Inhibitors – which include acids such as formic and sulphuric.

&#8226 Stimulants which include molasses, enzymes and bacterial inoculants.

Bacterial inoculants

An inoculant is a product containing lactic acid bacteria which produce lactic acid to aid silage fermentation, says Dr Keady. But he warns that inoculant is an umbrella term used to describe any culture of bacteria applied as an additive to grass at ensiling. Some contain a single strain of L. plantarum, he says, whereas others contain numerous strains of bacteria, enzymes, clostridiophages, rumen enhancers, and nutrients.

"It is important to be careful about which product you use to ensure the correct level of bacteria are applied to the grass," says Dr Keady.

"Effective inoculants should be applied at the recommended rate, the aim being to apply 1m colony forming units for every 1g of fresh herbage ensiled."

He suggests, then, that before using a particular inoculant, it is important to have evidence of positive trial results from independent research centres where animal performance was assessed under similar conditions to those in which the product will be used.

In 11 experiments at Hillsborough different inoculants were evaluated using herbages with low dry matter and sugar contents (see Table 1).

Inoculant treatment increased silage dry matter intake and milk yields by an average 1.3 litres but the response varied from an increase of two litres to a reduction of one litre – indicating the variability of the products, says Dr Keady.

Silage fermentation was not always altered by inoculant treatment. Other studies at Hillsborough have shown that the use of effective inoculant, used under conditions where the untreated silage was poorly preserved, increases daily carcass gain of beef cattle, from 0.46 to 0.52kg. A similar response was seen from a well preserved formic acid-treated silage (see Table 2).

Formic acid

Formic acid is an organic acid which is normally applied at 2.3-2.5 litres a tonne of herbage ensiled. It acidifies the crop and reduces the pH to about 4.7 at ensiling. The pH is then reduced further by the activity of the indigenous bacteria.

"Formic acid is the most tried and tested additive available on the market and the one with which all new products are normally compared," says Dr Keady. "But evidence indicates that, when untreated silage is well preserved, treatment with formic acid is unlikely to increase animal performance."

However, from work at Hillsborough and elsewhere, where formic acid treatment improved silage fermentation, as measured by decreases in pH and ammonia nitrogen concentrations, increases in silage intake and animal performance have been obtained.

For example, treatment with formic acid has, he says, increased liveweight gain of growing and finishing beef cattle by 22%, from 0.54 to 0.66kg/day (see Table 4) and increased daily carcass gain of finishing beef cattle by 8% (see Table 3) and 13% (Table 2).

Similarly, dairy cows trials show that where formic acid improved silage fermentation, it increased milk fat and protein yields.

Sulphuric acid

Sulphuric acid improves silage fermentation, but there is no evidence to show it improves animal performance, says Dr Keady.

The average of five experiments with finishing beef cattle (see Table 3), which included poorly preserved untreated silage, showed that treatment with sulphuric acid failed to improve carcass gain relative to untreated silage.

However, in the same comparisons, formic acid treatment increased daily carcass gain by 8%, from 0.49 to 0.53kg/head.

Also in two studies with dairy cows at Hillsborough, where material of low dry matter and water soluble carbohydrate contents has ensiled, treatment with sulphuric acid did not alter milk fat plus protein yields.


The main aim using molasses as a silage additive is to increase the supply of available energy for the bacterial growth to produce lactic acid and preserve the crop, says Dr Keady.

But from a review of 14 experiments, in which molasses treated silage were compared directly with untreated silages, treatment with molasses did not significantly alter animal performance.

Furthermore, even in those studies where molasses treatment improved silage preservation, it had little effect on liveweight gain, altering it from 0.54 to 0.58kg/day whereas formic acid treatment increased it by 22% to 0.66kg/day (Table 4).


Under difficult ensiling conditions enzymes have been shown to improve silage preservation. But from the results of four experiments enzyme treatment had no effect on milk fat plus protein output in lactating dairy cattle (Table 5).

Beet pulp

Inclusion of beet pulp improves silage fermentation. But in studies at Hillsborough (Table 6) in which sugar beet was used as an additive, at rates ranging from 40 to 120kg pulp a tonne of herbage ensiled, there were no differences in terms of animal performance whether the beet pulp was fed in the ensiled blend or offered as a supplement with the untreated silage the next winter.n


Dr Tim Keady:

"Use an inoculant unless ensiling conditions are tricky."

Formic acid treated silage improved liveweight gain of growing and finishing beef centres by 22%

Table 1. Effect of treatment with inoculants on silage fermentation and the performance of dairy cows (11 experiments)




Ammonia (g/kg N)10094

Silage DM intake (kg/d)9.410.3

Milk yields (kg/d)19.921.2

Source: Mayne and Steen 1993

Table 2. Effect of treatment of difficult-to-ensile herbage with an effective inoculant on silage fermentation and the performance of beef cattle




Ammonia N (g/kg N)912111182

Silage DM intake (kg/d)

Carcass gain (kg/d)0.460.520.52

Source: Keady and Steen 1994

Table 3. The effect of sulphuric acid on silage fermentation and performance of beef cattle (5 expts)


Untreated Sulphuric Formic



Ammonia N (g/kg N)947155

Silage DM intake (kg/d)

Carcass gain (kg/d)0.490.470.53

Source: Kennedy 1990

Table 4. Effect of molasses treatment of difficult-to-ensile herbage on silage fermentation and performance of beef cattle (6 expts)


Untreated MolassesFormic



Ammonia N (g/kg N)1459563

Silage DM intake (kg/d)

Daily liveweight gain (kg/d)0.540.580.66

Table 5. The effects of enzyme treatment of difficult-to-ensile herbage on silage fermentation and performance of dairy cows (4 expts)




Ammonia N (g/kg N)175137

Silage DM intake (kg/d)9.19.1

Fat and protein (kg/d)1.501.49

Table 6. The effect of the additions of beet pulp on silage fermentation and

performance of dairy cows


Untreated Ensiled blend (kg inclusion) Untreated silage plus beet pulp

silage 40801204080120


Ammonia N (g/kg N)19013710987

Total DM intake10.913.0 13.9 14.912.8 14.5 15.2 (kg/d)

Milk yield (kg/d)13.416.5 18.3 19.016.9 18.2 20.1

Source: Ferris and Mayne 1990