24 November 1995

A STANDARD FOR ANALYSIS

Silage analysis services urgently need standardising, according to one independent nutrition consultant.

Sue Rider reports his views

SILAGE analysis reports should be designed to meet the needs of livestock producers and not those of the adviser, compounder or additive rep for which the laboratories are compiling the reports.

So says Wolverhampton-based independent nutrition consultant Stuart Jones.

He believes that silage analysis reports are both varied and confusing. The variation arises because different labs use different systems of analysis and report different information. That same information is also reported in different ways.

He says farmers would benefit from labs adopting a standard approach to silage analysis. In this way advice would be presented in a more user-friendly format to show whether a grass crop has been preserved successfully as silage and to help the nutritionist plan diets.

All lab silage analysis reports should also be self-explanatory to a nutritionist, he says. At present that is not the case. "It is high time all scientists, lab managers, and consultants remembered that their main aim is to help the industry by helping individual producers."

The analysis

Mr Jones believes a silage analysis report should detail dry matter, fermentation and intake characteristics, and energy and protein contents. Anything else the service offers should be clearly separate from that basic information.

Assessments and determinations to be featured on the report should be carried out in the same way and expressed in standard terms on all reports together with an explanation of terms.

"We need to know how the silage has been preserved and how much animals are likely to eat," he says. "This information is important because it affects decisions about the level and type of the other feeds in the diet."

Mr Jones suggests the following information is all that needs to be included on a basic silage analysis report (see example).

Dry Matter (DM): Dry matter is the amount of useful material in the total fresh weight, the rest is water. DM is usually expressed as a %, but some laboratories use g/kg (DM% x 10).

Mr Jones believes dry matter is the most important measurement because nearly everything else is expressed per unit of dry matter.

Fermentation

pH: Is a measure of acidity. Well preserved and wetter silages normally have lower pH values than poorly preserved and drier silages.

To be interpreted well the dry matter content must also be known, says Mr Jones. For example, a well preserved wet silage (18-22%) will have a pH between 3.7-4.0 but a high dry matter silage (30-35%) can have a pH of 4.5-4.8 and still be well preserved.

Ammonia (ammonia-N as a % of total N): This is the single most important indicator of the control of fermentation, he says. It is an estimate of the amount of protein that has been degraded completely in the silage during storage. Higher values are associated with poorer quality fermentation. In well fermented silage the value will be less than 10. Under 5 and the crop will be very well preserved. This figure is a familiar one, but all too often confusion is caused by labs using other methods of reporting ammonia values.

Total Fermentation Acids (% in DM): Grass in the field has virtually no acid, so when you measure acidity in silage you are measuring the extent of primary fermentation during storage. That is what the TFA figure portrays. It is a measure of how much of the carbohydrates have been converted to acids. The more that has been converted the more likely it is that the material will have a low pH. In well fermented silage, TFAs are usually 8-12% of DM.

Lactic Acid – Good lactic acid content is desirable because it indicates a good primary fermentation, says Mr Jones. Sugars have been converted to lactic acid which prevents undesirable secondary fermentations (breakdown of protein). Look for a figure of 8-10% in the DM (when the crop is at 23-25% DM), he advises. This will mean there is sufficient acid to preserve the crop without risking excess acidity which might reduce intakes.

When the lactic acid content is over 80-85% of the total fermentation acids then there has been good control of primary fermentation and a stable silage product should result.

Mr Jones considers too much emphasis is placed on silage intake characteristics. "Few farmers make enough silage to be able to really encourage cows to eat as much as they want," he says. The biggest concern is to ensure that what they have made is preserved well.

Acetic Acid – This is not so important because it is difficult to interpret what it means. It can be desirable because it is a precursor of butterfat synthesis and encourages milk yields. However, because acetic acid can also be a by product of secondary fermentation its presence is not always welcome. It should not be higher than 2% of the dry matter, he says.

Butyric Acid – Silage should contain less than 1% in DM and ideally none for it is largely formed only as a by-product of secondary fermentation. Presence of high ammonia and high butyric acid levels indicate secondary fermentation has developed during storage and before the clamp has been opened.

Once the dry matter of the silage and its fermentation characteristics are known it is possible to determine whether silage-making techniques have produced a desirable fermentation.

"When a good fermentation is achieved the next step is to determine how to use the silage in rations," he says. "We need to be able to calculate how much energy and protein the animals will receive from the amount of silage eaten."

To do this the following feeding value details will be important.

Digestibility (D value): This is the content of digestible organic matter in the dry matter (DOMD). It is predicted by using NIR, MAD fibre or by digestion with neutral cellulose gammanese enzymes (NCGD).

ME metabolisable energy (MJ/kg): The energy available to the animal for the maintenance of body functions and to produce milk, weight gain, and foetal growth. The industry has been working to standardise its procedures for predicting ME.

M√E based on NIRS analysis is the best approach where available to determine silage energy values, says Mr Jones. Virtually all other systems do not predict ME with sufficient accuracy.

That said, it is important to remember the figure is still only an estimate.

For example, we can estimate ME, but the true value is likely to lie in a range 0.5 MJ/kg DM either side of that. So when the predicted ME of a silage is 11.0, the true value will probably lie between 10.5 and 11.5 MJ/kg DM.

FME: Is the amount of ME available for fermentation by the rumen microbes. New methods of predicting FME are being developed. It is necessary to obtain an accurate estimate to use the MP rationing system.

NDF (neutral detergent fibre): This is a measure of the cell wall contents (largely the carbohydrates in the silage that are not sugars). NDF is related to the digestibility of the silage and a silage with a D value of 70 and ME of 11.2 is likely to have a NDF of 48-50% of DM, suggests Mr Jones.

Sugars: Usually only about 1% in DM since the sugars of the original crop are fermented to acids during storage. However, this measure is not necessary as part of a routine analysis. The higher the DM of the ensiled grass the less fermentation required to preserve it, so more sugar will be found in the silage. Some additives also restrict fermentation, so such silages also tend to have higher sugar contents, says Mr Jones.

Ash: Content of fresh herbage is less than 10% so where the ash content of a silage is greater than 10% it is usually a sign of soil contamination. Ash levels at 12-13% in the dry matter reflect poor silage making techniques. It is likely that mud from tractor wheels is contaminating the crop or that the cutter bar is set too low. High ash contents will affect intake. Soil also encourages breakdown of protein (in secondary fermentation) since it contains the Clostridia organisms which otherwise are not normally found in herbage.

Total Nitrogen (crude protein): Is required to calculate diets, it is therefore essential to determine it because it can vary widely between silages. The degradability of crude protein reflects how much is available to the rumen bugs – in grass silages this will be about 80%.

Within that figure some protein will be very rapidly available (the "a" fraction), some becomes available more slowly ("b" fraction) and the rate at which the protein in the rumen is degraded is known as the "c" constant. All these factors are used to plan diets.

Sampling

If analysis results are to be used to advise a feeding plan, then they must provide the information needed by the adviser, says Mr Jones. The sampling method must be representative.

Mr Jones suggests that better assessments could be achieved by establishing a Co-ordinating Body or by asking an existing authority such as the UKASTA Forage Additives Committee to take on the role.


Example of a user-friendly silage analysis report

Dry Matter%

FERMENTATION

pH

Ammonia-N as a % of Total-N

Total fermentation acids% in DM

Lactic acid"

Acetic acid"

Butyric acid"

ENERGY

Predicted D-value

Est MEMJ/kg DM

Est FMEMJ/kg DM

NDF% in DM

Sugars (WSCs)% in DM

Ash% in DM

PROTEIN

Crude protein% in DM

Est degradability

of protein%

Est degradability factors

"a"% of CP

"b"% of CP

"c"rate/hour

Laboratories have the methods and equipment to provide far more accurate assessments of silage than 5-10 years ago. They must help the industry benefit from the new technology by providing clear, helpful reports to a largely standard format.

Have silage-making techniques produced a desirable fermentation? This should be clear from your silage analysis reports – but may not be.

Silage analysis reports should help estimate how the silage has been preserved and how much animals are likely to eat, says Mr Jones.

Nutrition consultant Stuart Jones calls for a co-ordinating body such as UKASTA to ensure that silage analysis reports are standardised to aid use.