Silage analysis

All You Wanted To Know About Ration Formulation But Were Afraid To Ask!!

Dr. Aidan Cushnahan, Greenmount Campus, College of Agriculture, Food and Rural Enterprise (CAFRE)

Advances in predicting the nutritive value of silage and increases in detail in ration formulation reports can at first glance appear to be confusing. The following article should give some guidelines on what to look for in a silage analysis or ration formulation report and what other terms mean.

"Intake value ="

This figure was derived from work at ARINI, Hillsborough and gives an indication of the potential intake of a silage. Factors which affect the intake value include dry matter, crude protein, digestibility, fibre fractions and ammonia N concentration. Figures range from 45 - 110.

Intake value interpretation

"Intake factor ="

Some silage reports will give this figure which was derived from work at the Scottish Agricultural Colleges (SAC). Figures range from 70 (very low) to 130 (very high) and are related to cluster index and neutralising value measurements.

This is a figure developed by SAC which attempts to classify silage into one of four catagories or "clusters".

Silage Cluster indices (SCI)

Some silages occur which are borderline between cluster groups. For example a silage may have a cluster index between 1 and 2. In such cases the report will describe the silage as 1/ 2 with a borderline statement.

"Neutralising value ="

This measurement gives an indication of the likely risk of reduced silage intake due to acids in the silage. It is only found on reports which use the SAC analysis system.

Neutralising value interpretation

Animals eating silage with a high neutralising value will have to produce a lot of saliva to neutralise the silage.

"Dry Matter (DM) ="

This is the quantity of material left in a feed after all the water has been removed by drying. Wet silages tend to have DM contents of less than 20 percent (200 g/KG). High DM silages have DM contents greater than 25 percent. The average DM content of a ration is around 35 percent. Diets for high producing animals may have a DM content as high as 55 – 60 percent.

"Crude protein ="

This is the term used to describe the total level of nitrogen in a feed. Grass silage normally has a crude protein concentration between 10 - 16 percent of the DM. The overall crude protein concentration in a ration will vary between 12 - 18 percent of the DM (120 - 180 g/KG DM) depending on the level of production of the animal.

"Ammonia - N ="

This figure can be used to determine the state of preservation of a silage

Ammonia N interpretation

"Metabolisable Energy (ME) ="

This is a measure of the energy content of a feed. The ME content of a silage is inversely related to stage of maturity at cutting.

ME interpretation

The overall ME content of a ration for a dairy cow will vary  according to stage of lactatn. Diets for high producing cows vary between 11.5 - 12.0 MJ/KG DM. The ME concentration of a diet for a dry cow is usually 9.0 MJ/KG DM depending on how much feed she consumes (80 - 100 MJ/day).

Fermentable Metabolisable Energy (FME) = This figure is used to describe the energy available to microbes in the rumen. Grass silages have FME concentrations ranging between 7 - 10 MJ/KG DM. Cereals have higher FME contents, for example wheat has an FME concentration of 12.8 MJ/KG DM.

"pH ="

This is a measure of the level of acidity within a silage. It will give an indication, in conjunction with the ammonia N content, of the level of preservation within a silage. Most silages have a pH between 3.8 - 4.2. Silages with a pH less than 3.6 are considered very acidic, while silages that have a pH greater than 4.5 maybe poorly preserved. However some well preserved wilted silages may have a high pH.

"Neutral Detergent Fibre (NDF) ="

This term is used as a measure of slowly digested material across a wide range of feeds. NDF levels in forage increase as the plant matures. Average levels of NDF in grass silage are around 55 percent DM (550 g/KG DM).

The content of NDF in a total ration can vary between 35 – 50 percentDM. Diets with less than 32 percent NDF may cause problems with acidosis. Diets that contain over 50 percent NDF may be restricted in their intake potential.

"Starch and sugar ="

Increases in the starch content of a diet will help to increase milk protein concentration. However too much starch in the diet can cause problems with acidosis. Usual levels of starch and sugar in a total ration vary between 12 - 22 percent DM (120 - 220 g/KGDM).

"Oil ="

Levels of oil in a diet tend not to exceed 6 percent DM (60 g/KG DM). Otherwise milk composition tends to be affected.

"Ash ="

Gives an indication of the mineral content of a silage. Normally ash concentrations are less than 10 percent DM provided that there was no soil contamination of the grass at ensiling.

"Soluble N percent (percent total N) ="

This figure refers to the proportion of nitrogen in a feed which is soluble in water and rapidly degraded in the rumen. Average levels of soluble N in grass silage are 60 percent

"Amino N percent (percent of soluble N) ="

This figure describes the percentage of amino acid N in soluble N. The higher this percentage, the lower the level of protein breakdown during silage fermentation.

"Lactic acid ="

This is the main acid produced by natural microorganisms in the grass during the silage fermentation. Well preserved silages have high concentrations of lactic acid. Silages which have been wilted or treated with formic acid may have lower levels of lactic acid. Poorly preserved silages have low concentrations of lactic acid and higher levels of other acids such as acetic and butyric acid.

Lactic acid interpretation

Bulk density (KG/cubic foot) = Figures will vary according to DM content. Silages with a bulk density less than 18 KG/ cubic foot are considered to be light. Silages with a bulk density greater than 23 are considered heavy, while average figures are considered to be around 20.