Forage Analysis and Interpreting the Results!!!

Forage is the foundation of any ruminant production system. Whether it is grazed or ensiled. The proportion of the diet forage makes can vary from 100% to 45% depending on the system and time of year. Interpreting what is affecting the animal now and what can be done to improve the performance, is important for any farming system.

The simplest way is to test the forage. This article looks at what the analysis means. It is a snapshot in time and should be viewed as an aid to decision making. The decisions made are all too often nutritional as these are often done as part of package with the feed provider. But the information provided shows how a plant has been grown and stored up to that point in time. How much nitrogen has the plant had or taken up, and the plant content of the sward. 

For silage the analysis should give an indication, of the feeding potential, the DM, the energy and protein, and the fermentation process.

The most used method of forage analysis is near infrared reflectance spectroscopy (NIRS). NIRS can determine the physical and chemical properties of forages including dry matter, crude protein, crude fibre, crude fat, micro-components including amino acid, and vitamins. NIRS analyses requires a sample that is exposed to an electro-magnetic scan over a spectral wavelength range of 1100 to 2500 nm (near infrared). Energy in this spectral range is directed on to the sample and reflected energy is measured by the instrument.

The main reason for using NIRS analysis is speed and cost. Most analysis can be done in 16 hours so results can be back with the farmer within 24 hours. This can be important when cutting fresh grass to ensile. This is compared to wet chemical forage analysis that can take up to one month to come back.

This long-winded explanation of the process is an important element in understanding why sometimes the forage analysis lets the farmer down. The accuracy of a NIRS forage analysis relies on a database used to calibrate the instrument. These data sets are normally built up from wet chemistry. This takes time and money therefore there is a limit to the number of sets that can be used in the calculation. When the calibration is out slightly the data coming back can alter slightly, hence it has been common to hear that forages aren’t feeding as they analyse.     

So why isn’t the process better? Simply put it is a combination of money and time. You can do wet chemistry to obtain more accurate results, but this takes time and costs you money. By the time this process has taken place the silage may have been used up or changed so the information is obsolete.

Above - A typical grass silage analysis 

Interpreting The Results

Dry Matter (DM) is the sample weight of the material after all the water has been removed. With very wet, or very dry silages, the dry matter intake (DMI) may be reduced.  This may be because of sheer bulk fill with wet silages or that the silage is just far too dry (50% +) and the cow cannot produce enough saliva to cope. This is important because nearly all the other measurements are expressed as per unit of dry matter.

Fermentation Characteristics
pH is a measure of acidity. The measurement varies depending on the dry matter of the silage. Wetter silages (18-22% DM) tend to be 3.7 – 4 and dryer silages (30-35% DM) tend to be 4.5-4.8.

Ammonia N is measured as a percentage of the total nitrogen. This is a measure of the amount of protein that has degraded during the fermentation process. Higher values above 10% are associated with poor fermentation.

Total Fermentation Acids (TFA’s) – Lactic, Acetic, and Butyric. Fresh grass has (virtually) no acidity. TFA’s measure the amount of carbohydrates that have converted to acids in the primary fermentation. The total TFA’s should be 8-12% of the DM. The lactic acid proportion should make up 80%.  This will show that the sugars have been converted, and there are fewer available in the silage for secondary.

Any butyric acid is formed through secondary fermentation, so the proportion should be less than 1%.

Ash is the total mineral content. Normal levels should be less than 10% (100g/kg), figures between 12/15% show that some form of contamination has occurred. This is normally soil that has been picked up during the silage making process.  High ash content figure increased the risk of poor fermentation from Clostridia bacteria in the soil.

Energy
Metabolisable Energy (ME) is measured in MJ/kg and is the energy available to the animal and is predicted from silage using industry standards.

Fermentable Metabolisable Energy (FME) is the amount of energy that is potentially available to the rumen microbes. This calculation is linked to the ability to digest and convert degraded (microbial) protein.

The D Value is an indication of the digestibility of the silage and is related to the maturity of the grass when it was cut. The figure is derived from the total digestible organic matter within the DM.

Neutral Detergent Fibre (NDF) measures the cell wall contents. The carbohydrates that are not sugars. NDF is closely linked to the D value and important to optimising dry matter intakes in rations.

Sugars are still shown on silage analysis even though the value should be less than 1% DM because of the fermentation process.

Protein
Crude Protein (CP) is calculated from the total nitrogen content. This includes true protein and non-protein nitrogen.

The protein degradability is an indication of how the protein will react once inside the rumen. These are displayed on an analysis as letters that represent how the protein will be processed in the ration software.  The ‘a’ represents the proportion of soluble protein. This is quickly available nitrogen in the rumen.  The ‘b’ is the non-soluble, but potentially degradable protein. The slowly fermentable protein.  The ‘c’ is the prediction of the rate that ‘b’ is degraded.

If you would like forage samples taken or assistance with a winter ration based on your results please contact Louise Cox on 07943 684215 or Allaster Dallas on 07496 760242