The costs to produce milk continue to increase. Feed is the largest cost on the dairy, so it is important for dairy employees and individuals in industries that support dairies to have at least a basic understanding of nutritional terminology.
This basic understanding should include the ability to read and interpret a feed or forage analysis.
Click here to download an example of a feed or forage analysis. (PDF, 1.8MB)
While the analysis will look different depending on where the sample is analyzed, the type of feed and the specific analyses conducted, they should all contain similar information.
Dry matter
One of the first numbers on most feed analyses is percentage of dry matter (DM). The percentage of DM is an indication of the amount of water in the feed ingredient.
For hay and other dry ingredients, this number will typically range from 80 to 90 percent, while silages will typically have values in the range of 30 to 40 percent.
As you go through the remainder of the report, most other nutrients will be expressed on a DM basis; this allows you to compare the nutritional value of feeds with different moisture levels, such as hay and haylage. Some reports may also report other nutrients on an “as-sampled” or “as-fed” basis.
For silages, it is a good idea to regularly monitor percentage of DM, as this value can change over time, and adjust the amount fed accordingly.
This can be done by sending samples to a lab or on the farm with a microwave oven or Koster dryer.
Protein
Crude protein (CP) is the most common measure of protein in feeds. It is calculated by multiplying the nitrogen content of the feed by a factor of 6.25. While CP is the most common measure of protein, it tells nothing about the availability of the protein or how well the protein meets the amino acid requirements of the dairy cow.
Other measures of protein found on some feed analyses include rumen-degradable and rumen-undegradable protein; these values are typically presented as a percentage of CP in the feed.
Rumen-degradable protein is available to the microbial population in the rumen and can be incorporated into microbial protein. Rumen-undegradable protein from the feed is not broken down in the rumen, and along with microbial protein, will pass out of the rumen and be used directly by the cow.
Metabolizable protein is not reported on feed analysis but is used in the formulation of most rations. This value is the protein (or amino acids) absorbed by the cow (a combination of microbial protein and rumen-undegradable protein) calculated by ration-balancing software.
Fiber and carbohydrates
Carbohydrates are classified as structural or non-structural carbohydrates. Structural carbohydrates generally increase as the growing plant matures and, as the name implies, give structure to the plant. Hemicellulose, cellulose and lignin are included in this group. Non-structural carbohydrates include starch and sugar and are energy storage for the plant.
The structural carbohydrates are represented on a feed analysis as neutral-detergent fiber (NDF), acid-detergent fiber (ADF) and lignin. NDF consists of hemicellulose, cellulose and lignin and can be digested in the rumen. DM intake and passage rate will decrease and rumen fill will increase as NDF increases.
Some feed analyses will also report the NDF digestibility at various hours, with 30-hour NDF digestibility being most commonly reported. ADF consists of cellulose and lignin. Forage digestibility is related to ADF; as ADF increases, digestibility decreases. The final fraction, lignin, is completely indigestible by both the cow and rumen micro-organisms.
Starch, a non-structural carbohydrate, is generally low in grasses and legumes. Starch values will typically range from 1 to 3 percent for grasses and 2 to 5 percent for legumes. In corn silage, starch values will typically range from 25 to 40 percent.
Minerals
The amount of ash, along with amounts of specific minerals, can also be found on feed analysis. Ash is a measure of all the minerals in the sample. High concentrations may be an indication of soil contamination. Corn silage will often have ash concentrations around 5 percent, while grass and legume hays may have values around 9 percent.
Calcium, phosphorus, magnesium and potassium are the minerals found in the highest concentrations in most feeds and are commonly reported in feed analysis as a percentage of DM. The amounts in the feed can be influenced by the amount of the minerals in the soil. Trace minerals, such as iron, zinc, selenium and copper may also be reported.
Energy
Total digestible nutrients (TDN) and net energy (NE) are commonly used to predict the energy content of a feed. TDN is calculated based on the amount of fiber, protein, lipid and carbohydrate in the feed.
Net energy is further broken down into NE for maintenance (NEM), lactation (NEL) and gain (NEG). Net energy lactation can be calculated from the ADF concentration, although the actual equation varies with feedstuff.
Net energy is a better indicator of energy for milk production than is TDN.
Collecting forage and feed samples
When a feed sample is sent to a lab for analysis, it is assumed that the sample is representative of the feed being fed to the cows.
A 40-foot-wide by 100-foot-long bunker silo can contain more than 900 tons of corn silage. From these 900 tons the dairy will send 1 to 2 pounds for analysis, and from that the lab may use a gram or less of material to determine the nutritional value. To ensure that the sample is representative of what is being fed, proper sampling is important.
A detailed description of proper sampling practices can be found online. However, in general, samples should be taken from multiple locations across the face of a bunker or multiple bales within baled hay or haylage. During sampling, spoiled feed should be avoided. PD
Mathew Haan is a dairy educator with Penn State Extension. He can be contacted by email.
This article is based on a presentation at the Penn State Extension Dairy Cattle Nutrition Workshop in November 2013. The complete workshop proceedings can be found online.
