Good quality forages are the main asset of any livestock operation and are crucial for the livestock industry.
In general, performance of grazing animals reflects forage quality. Forages contain nutrients that affect individual animal production (gain per animal) while the amount of forage produced affects production per acre.
You should make the decision whether to use conserved forage, such as hay, or to allow livestock to graze standing forages, as well as decisions around the selection and purchasing of hay, based on forage quality.
Forage analyses are important because they describe the forage quality. Forage testing is also a relatively inexpensive tool in your toolbox when estimating the nutritive value of forage to be grazed, hayed, purchased or marketed. Knowing what affects forage quality will also help in making appropriate selections of forages and supplements, resulting in economically optimum livestock performance.
When and What to Sample
Always try to sample the forage in question as near to the time of feeding or sale as possible.
Allow for enough time for the sample to be processed by the laboratory. This could range from one day to several weeks, depending on the tests requested, methods used and number of samples tested.
Extreme variation can occur in forage quality when harvested from the same field or lot of hay, etc.
There is a wealth of instructional information available on forage sampling techniques. Information is available on the sampling of hay bales from small to large, square to round, and do not forget the variety of sampling probes available (see options at bit.ly/hay-probe).
There is information on sampling cubes, pellets or ensiled forages, but what about sampling standing forage?
In a haying situation, standing forage should be cut at a height equal to the height setting on the swather from several areas throughout the pasture unit for a good representative sample.
In a grazing situation, take a “hand-plucked” sample by trying to select parts of the plants the grazing animal is or will be consuming. Remember, your forage analysis test will only be as good as the sample submitted.
Testing Forage Samples
It is important to select a forage laboratory that is a member of and certified by the National Forage Testing Association (NFTA). Many labs use near-infrared spectroscopy (NIRS) technology for analysis of forage samples. NIRS is a rapid, repeatable, nondestructive method of forage analysis. It measures the reflectance of near-infrared light to predict the quality parameters instead of chemicals used in a conventional “wet chemistry” method.
Components of Forage Quality
Normally, four measurements are taken for forage quality analyses:
- Crude protein (CP).
- Neutral detergent fiber (NDF).
- Acid detergent fiber (ADF).
Other forage quality components are calculated from these measured attributes, as shown in Figure 1.
Moisture content of the forage sample is usually reported in a wet and a dry matter (DM) basis. Wet basis indicates how much fresh forge would be required to meet the DM requirements of the livestock. Dry matter is calculated as if the forage had no water content. This calculation allows for the most accurate comparison among different forages. It will also vary depending on forage type and how the forage is fed (Table 1).
Crude Protein (CP, %)
Proteins are the most important nutrients for livestock. These nutrients support microbe activity in breaking down forage in the rumen. Proteins make up 60-80% of the total plant nitrogen. Proteins also contribute essential amino acids for the animal itself. Crude protein is an indirect measure of the nitrogen (N) concentration of the forage multiplied by 6.25. This calculation assumes thatNconstitutes about 16% of protein in the leaf and stem of the forage plant (100 ÷ 16 = 6.25).
Neutral Detergent Fiber (NDF, %)
NDF represents the total fiber fraction of the forage. Three fiber fractions make up the structural cell wall in a plant. These fractions are cellulose, hemicellulose and lignin. Values can range from 10% in grain to 80% in grass straw. NDF values for grasses are higher when compared to legumes. A high NDF content indicates a high fiber content in the forage. Therefore, the lower the NDF value of the forage sample, the better.
Acid Detergent Fiber (ADF, %)
The ADF value represents cellulose and lignin. These two structural cell wall components are partially digestible in the rumen over time. Forages can range from 3% in grain to 50% in grass straw. A high ADF value is associated with decreased digestibility. Therefore, a low ADF value is better.
Other Forage Quality Components
Total Digestible Nutrients (TDN, %)
TDN is calculated from the ADF value and represents the overall digestibility or energy value of the forage.
Relative Feed Value (RFV)
RFV is calculated from the ADF and NDF value and represents the forage’s digestibility and intake potential. RFV is only nutritionally applicable to alfalfa hay that is fed free-choice to dairy cows, and it is typically used in marketing all types of hay.
Relative Forage Quality (RFQ)
RFQ is a term that is similar to RFV in that it is used to rank forages according to their relative nutritive value. RFQ shares many of the properties of RFV. However, unlike RFV, RFQ takes into account digestible fiber and is more often used to describe the nutritive value of grass forages. It is a good index of how a forage will perform in an animal diet.
Factors Influencing Forage Quality
Forage quality can vary not only among different forage types but also within the same forage species or cultivars. Not every plant in a pasture will have the same nutritive value, leading to characteristics that can indirectly or directly affect forage quality.
Weather conditions and forage quality are the primary factors affecting the quality of standing forage. Maturity is the principal factor responsible for declining forage quality. As the age of plants within a stand advance beyond peak protein production (first several weeks of growth), stem growth increases along with the production of fibrous components, such as lignin, at the plant cell level. Lignin, a component of fiber, is essentially indigestible and acts as a barrier to rumen microbes working to break down the forage. If the forage is too mature and fiber is more prevalent, then the crude protein (CP) declines as well as forage digestibility (Table 2).
Additionally, conditions during harvest or poor storage practices can adversely affect forage quality by allowing the breakdown of soluble sugars. Other factors, such as harvest management practices, soil fertility levels, plant diseases and time of season can all affect forage quality.
Knowing what affects forage quality and how to interpret a forage quality test will aid in forage selection and supplements that will help match animal requirements and economically improve livestock performance.