Laboratory tests are needed to assess forage quality. The key is digestion of fiber in the forage. For each one-point increase in fiber digestibility there is a 0.37 pound increase in dry matter intake (DMI) and a 0.55 pound increase in milk production. The increased fiber digestion results in more energy to make milk and increases the concentration of butterfat and protein in the milk.
A lab test to assess forage quality is the in vitro (IV) 30-hour neutral detergent fiber (NDF) digestibility test. An artificial digestion system is set up in the lab where forage samples are incubated at 1020F with rumen fluid and buffer in vessels or bags. The forage is finely ground. Samples are incubated for time periods that approximate the residence time of digesta in the rumen at a level of feeding at maintenance (48 hours) or levels above maintenance (30 hours).
The use of NDF digestibility values can be used to estimate milk production potential from forages available to feed the lactating cow. In vitro NDF digestibility is also a useful tool when considering ration performance problems. The test is useful for ranking quality of forages on the farm to feed to various animal groups. For example, higher NDF digestibility forages might be directed to high-production cows in early lactation. NDF digestibility evaluations provide information for assessing forage purchasing decisions and provide qualitative information for characterizing forages and anticipating animal performance.
It is important to have an understanding of the averages and ranges of NDF digestibility values from various forage types used in the lab. Examples of 30-hour NDF digestibility values for Western alfalfa hay averaged 41 percent and had a range of 32 to 54 percent; and corn silage averages were 57 percent and had a range of 45 to 75 percent.
Comparisons can be made only as this information is available and understood by those people using the NDF digestibility information. The user of the information should have a good understanding of the basis and quality of the information coming from the lab. It should be recognized that significant differences may exist between labs in NDF digestibility procedures and that average values and ranges will differ.
The 30-hour NDF digestibility is a good measure of forage quality to estimate the value of the forage and assess the forage’s impact on animal performance. The lab test is a way of finding the best forage to increase DMI, milk production and a higher concentration of butterfat and milk protein. PD
Lawson Spicer, Independent Nutritionist, for Progressive Dairyman
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