How important is it to know your forage quality? Forage quality directly affects milk production and, therefore, your bottom line. The higher the quality of your forage, the more forage you can feed, supporting higher milk production and eliminating the need to purchase supplemental fiber.

Emery kathy
Dairy Nutritionist / Mycogen Seeds
Emery is a dairy veterinarian and practiced in Wisconsin for 11 years. Dr. Emery  completed a Foo...

Forage analysis is the critical first step to forage quality. The key to successful forage analysis is taking a representative sample on the farm. Take a sample from every hay or silage lot, and expect variation across fields. Test before feeding new-crop forages and monthly thereafter.

Here are some guidelines for forage sampling:

  • A core sampling will give the most accurate results when sampling baled hay. Take a minimum of 20 cores of hay from the center of the end of bales. Sample random bales or at random areas in the storage facility.
  • Filling is the best time to sample chopped forage. Take three to five scoops at random from several loads. Mix and subsample from this sample for analysis. Submit at least ½-pound of forage for sampling.

Haylage or silage samples can be vacuum-sealed and refrigerated until time of delivery to the lab. Mail these samples early in the week to prevent delays en route.

  • An unloader can be used to obtain a large representative sample from stored silage. Never pull samples from the face of a bunker silo because threat of silage shift is too dangerous. Scoop from the unloaded pile in several places, mix and subsample.

Measure NDFD to determine forage quality

The main factors affecting forage quality are forage variety, stage of maturity at harvest and the harvest and storage techniques used. Once you have done everything to maximize these factors, test your forages for neutral detergent fiber digestibility (NDFD) or fiber digestibility.


Digestibility measures the amount of forage absorbed as it passes through the animal’s digestive tract. NDFD can vary from 45 percent if forage is mature and contains a high percentage of stems to 90 percent if it is immature and leafy.

Research shows that for every percentage point increase in NDFD, cows will eat approximately 0.37 lbs more dry matter (DM), resulting in a 0.55-lb increase in milk production.

For example, cows consuming forage with an NDFD of 65 percent would be expected to produce 5 lbs more milk than cows eating forage with an NDFD of 55 percent.

I routinely observe the relationship between silage digestibility and milk production in real-world conditions. Recently I visited a dairy where two corn silage hybrids had been tested and fed.

The first forage sample was a conventional corn silage that tested at 55 percent NDFD; the second was a brown midrib (BMR) corn silage that tested at 70 percent NDFD. We monitored the milk production during the feeding of both forages and documented an increase in production of 7.5 lbs per cow per day with the BMR corn silage.

Know the limitations of your tests

Commercial labs routinely use one of two methods to measure NDFD: in-vitro digestibility and near infrared (NIR) spectroscopy.

Sometimes called “wet chemistry,” in-vitro digestibility is generally considered to be the best analysis to predict cow performance. It is a direct measurement using rumen fluid to digest the sample in a test tube. Although in-vitro is the gold standard of forage testing, it can take longer to receive results and is more expensive.

NIR spectroscopy measures the reflectance of infrared light of the sample and predicts the digestibility of the forage based on calibration equations. This gives an approximate measurement of NDFD.

Unfortunately, some labs are limited in the accuracy of their results because they do not have extensive types of forages included in their NIR calibration equations. Although NIR is less expensive than in-vitro testing and delivers results sooner, it is not an accurate test for NDFD.

To accurately determine the digestibility of corn silage hybrids, use in-vitro lab measurements. In my experience, when labs use NIR to predict the digestibility of corn silages, lower-quality silages appear better than they actually are and higher-quality forages appear worse than they actually are. All values are pulled toward the middle.

It is important to know what type of testing a lab uses. Since calibration equations are based on in-vitro results, sometimes they are categorized as in-vitro, resulting in confusion. Care should be taken to determine if your lab’s methods are truly in-vitro.

The National Forage Testing Association certifies labs with regard to accurate testing procedures. After finding a certified lab that uses in-vitro testing, stay with that lab.

Values can vary from one lab to another, although the range will remain the same. Using the same lab for each test will ensure consistency.

Forage testing is critical for ration formulation and to help you and your nutritionist understand the quality of the forage you are producing and feeding. Consult with your management team to create forage testing protocols that help to achieve your dairy’s goals. PD

Emery is a veterinarian and dairy nutritionist based out of Verona, Wisconsin.

Kathleen Emery