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Feed analyses for dairy producers

Charles Stallings Published on 06 April 2011

Today, more than ever, smart nutrient management is critical to overall feed value and efficiency of the dairy operation. The cost of keeping cows productive is more intense and challenging than ever, and getting the most from the ration is critical, no matter what the production goals are.

Testing for starch content in forages is not a new concept and most every nutritionist knows this. However, more often overlooked is how starch type influences the herd and what can be fine-tuned to maximize results from it.



It is important to understand ruminal starch digestibility to have the herd more efficiently and consistently utilize starch. Diets properly balanced for ruminally degradable starch can result in higher yields of milk, milk protein, improved efficiency and persistence of milk yield.

Understanding starch content allows dairy producers to precisely calibrate their herd’s ration. Dairy producers should be aware of short-term and long-term areas to monitor where variation is most likely to occur. Variation frequently occurs from bunker to bunker and truckload to truckload.

Regularly scheduled testing of each bunker and pile for total starch content and digestibility of the starch will provide analyses to assist producers in calibrating the proper ration needed for the herd at that point in time. Producers should also look to their dairy herd production numbers for short-term signs of abnormal fluctuation.

Roger Martens, a dairy nutritionist in Watertown, S.D., works closely with herds in his area to fine-tune their rations using new technology that measures ruminal starch digestion, thus providing more precise information for balancing rations. Roger has found it effective to help dairy producers see – and understand – the variation that can be found in feed relative to starch availability.

Rodney Elliott in Lake Norden, S.D., is one dairy producer with whom Martens works closely. Milking approximately 1,650 cows on their recently expanded operation, Elliott began looking at starch testing insights differently when they built the larger dairy.


“We always believe in keeping cows healthy with a forage-based diet,” says Elliott. “Our biggest expense is purchased feed, and we buy all of our forages.” Elliott knows there is a lot to learn about feed and how it can vary from one purchase to the next.

“By sampling the nutrients we buy, we have more control over the nutritional peaks and can better balance it,” he notes. “While we can pick and choose what we buy, still it is only as good as we can use it. It is critical to us to make sure that what we see on paper (through starch testing) is consistent with what the cows are getting and how they are doing with the feed.”

“We want more control over the inputs,” adds Elliott. “It is important to know more about the forage so we know how to feed it. Before using the new starch-testing technology to analyze the digestion more, we did not have control of the forage side. Now we are gaining more control, and it has helped us buy acres of ‘the right’ corn.”

Analysis and sampling make good business sense to Elliott. The dairy strives to balance rations toward the peak of the production curve. “We work to maximize inputs for optimized outputs,” he adds; “in turn, this optimizes the dollars we spend on feed.”

Gerhard Ter Denge is another dairyman that Roger Martens works with, based out of Millbank, S.D. Ter Denge milks 500 cows, and both grows and purchases feed.

“We started looking at the new starch technology in 2009,” says Ter Denge. “Our corn silage was bad that year, but through testing we were able to keep up milk production to 90 pounds per day. Our goals were to reduce feed costs and still keep cows healthier.


I have been able to achieve those goals, including lowering feed costs and even less veterinarian costs,” he adds. Ter Denge considers testing starch as a means of helping him achieve feed consistency in silage. “Every day, feed is the same, milk is the same, ingredients are the same,” says Ter Denge. “Consistency and cost – one follows the other, and not just feed costs.”

Technology has changed the way and speed with which feeds are analyzed. Reports list a wide range of nutrients; some measured, some calculated from other measurements. Before deciding what and how often to analyze, the following should be considered.

  1. Nutrient variation – Feeds that have a great deal of variation should be analyzed more often.
  2. Individual versus mixed feed – Usually we analyze the individual feeds for ration formulation, but we can check TMR for quality control.
  3. Method of analysis – Wet chemistry versus NIR.
  4. Nutrients to be analyzed – Dry matter, protein and fiber are most common.
  5. Need for dynamic measurements – Digestion over time is dynamic.

Sample collection

In order to have an accurate forage test for ration formulation, it is important to have a representative sample. The method of sampling varies with forage type.

Silages (corn or hay crop) can be sampled either at harvest or at feedout. There should be little change in dry matter, protein, fiber or energy during storage in most situations. If sampling at harvest, it is best to take three to four handfuls from every third load or more and place in a container with all samples from the same field.

After mixing the composite, a sub-sample can be taken for analysis. When sampling at feedout, it is advisable to take three to four handfuls at different times, mix the composite and take a smaller sub-sample for analysis.

In upright silos, there can be variation from top to bottom. Bunkers are different because common layers are encountered from front to back due to layering during storage.

When sampling hay, it is best to use a core sampler. Take 10 to 20 core samples from each hay lot, then composite, mix and sub-sample for analysis. Small rectangular bales should be sampled by coring from the end.

Large hay bales should be sampled from the front or back (not the sides) in order to get a cross- section of the rolled hay. A sampling protocol can be found at

Wet chemistry vs NIR

There are two ways forages are analyzed for nutrient content. Wet chemistry uses established laboratory tests to quantify protein, fiber, fat and minerals. Near-infrared reflectance spectroscopy (NIR) has been perfected to accurately and quickly measure nutrient content.

To do this, the NIR instrument must be calibrated to wet chemistry, which is the standard. Most typical forages can be analyzed with NIR, but unique forages may not be appropriate because no calibration set is available to standardize the equipment.

Also, total mixed rations are difficult with NIR because the composition of the mix can vary greatly from farm to farm. Laboratories will advise about which feeds can be analyzed with NIR based on the calibrations available to them.

Also, NIR technology uses light reflectance and works best with large compounds such as those that compose protein and fiber. Minerals are smaller and more difficult with NIR. It is generally recommended that minerals should be run with wet chemistry if precise levels are needed.

Basic wet chemistry tests

When a sample is received at a forage testing lab, a portion of it is weighed and dried in an oven to eliminate moisture. It is then reweighed and the dry matter content determined. The dried sample is then ground for analysis.

A portion of the sample is weighed into a tube for a Kjeldahl or nitrogen determination. The sample is digested with acid and then distilled with a base solution to convert nitrogen to ammonia, a form that can be trapped and analyzed.

We convert nitrogen to crude protein by multiplying by 6.25 due to the fact protein is 16 percent nitrogen (100/16=6.25). Crude protein measures all nitrogenous compounds present in the sample and does not distinguish true protein from nonprotein nitrogen.

This is fine for ruminants (cattle, goats and sheep), but can be a concern for chickens because poultry can’t utilize nonprotein nitrogen.

There are two types of fiber determinations typically run in forage testing labs. One uses an acid detergent solution to digest the dried feed sample and the other uses a neutral detergent solution.

The digested solution is filtered and the residue on the filter is the fiber. These fibers are termed acid detergent fiber (ADF) and neutral detergent fiber (NDF).

NDF is larger than ADF in plants and is considered to be the cell wall component. NDF is used to predict intake while ADF is used to predict digestibility. Both can be used to estimate energy.

Fat is commonly referred to as ether extract because it is removed with ether from feeds. Most forages are low in fat, but they can have 2 percent to 3 percent. Other feeds vary but the oil seeds such as cottonseeds and soybeans contain up to 20 percent fat. Elevated fat increases energy in feed.

Energy is not typically measured in forage testing labs. It is dynamic and changes with animal physiological conditions, unlike crude protein, fiber or fat. Energy can be estimated based on fiber content and there is an inverse relationship, with high fiber being associated with lower energy.

Different labs will many times report different estimates for energy because the equations used are different. Energy is usually expressed as kilocalories (1,000 calories) or megacalories (1,000,000 calories) of net, metabolizable or digestible energy. Also, total digestible nutrients or TDN is an indicator of energy content.

Expression of results

When expressing concentration of nutrients, it is necessary to define if the results are expressed on an actual (“as is” basis) or dry matter basis. Nutrient concentrations expressed on “as is” basis are less than when expressed on dry matter.

In species such as cattle and horses that eat high-moisture feed, rations generally are calculated on a dry basis. Chickens eat feeds that are dry and have approximately the same dry matter (88 percent to 92 percent) and will sometimes use the “as is” nutrient concentration.

The feed industry uses “as is” basis to express nutrient concentration on feed tags, unless stated otherwise. Therefore, it is important to know what basis nutrients are expressed before it is possible to use the results.

Crude protein, fiber, fat and macrominerals (calcium, phosphorus, etc.) are usually expressed as percentages (either “as is” or dry matter). However, microminerals (zinc, cobalt, etc.) are usually parts per million or mg per kg.

Energy will be as TDN (percentage) or calories, kilocalories or megacalories per lb. Vitamins are expressed as international units of activity per lb. Therefore, it depends on the nutrient type as to what units will be used.

The bottom line is forages should be analyzed regularly (every four to six weeks) for dry matter, protein, fiber and energy.

Digestibility is important

Sometimes even if intake is acceptable, rations do not support expected milk production. The problem will then likely be decreased digestibility of certain components of the diet. Digestibility considers what is consumed minus what appears in the feces.

For instance, if 50 lbs of dry matter is consumed and 17.5 lbs appears in the feces, the digestibility is 65 percent (50-17.5 = 32.5; 32.5/50 * 100 = 65 percent). In this case, 32.5 lbs of dry matter disappears from that consumed to what appears in the feces. This 32.5 lbs is considered available to the animal, although some nutrients will be excreted in the urine.

Most rations for high-producing cows will be greater than 65 percent digestible. It is possible to do an in vitro dry matter digestibility in a laboratory.

Fiber digestibility in corn silage and other forages is also important. Since most fiber digestion occurs in the rumen, we are mainly concerned with digestibility in rumen contents. Typically a 48-hour in vitro NDF digestibility is conducted.

Results indicate there are differences in fiber digestibility in corn varieties. Also, year-to-year variation most likely occurs in fiber digestibility due to more lignification during certain growing seasons.

Michigan State University research indicates that a 1 percent increase in ration neutral detergent fiber digestibility will result in a .17 kg per cow per day increase in dry matter intake (digestibility and intake are related) and .29 kg. more 4 percent fat-corrected milk.

If silage is harvested at a mature stage, the kernels will be hard and difficult for the cow to digest. Corn silages above 40 percent dry matter many times will have kernels that are hard and less digestible. Kernel digestibility will be reduced, but an analysis on the silage might indicate high energy content.

The reason for this is the lab grinds the feed and breaks the kernels and they are considered digestible. Therefore, a general lab analysis will not detect reduced digestibility of kernels. This is also true for reduced digestible fiber unless the lab incubates the sample over time and determines disappearance of the fiber.

The University of Wisconsin software program (MILK2006) uses dry matter content to estimate starch digestibility but also allows an actual digestibility to be used. Both starch and NDF digestibility are used in the MILK2006 program to calculate the net energy of the silage.

Finally, in hay crop silages and hays, there can be heating in the ensiling and storage process, resulting in protein bound in the fiber. This has been termed acid detergent fiber protein or acid detergent insoluble nitrogen.

There is a direct relationship with reduced protein digestibility and should be accounted for in balancing rations. There also appears to be a reduced amount of energy in heat- damaged forages. Some laboratories will calculate a reduced protein digestibility based on the amount of heat damage, measured as acid detergent insoluble nitrogen.

Reduction of digestibility of forage fiber, silage kernels or forage protein can result in failure of the ration to meet expectations. Year-to-year variation in corn silage quality can many times be related to fiber and/or kernel digestibility.

Laboratory tests do not always detect these changes unless the more refined in vitro or in situ measures of dry matter, fiber or protein digestibility are used. PD

References omitted due to space but are available upon request to .

Forage sampling labs

The following is a list of laboratories that analyze forage samples for nutritional quality. This is not an endorsement, nor is it all-inclusive. It is for informational purposes only.

• A&L Eastern Laboratory
7621 Whitepine Road
Richmond, VA 23237
(804) 743-9401

• Brookside Laboratories, Inc.
308 S. Main Street
New Knoxville, OH 45871
(419) 753-2448

• Cumberland Valley Analytical Services
14515 Industry Drive
Hagerstown, MD 21742
(800) 282-7522

• Dairy One Forage Lab
730 Warren RoadIthaca, NY 14850
(800) 496-3344

It is important that ruminal starch digestibility is assessed and measured in today’s dairy operation rations. New technology recently available on the market can help calibrate the likelihood of starch usage and in turn deliver precise information dairy producers and their nutritionists need to more effectively balance rations in today’s feed/milk price ration challenge.

Dairy producers should explore the options for various feed ingredients with their nutritionist, and find ways to reduce ration cost. Teaming with your nutritionist to better understand rumen starch digestibility might help identify alternative, convenient and less-costly options that would allow you to maximize production goals and further minimize feed costs. PD

Charles Stallings
  • Charles Stallings

  • Professor and Extension Dairy Scientist
  • Virginia Tech
  • Email Charles Stallings