Feed Testing: A Valuable Management Tool

The importance of a properly balanced ration in maintaining milk production and cow health is well accepted. An article in a recent Journal of Dairy Science indicated the economic value of a feed analysis program was $0.27/day/cow. An earlier publication showed that, after surveying 2700 dairy producers in the southeast United States, forage testing and ration balancing was found to return $136/cow compared to the management practices of DHIA and AI which returned $190 and $106, respectively. Regardless of how the economic benefit is calculated, it is clear that a feed analysis program is a valuable management tool.

 

The quantity and type of concentrates required in a ration depends upon the nutrient composition of the forage. The only way of accurately determining forage nutrient composition is through sampling and feed testing.

 

The most important factor in feed testing is making sure the sample or samples taken are representative of the total forage on hand. Inappropriate rations due to poor sampling can result in major ration and production problems. At least 15 core samples must be taken from each lot of hay to be tested. Core samplers are available from the local ag.rep offices. Mix the core samples in a pail and submit a one to two litre sample to a feed testing laboratory. If someone else takes the samples, make sure they too follow proper sampling procedures.

 

Silage can be sampled either at time of filling the silo or at feed-out. Sampling at time of ensiling allows for results to be available well before feeding. Take three samples from the middle of each load. Combine samples from the same field and/or variety and send immediately to a lab. If sampling at feed-out, take 3 to 5 samples over 3 feedings, combine and send to a lab. Silage should be retested as changes in composition become evident. Moisture content of silages and other high moisture feeds must be checked weekly.

 

Grain samples can be taken either at time of unloading into the grain bin or by using a grainery probe. Combine 15 - 20 samples and submit one litre to a feed analysis lab.

 

Feed analysis provides good information about forage and grain quality. The reports are only useful, however, if the resulting numbers are understood and used to formulate balanced rations for your dairy herd. The nutrients most commonly analyzed are described below.

% Moisture indicates the amount of water in a feed. % Dry Matter is calculated by subtracting the % moisture from 100. For example, a silage sample with 65% moisture would contain 35% dry matter. Most labs list nutrients on both an "as fed basis" (water in) and a "dry matter basis" (water out). Because forages vary so much in dry matter content, it is important to evaluate forages using only the nutrients given on a 100% dry matter basis. Nutrient requirements are also indicated on a 100% dry matter basis.

Crude protein is the primary protein analysis. It is calculated by multiplying the analyzed nitrogen content of the feed by 6.25%. Heat damaged protein estimates the amount of crude protein made unavailable to the cow due to heating. Generally, this number is insignificant (less than 10% of crude protein). The available protein is crude protein minus heat damaged protein. Soluble protein is the protein fraction which is available immediately to rumen microbes. No more than approximately 30% of the total dietary crude protein should be present as soluble protein. Lab analysis of undegradable protein is not routinely available but is available at several laboratories including Dairyland and DHI Stearns Central Laboratory.

Acid detergent fibre(ADF) includes the cellulose and lignin fractions of the plant and is closely associated with forage digestibility. Rations for high producing dairy cows should contain a minimum of 18% ADF. Neutral detergent fibre(NDF) is composed of the hemicellulose, cellulose and lignin fractions of the plant. Because NDF incorporates all three fibre components, it has been used to estimate intakes. The Agriculture Canada station at Lethbridge suggests that barley based rations contain approximately 35% NDF.

Energy values are often reported in several ways - Total Digestible Nutrients (TDN), Digestible Energy (DE) and Net Energy (NE) of maintenance, gain and lactation. All energy values are calculated from the analyzed ADF value. There is no commercial method for analyzing the energy content of a feedstuff. Net Energy of lactation (NEL) is the energy measurement used for dairy cows.

Relative feed value (RFV) is a number calculated from ADF and NDF. Protein is not included in the calculation. RFV's express the nutritive value of a forage in a single number. This allows for easy comparison of forages. RFV's have no specific nutritional meaning.

The above nutrient analyses are usually done by NIR and are very accurate. However, mineral levels obtained from NIR analysis should be used as guidelines only. Calcium, phosphorus, magnesium and potassium are the elements commonly included on NIR report sheets. Minerals may be associated with organic complexes in the feed and it is this fraction which is detected by NIR. However, the elements also exist in inorganic complexes which are not detected by NIR. In addition, these complexes are not static, and this can further contribute to variability in NIR mineral results.

 

If precise mineral results are required, for example in dairy rations or when troubleshooting nutritional problems, NIR is not the method to use. Minerals should be analyzed by wet chemistry. Norwest Labs offers two "add-on" packages to their general NIR analysis. NIRX ($29/sample) provides calcium, magnesium, phosphorus and potassium by wet chemistry. The NIRXT ($41/sample) provides by wet chemistry, the above minerals as well as copper, iron, manganese, molybdenum and zinc.

 

Mineral analysis provided by NIR can be viewed with the same degree of accuracy as "book values". It is likely adequate in situations where mineral premix is being consumed free choice and precise ration balancing does not take place.

A package, such as NIRX from Norwest Labs, is routinely recommended for dairy forages. This provides dry matter, moisture, crude protein, heat damaged protein, ADF, NDF, energy estimates, RFV by NIR and calcium, phosphorus, magnesium and potassium by wet chemistry. Checks of trace mineral status should be done periodically with an NIRXT package. Soluble protein should also be requested for each forage, particarly those which have been ensiled. Grains should be checked for a minimum of dry matter and crude protein.