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Testing forages for beef cattle rations

Rick Rasby Published on 31 December 2010
moving hay

Forages are a primary energy feed source. From a forage standpoint, as plants mature, fiber increases. Fiber is less digestible than other plant parts and fiber digestibility declines as plants mature.

These factors cause the concentration of energy in plants to decline as maturity advances. In addition, as plants mature, the increase in fiber and bulkiness reduces the amount of forage an animal can consume.

As the plant matures, ADF (acid detergent fiber) and NDF (natural detergent fiber) increase. Protein concentration also declines as plants mature. When designing diets using harvested feeds, many rations are balanced using average values and these “book values” often result in overfeeding or underfeeding of certain nutrients.

More economical and better-balanced rations can be formulated using nutrient concentrations determined from feed analysis.

Each sample must represent only one “lot” of forage. A “lot” of forage consists of forage harvested from one field at the same cutting and maturity. All forage from the same “lot” should be similar for:

• type of plant(s)
• field (soil type)
• cutting date
• maturity
• variety

Variation in any of these characteristics can cause substantial differences in the nutrient value of the forage.

Forage testing laboratories will not accept a “grab” sample of a baled forage. Sample baled hay after curing (usually 17 to 21 days after baling), using a core sampler or probe. Such an instrument is essential for collecting a representative sample.

For large round and square bales, the probe should penetrate at least 18 inches into the bale and have an internal diameter of at least three-eighths of an inch. If the probe is 18 inches long or longer, 15 large round bales should be adequate if the “lot” size is 30 to 40 bales.

Collect one sample from each bale by coring straight in from the center of the end of square bales and from the wrapped circumference of round bales. Place the entire sample into a plastic bag and seal tightly.

For loose or compressed hay stacks, use a hay probe at least 24 inches long to collect 15 or more samples from each lot. Sample loose hay stacks from the top and from the side of the stack. Compressed loaf stacks require six sampling locations: top front, top middle, top rear, lower front side, lower middle side and lower rear side.

Label the sample bag with your name, address, lot ID and type of material. Most testing labs provide a description sheet to report this information and to request the desired tests. Place samples in polyethylene freezer bags, squeeze the air out of the bag and seal tightly.

If you are sending a sample of silage, double-bag silage samples for extra protection. Use extra caution if subdividing a large hay sample, because subsampling dry hay can result in loss of fines and leaves. Freeze samples containing over 15 percent moisture until shipping; store dry samples in a cool location.

Most forages are analyzed using near-infrared reflectance (NIR) spectroscopy. NIR is a rapid, reliable, low-cost, computerized method to analyze feeds for their nutrient content. It uses near-infrared light rather than chemicals to identify important compounds and measure their amount in a sample.

Feeds can be analyzed in less than 15 minutes using NIR, compared to hours or days for chemical methods. This rapid turnaround and the resulting cost savings in labor make NIR an attractive method of analysis.

Because the NIR method uses a “library” to compare the reflectance collected for the sample to compare the reflectance spectra collected for a known sample in its library, it is very important to label the sample correctly (alfalfa, cool-season grass hay, millet, corn silage, etc.) so the computer knows what library to use.

NIR does not do a good job of measuring minerals, although it does a decent job measuring calcium (Ca) and phosphorus (P), likely because these minerals are tied closely to organic matter. If you think you need a mineral analysis, consider using the chemical method.

What nutrients should producers with cows analyze forages for?

Dry matter
Percent dry matter (DM) is the percentage of feed that is not water. In contrast, moisture is a measure of the amount of water in the feed on an “as is” or “as fed” basis, and is important because moisture dilutes the concentration of all nutrients.

This is an important number because beef cow diets are formulated on a dry matter basis, then using the DM percentage, are converted to the amount of feed needed to be fed.

As an example, if the diet calls for grass hay to be fed at 25 pounds per head per day and the hay is 15 percent moisture; therefore 85 percent dry matter, you would need to feed each cow 29.5 pounds per head per day (25 pounds/0.85) to account for the water in the hay.

Percent crude protein
Percent crude protein (CP) measures nitrogen concentration. However, CP will measure both true protein and non-protein nitrogen because the actual measurement is percent N.

The new Metabolizable Protein (MP) System, introduced in the 1996 NRC for beef cattle, incorporates degraded intake protein (DIP) and undegraded intake protein (UIP or bypass protein).

The DIP and UIP percentages must total 100 percent of the protein and are normally expressed as a fraction of the CP. So if the DIP of a forage is 70 percent, then UIP is 30 percent. If the CP of that same forage is 10 percent, then DIP is 70 percent of DM (10 percent x 0.70 = 7 percent DIP) and the UIP is 30 percent (10 percent x 0.3 = 3 percent UIP).

Because NIR does not measure DIP and UIP, use book values from the 1996 NRC. If the CP and adjusted crude protein numbers are not the same, it indicates that there has been some heat damage in the forage. Use the ACP value in developing diets for this feed.

Total digestible nutrients (TDN)
TDN represents the total of the digestible components of crude fiber, protein, fat (x 2.25) and nitrogen-free extract in the diet. This value is calculated from ADF in the NIR analysis. TDN is still used to calculate beef cow rations where the diet is primarily forage.

Relative feed value (RFV) combines digestibility (ADF) and intake (NDF) into one number for a quick, easy, effective way to evaluate the quality of alfalfa or haylages. It is used primarily with legume or legume/grass forages.

Relative feed value is most valuable for formulating diets for dairy cows and not really useful in balancing diets for beef cattle. RFV provides an index to rank a forage according to its digestible energy intake potential. RFV also has been used widely in hay marketing, but is not used in developing cow diets.

Knowing the nutrient needs of the cattle you are feeding and the quality of the forage can save you money; you may or may not need additional feeds or supplements to meet their requirements.  FG

—Excerpts from University of Nebraska Agriculture and Natural Resources website

PHOTO: When designing diets using harvested feeds, many rations are balanced using average values, and these “book values” often result in overfeeding or underfeeding of certain nutrients. Photo by FG staff.

Rick Rasby
Animal Science Professor
University of Nebraska

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