Know Your Forages

Deanne Morse, Extension Dairy Specialist
Kevin Sedivec, Extension Rangeland Management Specialist

Animal consumption of hay or silage depends on many things. One major determinant of forage intake is its quality. The nutrient composition of a forage helps to explain expected animal performance.

Determining the chemical composition of forages is important. The amount of each nutrient in a forage, energy, protein, and mineral composition, needs to be determined before a diet can be balanced. Fiber content enables a nutritionist to estimate feed intake, digestibility of feed, energy content, and expected animal performance.

Why have forages analyzed?

Most livestock producers in North Dakota try to limit feeds to home grown feedstuffs. In most cases that means cows get some combination of prairie, native, or alfalfa hay, corn, sorghum, oat or barley silage, and grain. If hay and grain are fed, what is needed to increase their production? Unless your feeds have been analyzed, answering this question is virtually impossible. If the nutrient composition of the feed is not known, determining what nutrients are limiting production is also unknown. Everyone wants to know how to improve production with the least amount of input. It cannot be determined what the cows need without knowing what nutrients they are already getting.

What components of forage do I need to know?

The most important components to identify are dry matter (DM), crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF), calcium (Ca), and phosphorus (P). Energy values can be calculated from ADF values.

Knowing forage dry matter (DM) content is important, especially when silage or haylage are fed. The actual amount of silage DM consumed by each animal depends on the quantity of silage consumed and its DM content. Typically, silage is about 32 percent DM, but may vary from 23 to 42 percent DM. If a cow receives 40 pounds of silage (32 percent DM) a day, she receives 12.8 pounds of DM. If corn is wet when ensiled, DM may be as low as 25 percent. Feeding 40 pounds of silage a day (25 percent DM) would only deliver 10 pounds of DM. If corn is 38 percent DM when harvested, the cow would receive 15.2 pounds of DM.

When underestimating DM content of silage or haylage, cows receive more than the estimated amount of DM from silage. This reduces intake of energy-dense grains The end result is reduced milk production in dairy cows.

When overestimating DM content of silage or haylage, cows receive less than the desired amount of DM. Overestimating DM can be detrimental in high producing cows, because high producing cows consume a larger amount of DM from concentrate feeds. The forage component represents a smaller portion of daily intake (30 to 40 percent on a DM basis). By overestimating DM, the actual amount of DM from the forage can drop to as low as 20 percent of the diet. Metabolic upsets (acidosis, displaced abomasum) as well as milk fat depression in dairy animals may occur more frequently.

Crude protein (CP) represents the nitrogen fraction of the forage. True proteins (amino acids) and non-protein nitrogen comprise this fraction. This value does not differentiate between available and unavailable nitrogen. Some proportion of nitrogen in a feed is not available to the animal. Less than 3 percent of the protein in normal forages is unavailable. If hay is baled too wet, or oxygen is present in silos, heat damage can occur. Heat damaged forages are characteristically brown to black in color and have a sweet carmel-tobacco aroma. If forage is heat damaged and represents a large proportion of daily feed, lab analysis should be conducted to determine the amount of damage. Unavailable protein in heat damaged forages can exceed 10 percent of the total protein. Milk production in dairy cows is reduced if protein is limiting in the diet. In such cases, the protein content of the forage should be discounted prior to balancing a diet. To complicate matters further, intake of a forage may be reduced if heat damage is extensive. This results in decreased animal performance.

Fiber content of a forage is a very useful tool and must be determined. The rule of thumb is that as the fiber content increases in a feed, the energy value decreases and feed intake potential by cattle decreases. A minimum amount of fiber must be provided to maintain proper rumen function. Some maximum amount of fiber intake also exists for each animal. Beyond that maximum amount the animal is unable to consume more feed.

Fiber is determined by two lab procedures. The proximate analysis system uses crude fiber to estimate poorly digestible carbohydrates. Crude fiber is not a true chemical component, it described "the fiber fraction that is resistant to degradation in acid and alkali." This fraction represents a partial recovery of cellulose and lignin (Figure 1).

The newer and preferred system used to describe fiber constituents is the VanSoest analysis. Two fiber fractions are determined during this procedure. Neutral detergent fiber (NDF) represents the cell wall material. This value includes all components of the acid detergent fraction plus hemicellulose (Figure 1). Acid detergent fiber (ADF) consists of cellulose, lignin, heat damaged proteins, and acid insoluble ash (some minerals). Lignin is almost completely indigestible. The amount of fiber which can be degraded by rumen bacteria is inversely related to the amount of lignin in a forage. So, as lignin content increases, digestibility of fiber decreases. Reducing the amount of lignin in a forage maximizes digestibility.

"Total digestible nutrients" (TDN) is a calculated estimate of energy in a feed, based on proximate analyses. In the VanSoest system, TDN can be estimated by acid detergent fiber. Labeling forage is extremely important. For instance, if the ADF content was 30 percent. The estimated TDN content of the feed could be 61.85 percent (legume) or 66.84 percent (corn silage).

Another estimate of energy in a forage is net energy of lactation (NE_L). The energy in a feedstuff is as useful for maintenance as for production. However, use of energy in a feedstuff differs for deposition of body tissue and maintaining body tissue. More energy is required to deposit body tissue than to maintain body tissue. For lactating dairy cows, NE_L, is needed when balancing rations. NE_L is usually expressed as megacalories (Mcal) per pound or per 100 pounds of feed DM. Estimates of NE_L, (Mcal) per pound can be made similarly to those for TDN: ADF in the calculation to estimate NE_L. A forage with 30 percent ADF may have .675 Mcal/lb (legume) or only .651 Mcal/lb (legume/grass mix).

Since TDN and NE, values are both estimated by ADF, it is critical that ADF is determined as precisely as possible. Identification of sample type is also important. If a forage is labeled as alfalfa but is an alfalfa/grass mix, a different set of equations should be used to estimate the energy values. It is the responsibility of the person submitting the sample for analysis to identify it appropriately. Label the sample as accurate as possible. If the pasture was half weeds, it should be labeled as such. Do not label a forage sample by what it should have been.

Equations used to estimate TDN and NE, should be determined for each region. Forages grown in various parts of the country differ dramatically in nutrient composition and availability. Although the equations used to estimate energy are beyond the hands of the producer, it is up to nutritionists to use reliable laboratories for feed analysis.

Taking a representative sample

The most important part of analyzing forages is taking a representative sample. Hay probes are effective when sampling dry forages. Take at least one core sample from each large bale in question (if you have less than 15 bales). If you have more than 15 bales, sample up to 15 bales from a field for each cutting. Empty the probe contents into a paper bag or box. After all bales have been sampled, mix the contents of the bag or box. Do not separate out less desirable portions of the hay. If you feed the entire bale of hay to your animals, you need to send vr a sample that represents the entire bale of hay. Fill a sea able bag with a portion of the contents of the box. Press out as much air as possible. Seal the bag. If bales come from different fields and cuttings, it is best to have bales from each field and cutting analyzed separately because nutrient composition can vary greatly. Try to obtain core samples from as many bales as possible. The more bales included in the combined sample, the more representative the sample will be.

If there are one or two bales of quality hay and the remaining bales of hay are mediocre, sample the mediocre bales. The results received will represent a majority of the bales. The animals should receive the high quality hay occasionally (the few bales not sampled) and not as portions with the poor bales. If some of the bales are good and some are not, separate the hay accordingly, store separate, evaluating both types. It is not uncommon to feed the higher quality hay to dairy cows in early lactation or during the third trimester of pregnancy of other classes of livestock, and the poorer quality hay in later lactation and early pregnancy.

Sampling silage and haylage requires more effort than sampling dry hay. Once again, collect samples from many locations in the silo. Nutrient composition of silage retrieved from an upright silo will differ from top to bottom. Obtain a composite sample of the feedstuff, taking a portion of the sample and fill a plastic bag. Immediately remove as much air as possible and seal the bag. Take care to minimize exposure of the sample to the sun or open air, as both act to dry the sample. The result would be overestimated dry matter. This translates to under feeding cows. Samples of silage should be retrieved from at least an elbow's depth to reduce the possibility of a sample that is too dry or too wet.

Where can I get my forages evaluated?

Once you have obtained a feed sample, immediately send it to the laboratory. Feed companies, elevators, and private laboratories analyze feeds. Your NDSU Extension Service county agent should be able to provide a partial list of laboratories capable of evaluating feeds.

Some values obtained are more reliable than others. Traditionally, feeds were analyzed by wet chemistry techniques, which were time consuming and costly. Dry matter, crude protein, fiber content, and mineral values from wet chemistry analysis are useful for diet formulation. More recently, "near infrared reflectance spectroscopy" (NIR) has been used for evaluation of feedstuffs. Dry matter, crude protein, and acid detergent fiber values from NIR analysis can be useful if the forage sample was predominantly one forage and labeled appropriately.

Many producers have questions about NIR. The precision of the results obtained from NIR relate directly to laboratory procedures. This can best be understood by describing the NIR procedure. Samples of feeds are ground to a very fine powder which is placed in a sample cup and loaded into the machine. Nutrient composition of the sample can be determined in seconds. This is much quicker than wet chemistry, which requires days for analyses. The chief disadvantages of NIR include expensive instrumentation, dependence on calibration procedures, method of data treatment, and lack of sensitivity for microminerals. Calibration of the machine used is a key point. Furthermore, equations must be programmed into the NIR machine to evaluate each feed type. It is important that laboratories compare results from NIR analysis to results from wet chemistry on a regular basis. If a feed is mislabeled the information provided by the machine will be incorrect. In many cases such differences will be dramatic.

The next step involves the equations used to estimate energy value TDN and NEL. An equation must be established for each feed type evaluated. In many laboratories, the equations used were derived for forages grown in the Northeast. Nutrient composition of forages in the Northeast is quite different from similar forages grown in the upper Midwest. Using equations developed for New York forages on North Dakota forages may be quite imprecise.

A pointer or two: When a sample is not "pure" it is important the label the sample appropriately, i.e., 50 percent alfalfa, 50 percent weeds. In most cases, forages that are of mixed varieties should be evaluated via wet chemistry. If 2 or 3 pounds of a mixed hay are consumed daily, it may not be as critical to know the exact nutrient composition of the forage. If a total mixed ration is required, send in individual feeds. Do not send a sample of a total mixed ration for analysis. It is extremely difficult to get a representative sample of a mixed ration, and the dry matter content of the ration can change hourly and daily.

Recommendations

Once the results are known, how should they be used? This will depend on the type of analysis. If a representative sample of the forage was evaluated by wet chemistry, the reported values should be reliable. Dry matter, crude protein, `fiber, estimates of energy, and mineral values should be used to determine diet composition of individual feed ingredients. If a representative sample of the forage was not collected, use the results with some understanding that they do not represent the feed your animals will consume.

What about using results from NIR analysis? Again, a representative sample must have been labeled correctly and sent to the lab. If the forage was a mixture of different grasses, ask if the NIR machine has the needed equations to determine nutrient composition . A nice computer print-out from lab analysis does not mean the numbers on the print-out are valuable. If your hay was 90 percent of one type and sampled appropriately, there is more confidence in the values you receive for crude protein and ADF. Values for macro minerals (calcium, phosphorus, potassium, and magnesium) may or may not be precise.

Certain feedstuffs can be evaluated successfully with NIR while other feedstuffs should be evaluated under wet chemistry conditions (Table 1). Once a representative forage sample is obtained, be sure the method of analysis will provide useful nutrient composition data.

Feed type	NIR	Wet chemistry

Silage (pure)	yes	yes
Silage (mixed)	no	yes
Pure hay	yes	yes
Mixed hay	depends¹	yes
Grain	yes	yes
Grain mix	depends²	yes
Grain mix w/mineral³	no	yes
Total mixed ration⁴	no	yes

¹Label sample mixed hay. Find out if lab can evaluate properly.
²Labei sample mixed grain and list types.
³Use caution to obtain a representative sample.
⁴TMR's can be evaluated via wet chemistry. However, it is difficult to obtain a representative sample. See text discussion.

Knowing what's in the forages will allow you to formulate diets and maximize production. If the nutrient composition of - the forage is not known, wasting nutrients may occur. Spending $15 to $20 on forage analysis will provide added savings to livestock operations. In many cases, once forages are analyzed and diets are formulated to provide necessary nutrients at least cost, feed costs are reduced up to 30 percent and production is increased. Assistance in developing complete diets is available at your local NDSU Extension Service office.