Know Before You Feed - The Value of Testing Forages

Know Before You Feed – The Value of Testing Forages by Kristin Thompson, Ruminant Nutritionist with New Life Mills

Forage quality has a direct impact on a producers’ profitability through influencing production performance, milk yields and reproductive efficiency. The quality of a forage is determined by the amount and availability of nutrients it contains. And when forages are the main component in beef cattle rations, this quality is important to evaluate in order to develop an effective feeding strategy. This is where analytical testing comes into play which allows us to formulate rations to meet the animals’ nutrient requirements and offer a prediction on animal performance. Three of the main forage quality components are protein, fiber and energy.

Forage Quality

Weather conditions and forage maturity are the main factors impacting forage quality in a hay stand or pasture. When adequate rain and limited environmental stressors are present, plants continue to grow throughout the growing season leading to a high leaf to stem ratio. However, rainfall during harvest can lead to mold growth, leaf loss and nutrient leaching. In contrast, hot, dry weather, particularly at the start of the growing season, will result in stunted plant growth and increased plant lignification. With increasing plant lignin content, the fiber component increases which corresponds to a reduction in fiber digestibility.

The forage quality can be further impacted by harvest and storage conditions after cutting. The longer a forage remains in storage, the more potential there is for nutrient breakdown and leaching, particularly if the forage is stored outdoors with little to no weather protection. In this situation, the outer layers of the forage become less palatable and have reduced digestibility.

Analytical laboratories will typically offer a number of different analysis packages that can be grouped by sample type or livestock category. At a minimum, forage should be tested for moisture or dry matter (DM), crude protein (CP), acid detergent fiber (ADF), neutral detergent fiber (NDF) and an estimation of the energy content (Total Digestible Nutrient (TDN); Net Energy for Maintenance (NEm) or Net Energy for Gain (NEg)). The macromineral content of the forage can also be tested and depending on the different farm situations, other nutritional parameters could also be chosen for analysis to cover the requirements. For example, a farm that has previous issues with copper deficiency, might want to evaluate the molybdenum levels in forages, as molybdenum is a known copper antagonist. Figure 1 shows an example of a Certificate of Analysis.

Protein

The crude protein value on a forage analysis report represents the total nitrogen (true protein and non-protein nitrogen (NPN)) in the feed. The crude protein level can be further fractionated into soluble protein (rapidly available in the rumen), Neutral Detergent Insoluble Crude Protein (ADICP) and Acid Detergent Insoluble Crude Protein (NDICP). The NDICP value represents the portion of protein that is slowly degraded in the rumen and consists of a large portion of the ruminally undegraded protein. The portion of protein that is undigested and thus, unavailable to the animal, is reported as ADICP. Unless the forage has heatdamage, the ADICP level should be below 12%. Some forage analysis reports will include an Adjusted Crude Protein value. This value is the CP% adjusted to remove the ADICP and should be the value used in ration formulation. It should be noted that the CP value is typically not adjusted until ADICP is 10% or higher as a percent of CP.

Research has shown that when a feed tests below 7% CP, microbial fermentation is reduced. This can result in reduced feed intake and rumen passage rate. When this based on the rate of ruminal breakdown

type of low-quality forage is being fed, a high-quality protein supplement such as by product feeds (distillers’ grains, canola meal) or high-quality legume forage, should also be included in the ration to improve nutrient utilization.

Non-protein nitrogen sources can be a cheap option for inclusion in rations when protein is limiting. Urea, a common NPN source, is often also included in commercial protein supplements.

However, when considering NPN sources, such as urea, it is important to remember that NPN is utilized less efficiently in the rumen than true protein. There is also the concern of toxicity when high levels of urea are included in the diet, particularly if the ration is not balanced adequately for energy. Therefore, it is important to ensure the correct amount of urea is being fed per head per day. The recommendation for urea is no more than 0.05 lbs for dry cows, 0.1 lbs for lactating cows and 0.25 lbs for feeder cattle. Calves less than 500 lbs should not be fed urea.

Fiber

When it comes to fiber, the majority of forage analysis reports will provide a crude fiber value for the feed tested. However, it is recommended that the report also include ADF and NDF measured separately. Neutral Detergent Fiber is an estimation of the total fiber constituents of the forage as it contains the cell wall components hemicellulose, cellulose and lignin. These components make up the fibrous bulk of the plant and are negatively correlated to dry matter intake (DMI). Acid Detergent Fiber is a measurement of the indigestible components of the plant cell wall and is negatively correlated to forage digestibility. Generally, a high ADF value is associated with a more mature forage.

In a situation where a forage sample has tested high in NDF, this results in a reduction in intakes along with a reduced digestibility of the fiber being consumed. To combat this, supplemental protein and energy may be necessary as cattle may not physically be able to consume enough forage to meet nutrient requirements. This is especially important for bred replacement heifers, who already have a reduced rumen capacity and increased nutrient requirements.

Energy

Traditionally, the energy value of a forage is reported as TDN, which is a calculated value dependent on the digestible portions of a feedstuff and is based on ADF. However, TDN underpredicts the energy value for concentrates (grains). Therefore, TDN can be a useful tool when formulating a ration that is primarily forage based, but it may be more advantageous to formulate based on Net Energy (NEm and NEg) values rather than TDN especially when concentrates are included in the diet.

Both NEm and NEg are an estimation of the forages ability to meet the animals’ energy requirements, expressed as megacalories (Mcal) per unit of forage (lb or kg) and are based on ADF. Net energy of maintenance is the energy available to meet maintenance requirements, while NEg is the energy available in the forage for growth (weight gain). The energy in forages is more efficiently utilized for maintenance rather than growth, therefore, the NEm value will always be higher when compared to NEg. The NEg value should be used when estimating the forages ability to put weight on growing cattle.

If the energy value of a forage is low, (<50% TDN; <0.5 Mcal/lb NEm; Figure 1) the diet should be supplemented with another energy source in order to meet animal requirements. The most common energy supplement is a high starch grain such as corn or barley. However, other options include high quality forages or other feed sources high in digestibly fiber Figure 2: Interrelationship in forage quality parameters and animal performance.

such as beet pulp or soybean hulls. The use of energy supplements can also allow for the extension of forages if supply is low.

Conclusion

Forages are a valuable feed source for utilization in beef cattle rations. However, proper evaluation of forage quality is essential in developing an effective feeding strategy. Protein, fiber and energy are three parameters that can be used to determine the basic nutrient availability of a forage. B

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