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Understanding your forage analysis: Value ranges and definitions

Progressive Forage Editor Lynn Jaynes Published on 10 January 2018
alfalfa bale with nutritive values

Problem: You’re frustrated – you send hay samples to multiple labs and the variability between them is high. What gives?

Yes, variability among labs exists; no one denies that. Before we lay blame on the lab, however, let’s make sure you understand the analysis.

Are you clear on the results?

Understanding the values on the hay report and the acceptable variation within the values is critical. Presenters at the Alfalfa Hay Quality Workshop, held recently in Reno, Nevada, gave guidance on value ranges for lab analyses, for alfalfa hay.

Keep in mind acceptable variation is 3 percent of the value (not 3 percentage points).

The ranges listed here follow a bell curve – the standard deviation (or the bottoms of the bell curve) has been eliminated. The ranges listed here represent the “middle hump” of the bell curve (Figure 1).

bell curve

Dry matter (DM) – Normal range: 82 to 90 percent. Definition: The percentage of the sample that is not water. Dry matter is the most important determination – if it’s wrong, then everything else is wrong, as nutrients are based on dry matter.

Protein terms

Crude protein (CP) – Normal range: 18 to 24 percent; if it’s anything outside of that range, you should ask questions. CP is nitrogen concentration times 6.25. This is the most common and dependable test of total protein. High protein is desirable. It can be obtained by harvesting at an early growth stage. Can it be too high? Yes. Any number above the 24 percent range means the nutritionist will discount it because it is likely non-protein nitrogen.

Available protein – This value should be as close as possible to CP and is calculated by adjusting total CP for heat damaged protein, which is unavailable protein and may pass through the animal without absorption or benefit to the animal. Nutritionists want some protein modified in the rumen, but if too much is passed through the animal, it simply goes into ammonia production.

Soluble protein – Normal range: 35 to 47 percent. Definition: The percent of CP that is soluble and quickly degraded (sometimes instantly). This is usually expressed as a percent of CP (not DM).

Degradable protein – Normal range: 65 to 73 percent. Definition: Protein that is degraded in the rumen.

Neutral detergent insoluble CP – Normal range: 2.4 to 4.3 percent. Definition: The portion of undegradable protein that is available to the animal. It is associated with the plant cell wall. It slowly degrades in the rumen, but a big portion may escape and be digested in the small intestine.

Acid detergent insoluble CP – Normal range: 1.1 to 1.7 percent. This is CP not available for digestion; greater than 1.5 percent indicates heat damage. Nutritionists watch this closely and discount hay accordingly, even if it’s palatable, because too much protein is tied up in the Maillard effect (browning).

Fiber terms

Neutral detergent fiber (NDF) – Normal range: 33 to 44 percent. Definition: Indigestible and slowly digestible components of the cell wall (cellulose, hemicellulose, lignin). NDF increases with plant maturity and will reduce animal intake.

Acid detergent fiber (ADF) – Normal range: 26 to 34 percent. This fiber includes plant cellulose and lignin, and is the least digestible portion of roughage. High values decrease digestibility.

Lignin – Normal range: 6.5 to 8.4 percent. This plant component is indigestible and increases with plant maturity and higher temperatures.

Non-fibrous carbohydrates (NFC) – Normal range: 27 to 34 percent. Lower non-structural carbohydrates and NFC content indicates stress or immaturity; higher levels indicate higher dry matter intake. NFC is what we should be focusing on instead of fiber. Fiber is a negative except for use in rumen motility. Energy comes from non-fibrous carbohydrates.

Crude fat – Normal range: 2 to 2.8 percent. Definition: True fat containing triglycerides.

Total digestible nutrients (TDN) – Normal range: 57 to 63 percent. Definition: The sum of digestible CP (crude fat x 2.25), nonstructural carbohydrates and NDF. TDN is the amount of nutrients that are digestible. It’s the energy value of forage. TDN is often estimated by calculation from ADF, but with much greater error.

Energy terms

Forage tests for energy are expensive and are not routinely done.

Net energy for maintenance (NEm) – Normal range: 0.53 to 0.62 Mcal per pound. Definition: An estimate of the energy value of a feed used to keep an animal in energy equilibrium.

Net energy for gain (NEg) – Normal range: 0.27 to 0.36 Mcal per pound. Definition: Feed used for bodyweight gain above maintenance needs.

Relative forage value (RFV) – Normal range: 120 to 190. Definition: A calculation of cool season grasses and legumes based on intake of digestible energy – higher RFV equals higher quality. Feeder hay is considered less than 160 RFV, and dairy hay is greater than 180.

Relative forage quality (RFQ) – Range: 100 to 200 (this is not a percentage, but a numeric scale). It is based on intake of TDN of all forages. The higher the RFQ, the better the quality. This value is recommended with any mixed hay with grasses. RFQ uses digestible NDF (that’s the difference between it and RFV). RFQ is the newer measurement, and it is recommended over use of RFV.

Neutral detergent fiber digestibility (NDFd) – The range is determined by an in vitro incubation; 30 hours – 33 to 45 percent; 48 hours – 41 to 53 percent. Definition: The portion of NDF that is digestible. Higher values equal higher intake and animal performance. The challenge is that you need to know the time factor and compare it accordingly. The time factor varies between labs.

Some dairy nutritionists argue that 48 hours is too long and suggest a 24-hour time period. So be aware that there is disagreement on time length. Just remember that time periods can’t really be compared unless you calculate the rate of digestion (TTNDFD or total tract neutral detergent fiber digestibility). Rate of digestibility becomes really important for dairy cows, and as a hay producer, you’ll need to ask the lab what the time period is for this test.

In vitro true digestibility – This test is also time sensitive, but the optimal range for a 30-hour test is 70 to 80 percent. This test of anaerobic fermentation is conducted in a lab to simulate digestion in the rumen. It measures digestibility to determine energy. Incubation of forage is conducted in a beaker with rumen fluid from a donor cow for 24 to 48 hours. Heat, time and source of the inoculant are all important and this creates variability among labs.

Ash – Normal range: 10 to 11 percent or less. Definition: Ash is comprised of soil (silica and other inorganic matter) contamination and mineral content. Ash reduces energy at a 1:1 ratio, and cows can’t produce milk or beef from dirt. Ash is becoming more important with the use of rotary mowers, and nutritionists are starting to discount these forages considerably.

Perhaps most importantly, be realistic: Arguing over a few points of RFV or 0.5 percent TDN or 1 point NDF is a waste of time. Accept a “plus-or-minus” surrounding each number due to sampling variation or lab variation.  end mark

Lynn Jaynes
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PHOTO: An alfalfa entry from an Oregon hay contest was used as a training tool at the Alfalfa Hay Quality Workshop in Reno, Nevada. Photo by Lynn Jaynes.

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