When evaluating forage nutritive value, we often hear about relative feed value (RFV) and relative feed quality (RFQ). These measures provide a quick snapshot of forage quality and can be helpful for comparing feeds, especially when buying or selling. But while RFV and RFQ are helpful benchmarks, they do not tell the whole story. To fully understand a forage’s feeding value, it is important to look deeper into the forage analysis.
RFV and RFQ: Useful but limited
RFV was developed in 1978 to help producers rank cool-season grasses and legume forages based on predicted digestible dry matter intake relative to full-bloom alfalfa. But it was soon recognized that the RFV equation did not account for important factors such as protein concentration or fiber digestibility. This made it difficult to accurately compare different forage types like grass hay versus alfalfa hay.
These issues spurred the creation of the RFQ metric, which incorporates total digestible nutrients (TDN) and fiber digestibility, making it a more complete indicator of forage quality. RFQ also uses different equations for legumes and grasses, helping better reflect differences in digestibility. It is important to remember that neither RFV nor RFQ adequately reflects starch digestibility and should not be used to evaluate corn silage.
Overall, RFQ is a better snapshot of forage feeding value than RFV. But RFQ is still an index value. It simplifies a complex feed into a single number and does not capture all the information needed for sound feeding decisions. Think of RFQ as a starting point, not the final answer. Digging deeper into metrics like dry matter, pH, silage acids, protein, fiber and ash concentration can help you understand how a feed will behave during storage and feeding.
Dry matter
A forage’s dry matter (DM) content is not fully reflected in RFV or RFQ, but it has important implications for both forage management and feeding performance.
Silages that are too dry can be difficult to pack, allowing more oxygen infiltration and increasing the risk of heating and spoilage. Silages that are too wet are more prone to seepage, clostridial fermentation and increased dry matter losses. For hay, very low moisture increases leaf loss and stem brittleness, while overly wet hay raises the risk of spoilage and spontaneous heating.
At the feedbunk, forage moisture matters as well. Two forages with the same RFQ can perform very differently if their dry matter concentration differ. Very wet silages contribute to wetter total mixed rations (TMRs), which tend to heat faster in the feedbunk and can reduce feed intakes. Moisture also affects purchasing and transportation decisions since wetter bales or silage will weigh more and can increase hauling costs.
Forage pH
For ensiled forages, pH provides a useful snapshot of fermentation efficiency. When properly fermented, forages will be more digestible, palatable and have less dry matter loss. This can increase dry matter intake and improve animal performance. Immediately after harvesting, silage typically has a pH between 5.5 and 6. Well-fermented haylage will generally have a final pH between 4.0 and 5.0. Final silage pH is strongly influenced by the forage’s buffering capacity. Crops with higher ash and protein concentrations have greater buffering capacity (i.e., they resist changes in pH). This is why legume silage typically has a higher final pH than corn or grass silage. In general, lower final pH values indicate a more efficient fermentation process and a lower risk of spoilage.
Silage acids
Looking at silage acids can help explain how fermentation went and identify potential spoilage risks. Three acids are commonly reported: lactic, acetic and butyric acid.
Lactic acid is the main acid responsible for dropping silage pH. It is the main fermentation acid found in well-preserved silages. High lactic acid concentrations usually indicate desirable fermentation. Acetic acid is also common, but usually at lower levels than lactic acid. Acetic acid has antifungal properties that help limit spoilage and improve stability. However, high levels of acetic acid can indicate less efficient fermentation. It is important to note that silages inoculated with Lactobacillus buchneri (designed to improve silage stability) will have higher acetic acid concentrations. Lastly, butyric acid results from clostridial fermentation, which is often the result of harvesting forages that are too wet. Elevated butyric acid can raise serious feeding concerns, and prolonged clostridial fermentation can reduce feed digestibility.
Protein
Crude protein (CP) is included in the RFQ equation, but CP alone does not tell you how much protein is actually available to the cow. Heat damage during harvest or storage can bind protein to fiber, making it unavailable for digestion. Acid detergent insoluble crude protein (AD-ICP), also listed as ADIP, ADIN or ADFP, can be used as a quick and reliable indicator of heat damage. While all forages contain some AD-ICP, values greater than 10% of crude protein indicate that heat damage has reduced protein availability. This represents protein that has been grown, harvested and paid for, but cannot be used by the cow. Metrics such as adjusted crude protein (ACP or Adj. CP) account for this unavailable protein and provide a clearer picture of a forage's protein value.
Fiber
One of the strengths of RFQ is that it accounts for fiber digestibility. But it is still important to look at the actual fiber values of a forage. Neutral detergent fiber (NDF) plays a key role in intake, energy supply and rumen fill.
NDF is composed of hemicellulose, cellulose and lignin, which range from moderately to poorly digestible. Forages with higher NDF concentrations have a greater rumen filling effect and therefore, often limits intakes. Lower NDF forages typically are more digestible and support higher intakes. Both NDF concentration and digestibility are strongly influenced by plant maturity at harvest.
Fiber digestibility is often reported at multiple time points, such as 30, 48 and 120 hours. Another useful metric is undigested NDF at 240 hours (uNDF240), which represents the fraction of fiber that is undegraded after 240 hours (10 days) of fermentation. This portion is considered completely indigestible. High uNDF240 concentration will increase rumen fill, reduce intake and lower the forage’s nutritive value. Forages with lower uNDF240 generally provide more usable energy and support better performance.
Ash
Ash represents the mineral portion of a forage and provides no energy value. As ash value increases, RFQ decreases. Ash can also introduce undesirable microorganisms to feed, decrease nutrient absorption and cause health issues.
Normal ash levels in legume-grass forages are around 9%. Elevated ash levels may result from low cutting height, plant lodging, or handling and storage practices. The mineral composition of ash is also important to consider. High-potassium forages can increase the risk of grass tetany or milk fever. Forages with high sulfur concentration can interfere with overall mineral absorption.
Take-home
RFV and RFQ are convenient values that provide a quick way to compare forage nutritive value. But do not stop there. Looking beyond RFV and RFQ will provide valuable insights into harvest management, fermentation success, storage conditions and how a forage will perform in the ration. The best feeding decisions come not just from understanding the number at the top of the report but from diving deeper.
References omitted but are available upon request by sending an email to the editor.
