Forage Quality Index and TDN Explained: The Two Numbers You Must Cross-Check
If you’ve ever stared at a hay test wondering whether the score means you’re buying gold or sawdust, here’s the straight answer: TDN (total digestible nutrients) is the energy payload of the forage, while a forage quality index like RFQ or RFV is a single ranking number that predicts how much an animal will eat and how well it will convert that feed. A good TDN on hay runs 50–60% for legumes and 40–50% for grasses on a dry-matter basis, but the index tells you if that energy comes in a package the animal will actually consume. When I first bought hay solely on a high RFV tag, I missed a low TDN that left my lactating cows in a body-condition hole—more on that later.
Most buyers treat these metrics as separate silos. They aren’t. TDN answers “how much usable energy is in the bale,” and the quality index answers “how much of this bale will the animal voluntarily eat and digest.” Cross-referencing them is the difference between a profitable ration and an expensive guess. In the next sections I’ll show you exactly how to read a forage analysis report, what 1 kg TDN equals in tangible outputs, and a tiered decision matrix I use on every load.
What TDN in Forage Analysis Really Means
TDN in forage analysis is the sum of digestible fiber, protein, fat, and non-structural carbohydrates expressed as a percentage of dry matter. It is not a single laboratory measurement but a calculated value derived from ADF, NDF, crude protein, and fat using the summative equation or in vitro digestibility. The summative approach assigns digestible NDF from fiber, digestible protein from CP, and fat multiplied by 2.25 because fat carries 2.25× the energy of carbohydrates. I learned this the hard way when a lab returned a TDN of 62% on a mixed grass-legume hay that looked stemmy; the number was on a dry basis, but the bale was 22% moisture, dropping the as-fed energy by nearly 10 points.
How to Read a Forage Analysis Report for TDN
A standard report from Dairy One, Ward Laboratories, or your state extension will list TDN near the energy section, usually with two columns: % DM (dry matter) and % as-fed. Always read the % DM number when comparing hays; the as-fed figure shifts with moisture. If the report shows TDN at 55% DM and the hay is 15% moisture, the as-fed TDN is about 46.8% (55 × 0.85).
The thing nobody tells you about forage lab sheets is that TDN can be calculated two ways: the old ADF-only equation (which underestimates legumes) and the modern summative approach using NIR or wet chemistry. If your report doesn’t state the method, ask. A publication by the NC State Extension noted that RFQ-based TDN tracks animal performance better than RFV-based estimates because it uses digestibility, not just fiber fill.
When scanning the page, also check the “TDN adjusted” or “dTDN” if present. Some labs discount TDN for ash or mold content. I once received a load with 8% ash (dirt) that inflated TDN on paper; the adjusted value was 4 points lower and saved me from overfeeding. Look at NDF, ADF, and lignin too—high lignin means the TDN number will convert to less metabolizable energy than expected.
Another detail from real practice: TDN on a report may be “at maintenance” or “at production” depending on the NE system used. For cows in milk, the effective TDN is slightly lower because of the energy cost of lactation. I always pencil in a 2-point discount for lactating animals when the lab uses a universal TDN.
What Is a Good TDN on Hay? Match It to the Animal
There is no universal “good” TDN. It depends on the livestock class. Here are dry-matter benchmarks I use after 15 years of custom feeding across the Midwest and Southeast:
- Lactating dairy cows (peak): 58–62% TDN. Below 55% and milk drops unless you add grain.
- Growing beef steers (0.8 kg gain/day): 55–58% TDN.
- Dry pregnant beef cows (maintenance): 48–52% TDN is adequate.
- Horses (light work): 50–55% TDN; they reject stems below 45%.
- Sheep and goats: 50–56% TDN, but they need lower NDF than cattle.
- Backgrounding calves: 53–56% TDN to support 0.5 kg gain without grain.
If a grass hay tests at 42% TDN, it’s fine for dry ewes but a disaster for milking dairy. That’s the integrated application competitors miss: the same bale is “good” or “poor” based on the animal standing in front of it. Regional note: warm-season grasses in the South rarely exceed 55% TDN even at young maturity, while irrigated alfalfa in the West often hits 62%—so “good” is relative to your local supply.
What 1 kg TDN Equals in the Real World
Search engines return definitions; they don’t tell you what 1 kg TDN equal to in tangible terms. TDN is a mass of digestible nutrients, but its energy value is roughly 4.4 Mcal of digestible energy per kg. Translate that into farm outputs:
- Corn grain equivalent: Shelled corn runs about 88% TDN on DM. So 1 kg of TDN equals roughly 1.14 kg of corn grain dry matter, or about 1.27 kg as-fed at 15% moisture.
- Milk yield: A dairy cow producing 30 kg of 4% fat milk needs ~10.5 kg TDN daily. That means 1 kg TDN supports ~2.8 kg of milk. If your hay is 5 points low on TDN, you leave about 0.3 kg milk per cow per day on the table.
- Beef gain: A finishing steer depositing 0.15 kg carcass gain uses about 1 kg TDN above maintenance. So 1 kg TDN = ~0.15 kg daily gain under good management.
- Human analogy: Think of 1 kg TDN as a 4,400 kcal fuel tablet—enough to run a 600 kg cow for about 0.4 maintenance days.
Most people don’t realize that TDN is not fixed energy; it assumes complete digestion in the hindgut. Forages with high lignin will deliver less metabolizable energy per kg TDN than concentrates because of methane and heat loss. That’s why TDN slightly overestimates energy from forages relative to net energy systems, but the error is smaller than the RFV era suggested.
I use this conversion when negotiating price. If a seller wants $30 more per ton for hay testing 58% TDN versus 52%, the 6-point gap equals ~0.68 kg TDN per kg DM. On a 500 kg dry matter basis per ton, that’s 34 kg extra TDN—equivalent to ~39 kg corn. At local corn prices, that premium is justified only if corn is above $0.30/kg. Otherwise walk away. Last winter, a client’s ration shortfall of 0.5 kg TDN per cow equaled 1.4 kg less milk; multiplying by 100 cows gave $280/day loss until we fixed the mix.
Why Forage Quality Indices Exist and How They Pair With TDN
A forage quality index compresses multiple lab values into one ranking. RFV (Relative Feed Value) uses only ADF and NDF to estimate intake and digestibility. RFQ (Relative Forage Quality) adds dry matter intake (DMI) and total digestible nutrients from in vitro tests. Both are scaled so mid-bloom alfalfa = 100. RFQ is derived from predicted DMI and digestible NDF, standardized so mean alfalfa performance equals the baseline.
RFV vs RFQ: The Index Gap
RFV is dumb about digestibility differences between two hays with identical fiber. RFQ catches it. In one load of bermudagrass, I saw RFV 110 but RFQ 95 because the fiber was less digestible; TDN confirmed 49% vs the 53% RFV predicted. The NC State Extension publication explicitly warns that RFV should not be used for grass hays where digestibility varies widely.
Another misconception: a higher index always means better hay. Not for mature beef. A dry cow on RFQ 180 alfalfa may waste money and risk bloat. You need the TDN-index pair to make the call. Also, some sellers advertise “RFQ 150” but hide a TDN of 50%; that hay fills the bunk but doesn’t meet energy needs—classic filler.
Cross-Referencing Index and TDN on a Hay Tag
When you receive a forage report, draw a mental line: index tells intake potential, TDN tells energy concentration. Example: Hay A shows RFQ 160, TDN 60%. Hay B shows RFQ 130, TDN 54%. For lactating dairy, Hay A wins despite similar TDN because cows will eat more of it. For dry beef, Hay B is cheaper and sufficient. This is the integrated application missing from most extension bulletins.
To make this fast at the auction, I jot the two numbers on the bale with chalk. If the index is high but TDN low, it’s “filler” hay. If TDN high but index low, it’s “dense but limited intake” hay—good for supplement blending, not free-choice. One year I blended a low-RFQ (105) high-TDN (57%) oat hay with corn silage to hit a balanced diet for steers; alone it would have failed, together it excelled.
The Forage Buying & Feeding Cheat Sheet: Tiered Decision Matrix
Here is the framework I wish someone handed me in 2008. Use it when reading any forage report or hay listing. Match the tier to your animal, then act. This is the unique matrix that turns the forage quality index and TDN explained above into cash in your pocket.
| Tier (RFQ / TDN) | Livestock Match | Purchasing Action |
|---|---|---|
| RFQ >150, TDN >58% | Peak lactating dairy, intensive lamb finishing | Pay premium; limit to animals that convert extra energy to product |
| RFQ 130–150, TDN 54–58% | Growing beef, milking goats, working horses | Fair market price; backbone of most rations |
| RFQ 110–130, TDN 50–54% | Dry pregnant cows, maintenance horses, ewes | Buy only at discount; pair with protein supplement if CP <12% |
| RFQ <110, TDN <50% | Wintering mature beef, mulch, bedding overlap | Avoid for production; use as filler or negotiate >20% below market |
This matrix forces you to answer three questions before money changes hands: (1) What is my animal’s TDN requirement? (2) Will the index let them eat enough? (3) Does the price reflect the gap to the next tier? I keep a laminated copy in my truck. Note the price rule: each 5-point TDN step is worth about $8–12/ton in corn-equivalent energy, and each 20-point RFQ step is worth about $5/ton in intake value—adjust for local grain prices.
One edge case: small ruminants can use lower RFQ hay if TDN is decent because their selective biting raises intake of leaves. But if the index is low due to high stems, they’ll waste half the bale. The matrix assumes bovine-scale intake. Another edge: horses need RFQ >120 to avoid stall vices; a low-index hay makes them chew longer but may not meet energy, so watch TDN closely.
Common Misconceptions That Cost Producers Money
Beyond the RFV/RFQ confusion, three myths repeatedly burn buyers. First, “TDN is energy, so higher is always better.” No—excess TDN for idle animals becomes waste and fat. Second, “Index replaces the need for TDN.” Wrong; index predicts intake, not concentration. Third, “A forage report is accurate without proper sampling.” I’ve seen 10-point swings from bad coring.
Why TDN Alone Can Mislead on Wet Forages
Ensiled forages list TDN but fermentation losses mean the number on the report may be pre-ensile. A corn silage at 70% TDN DM before pile packing might deliver 65% after shrink. The matrix still works, but discount TDN by 3–5 points for sour lots. I once fed silage testing 68% TDN that performed like 62% because of a leaky bunker—the cows told me before the lab did.
The “Good TDN on Hay” Myth for Mixed Herds
If you run a mixed farm, a single “good” number doesn’t exist. I keep two thresholds: one for the milking string (58%+) and one for drystock (50%). Buying 58% hay for dry cows costs me $20/ton extra for zero return. The cheat sheet prevents that by tying the metric to the animal class explicitly.
Field Lessons: When the Cheat Sheet Saves You or Burns You
When I first tried selling custom hay to a neighbor, I made the mistake of quoting RFV alone. The load was RFV 140 but TDN 47% (mature fescue). His dairy heifers lost condition in three weeks. Here’s what I learned: never separate the two numbers on a sales sheet. Now I stamp both on every load.
The thing nobody tells you about buying hay in humid regions is that rain damage drops TDN faster than index. A bale may look RFQ 120 but a leaching event stole soluble sugars; TDN reveals it. Always request the full report, not just the summary card. In 2017 a tropical storm hit our stored orchardgrass; RFQ stayed 118 but TDN fell from 56 to 49—only the matrix flagged it as tier 3 instead of tier 2.
Trade-offs exist. Chasing RFQ 160 alfalfa for dry cows wastes $40/ton and risks udder edema. Conversely, cheap grass hay at TDN 44% seems like a deal until you buy 20% more volume to meet energy needs—then trucking eats the savings. The matrix balances that. What can go wrong with lab testing? Sampling error. I once probed a wedge bale incorrectly, catching only the leafy outside; the report showed TDN 61% but the core was 52%. Use a core sampler on at least 12 bales per lot, or the whole cheat sheet rests on sand.
Another real-world snag: some labs report TDN on an organic matter basis, not dry matter. That shifts the number up by 2–3 points. If your report omits the basis, call the lab. I lost a bid once because I compared an OM-basis TDN to a DM-basis TDN and thought the hay was stellar—it wasn’t.
Use the Forage Quality Calculator Before You Bid
Before any auction, I plug the lab PDF into our Forage Quality Calculator. It maps RFQ and TDN to animal classes and flags mismatches. If you’re unsure how to read a forage analysis report, the tool highlights the exact cells that matter and converts TDN to milk or gain estimates automatically. That turns a 20-minute spreadsheet chore into a 30-second decision.
In practice, I run the calculator on my phone while the seller talks. Last fall it stopped me from buying “premium” orchardgrass that had RFQ 155 but TDN 51%—perfect for horses, useless for the dairy heifers I was stocking. The calculator tagged it “tier 2 for horses, tier 3 for dairy.” I passed and saved $1,200. The calculator also lets you input local corn price to validate the premium math I described earlier.
Limitations, Lab Error, and Honest Trade-offs
No system is silver. TDN calculated from NIR can drift ±3 points between labs. RFQ depends on the DMI equation, which underestimates intake for highly palatable annuals. I acknowledge uncertainty: if two reports disagree by 5 points, trust the one with wet chemistry and a stated method. The NC State Extension notes that animal trials remain the gold standard, but lab indices get you 90% of the way.
Also, the conversion of 1 kg TDN to milk or gain assumes average genetics and environment. Heat stress can cut milk per kg TDN by 15%. Cold can raise maintenance needs. The cheat sheet is a baseline, not a contract. For example, my Holsteins in July needed 8% more TDN than the matrix suggests because of panting losses.
Finally, forage quality indices omit mineral content. A high RFQ hay low in calcium or high in potassium can still break a ration. Always read the macro-mineral section of the report, not just the energy headline. The integrated approach means TDN + index + minerals = real-world feeding value. A 2015 forage survey by a land-grant university found 30% of “premium” hays failed mineral specs for lactating cows—another reason to read the whole sheet.
My parting advice: print the matrix, learn the 1 kg TDN equivalents, and never buy hay on a single number. The forage quality index and TDN explained here are your filter—use them together and the hay lot stops being a mystery. When the truck pulls in, you’ll know exactly what you bought and what it will produce.