What dry matter intake is and why every ration is built on it
Dry matter intake is the weight of feed an animal eats per day after every drop of water is removed. It is the single most important number in animal nutrition because it is the denominator of every requirement you will ever look up. A dairy ration is not specified as "4 pounds of protein"; it is specified as "16.5% crude protein of the dry matter". Until you know how many pounds of dry matter the cow will eat, that percentage cannot be turned into pounds of soybean meal.
The reason nutritionists insist on the dry matter basis is that moisture varies wildly and carries no nutrition. Corn silage might be 32% dry matter one week and 38% the next after a dry spell in the pile. A cow eating 100 pounds of the wetter silage takes in 32 pounds of dry matter; the same 100 pounds of the drier silage delivers 38 pounds. Nothing about the crop changed, but the animal's intake moved by nearly a fifth. Balancing on an as-fed basis quietly re-formulates your ration every time the weather changes.
This calculator does two jobs. First it predicts dry matter intake, either with the NRC (2001) equation for lactating dairy cows or with a percent-of-body-weight figure for every other class. Then it inverts the dry matter percentage to give you the as-fed weight the loader operator must actually put in the mixer, and multiplies by head count so you can plan deliveries. If you are buying that feed as hay, pair this with the hay bales needed calculator and the round bale weight calculator.
The NRC intake equation, term by term
The NRC (2001) lactating-cow prediction has three parts, and each one answers a different question about the cow.
The milk term, 0.372 × FCM. Fat-corrected milk is milk yield adjusted to a common 4% fat content, so a Jersey and a Holstein can be compared on one scale. Compute it as 0.4 × milk pounds + 15 × milk fat pounds. The coefficient says a cow eats about 0.372 kg of extra dry matter for every extra kilogram of 4% milk she ships. That is the demand side.
The body-size term, 0.0968 × BW0.75. Metabolic body weight, body weight raised to the three-quarter power, is the standard scaling for maintenance energy across mammals: a heavier animal needs more feed, but not in direct proportion, because heat loss scales with surface area rather than mass. Raising 635 kg to the 0.75 power gives 126.5, so the maintenance part of the prediction is about 12.2 kg of dry matter regardless of milk yield.
The lag factor, 1 − e−0.192(W + 3.67). A fresh cow cannot physically eat what her udder demands. Rumen fill, a shrunken abdominal cavity and post-calving metabolism all restrict intake for the first month or two, and the exponential term models the recovery. Because the exponent becomes more negative as the week number climbs, the factor increases with every week of lactation and approaches 1.0: at week 1 it is 0.592, at week 12 it is 0.951, and by week 20 it is 0.989. That is the shape of the classic negative-energy-balance problem, where peak milk arrives weeks before peak intake.
For beef cows, growing cattle, sheep, goats and horses, no equation of this kind is in general use, so the calculator falls back on the percent-of-body-weight method: multiply body weight by an intake percentage appropriate to the class and the forage quality. Higher-quality, more digestible forage passes out of the rumen faster and permits a higher percentage; coarse mature hay permits less.
Worked example: a 1,400 lb cow at 75 lb of fat-corrected milk, week 12
Take a Holstein weighing 1,400 lb, shipping 75 lb of 4% fat-corrected milk, twelve weeks fresh, eating a total mixed ration tested at 45% dry matter. There are 120 cows in the pen.
- Convert to metric. 1,400 ÷ 2.20462 = 635.03 kg body weight. 75 ÷ 2.20462 = 34.019 kg of FCM.
- Metabolic body weight. 635.030.75 = 126.50 kg0.75.
- Body-size term. 0.0968 × 126.50 = 12.245 kg.
- Milk term. 0.372 × 34.019 = 12.655 kg.
- Add them. 12.245 + 12.655 = 24.901 kg, the intake an unrestricted cow of this size and yield would reach.
- Lag factor. 12 + 3.67 = 15.67; 0.192 × 15.67 = 3.0086; e−3.0086 = 0.04936; 1 − 0.04936 = 0.95064.
- Dry matter intake. 24.901 × 0.95064 = 23.672 kg, and 23.672 × 2.20462 = 52.19 lb of dry matter per day.
- Percent of body weight. 52.19 ÷ 1,400 = 3.728%.
- As-fed weight. 52.19 ÷ 0.45 = 115.97 lb of ration as delivered.
- Whole pen. 115.97 × 120 = 13,916 lb per day, which is 13,916 × 30 ÷ 2,000 = 208.7 tons over thirty days.
Notice what happens if the silage dries down and the mix tests 48% instead of 45%. The cow still needs 52.19 lb of dry matter, but the as-fed call drops to 52.19 ÷ 0.48 = 108.7 lb. Feeding the old 115.97 lb would push 3.4 lb of extra dry matter per cow into the bunk, which over 120 cows is 408 lb of dry matter a day going into refusals or over-conditioning.
How to read the result against real bunk data
Treat the prediction as a starting point for formulation, not as a measurement. The honest test is the bunk: weigh what you deliver, weigh the refusals, subtract, and divide by the number of head. If your measured intake and the prediction disagree by more than a few pounds, the prediction is not what is wrong — something in the pen is.
Percent of body weight is the fastest sanity check. Lactating dairy cows in mid-lactation commonly land between 3.0% and 4.0% of body weight; the calculator flags anything above 4% or below 1.5% for a second look, because those regions usually mean a mistyped body weight or a milk yield entered as a monthly rather than daily figure. Dry cows, mature beef cows and maintenance-level animals sit far lower, and growing animals on high-quality forage sit higher than mature ones of the same weight.
Three things commonly depress real intake below any prediction: heat stress, a sorted ration that lets cows leave the long fibre behind, and crowding at the bunk. Two things commonly raise measured intake above the prediction: under-counting the animals actually eating, and refusals that are wetter than the delivered ration, which makes the as-fed refusal weight overstate the dry matter left behind. Always dry down a refusal sample before you subtract it.
Typical dry matter of common feeds as delivered
| Feed | Typical dry matter | As-fed lb per 10 lb of dry matter |
|---|---|---|
| Dry shelled corn | 88% | 11.4 |
| Soybean meal | 89% | 11.2 |
| Grass or legume hay, baled dry | 88% | 11.4 |
| Haylage / baleage | 45% | 22.2 |
| Corn silage | 35% | 28.6 |
| Wet brewers grains | 25% | 40.0 |
| Whole-plant high-moisture ear corn | 70% | 14.3 |
The third column is simply 10 ÷ (dry matter ÷ 100). Dry matter of ensiled feeds moves through a pile and through a season; test monthly at minimum.
Mistakes that make an intake figure wrong
- Balancing on an as-fed basis. Every requirement table in animal nutrition is written on a dry matter basis. Converting once, at the end, is the only safe order of operations.
- Entering milk instead of fat-corrected milk. A Jersey at 60 lb of 5% milk is not the same intake driver as a Holstein at 60 lb of 3.5% milk. Convert with 0.4 × milk + 15 × fat pounds first.
- Using a book dry matter for silage. The single largest error source in this calculation is an assumed silage moisture. A Koster tester or a microwave dry-down costs minutes; a three-point dry matter error on a 60 lb silage feeding is nearly two pounds of dry matter per cow per day.
- Ignoring refusal moisture. Weigh-backs pick up water in the bunk. Dry the sample before you subtract, or you will overstate what the cows left and understate what they ate.
- Applying a lactating-cow equation to dry cows. The NRC lactating equation is not valid at zero milk; use a controlled-intake percent-of-body-weight target for far-off and close-up groups.
- Treating the prediction as a feeding instruction. Predictions size a ration. Bunk scores, weigh-backs and body condition change it.
Where this sits among the other intake models
The NRC (2001) equation is the seventh revised edition of Nutrient Requirements of Dairy Cattle, and it remains the most widely programmed intake prediction in on-farm software because it needs only three inputs a herd manager already has. Its successor, the eighth revised edition published by the National Academies in 2021 as Nutrient Requirements of Dairy Cattle, revises the intake model and adds separate treatment of dry cows and heifers; if you are working to that edition, use its equations and treat this page as a cross-check.
For beef cattle, the analogous reference is Nutrient Requirements of Beef Cattle, and intake there is normally expressed against body weight and diet energy density rather than milk. For sheep and goats the small-ruminant NRC volume plays the same role. Across all of them the arithmetic on this page — dry matter to as-fed, per head to per group — is identical; only the intake prediction changes.
Once you have the intake figure, the next steps are ration composition and cost. Use the Pearson square feed ration calculator to blend two ingredients to a target protein or energy level, the feed conversion ratio calculator to see what that feed is buying you in gain, and the pasture grazing days calculator when the dry matter is coming out of a paddock rather than a mixer.