Article — Dry Matter Calculator
Dry matter calculator: forage, silage, and feed analysis
Dry matter (DM) is the fraction of a feed left after all the water is removed. The formula is simple: DM% = (dry weight ÷ fresh weight) × 100. A sample of corn silage that weighs 100 g fresh and 33 g after drying contains 33 percent DM and 67 percent moisture. Dairy nutritionists balance rations on DM basis because all the protein, energy, fiber, and minerals are in the dry matter, not the water. A typical dairy cow eats 3 to 4 percent of body weight in DM daily — about 21 kg DM for a 600 kg cow.
Knowing DM matters in three practical situations: harvesting forage at the right moisture for storage, balancing rations between wet (silage) and dry (hay, grain) ingredients, and pricing feed fairly. Two loads of corn silage at the same price per ton can deliver very different amounts of nutrition if their DM differs by 5 percent.
What is dry matter?
Every feed has two fractions: water and dry matter. Water provides no nutrition. Dry matter contains everything else — protein, fiber, starch, fat, minerals, vitamins. The proportion of each depends on the feed type. Pasture is 78 to 82 percent water, fresh-cut alfalfa 75 to 78 percent water, corn silage 65 to 70 percent water, dry hay 10 to 15 percent water, grain 12 to 15 percent water.
Because all the nutrition is in the dry matter, animal nutritionists report nutrient analysis on a DM basis. A hay analysis might read "16% crude protein, DM basis." That means 16 grams of protein per 100 grams of dry matter. The same hay as-fed (still containing 12 percent moisture) has only 14 percent protein because the water dilutes the protein measurement.
The cellular composition of plants is about 50 percent water by fresh weight — even in dry hay, the carbohydrates and proteins themselves contain bound water in their molecular structure. Lab dry matter analysis only removes "free" water, the kind that evaporates at 60 to 105°C. That's why DM analysis is reported to one decimal place — the precision below that depends on temperature, time, and sample preparation.
How to measure dry matter on-farm
Three methods give usable on-farm results. The lab oven method is the gold standard — sample dried at 60 to 105°C for 24 to 48 hours until constant weight. Accurate to ±0.5 percent. Most feed labs use this. Turnaround is 2 to 5 days and costs $20 to $50 per sample.
The microwave method is fast and reasonably accurate (±2 percent). Weigh 100 g of sample on a cheap kitchen scale, microwave on a paper plate at full power for 2 minutes, weigh again, microwave another 2 minutes, weigh again. Continue until two consecutive weights match. Add a small glass of water in the microwave with each round to prevent scorching. The whole process takes 15 to 30 minutes.
The Koster moisture tester is a purpose-built farm device — a small heater with a basket. Takes 30 to 60 minutes per sample. Accurate to ±1 percent. Once-off cost about $200 to $300. Useful for farms checking silage and haylage moisture multiple times per harvest.
Sample size matters more than method. For forage and silage, mix material from at least 5 to 10 spots in the pile or bale and take a single 100 to 200 g sample from the mix. A single grab from one spot can easily be 5 to 10 percent off the truck average. For grain, a sampling probe pulls a vertical core from a bin or bag.
Dry matter targets for forage
Each type of stored forage has a target DM range that ferments and stores well.
Fresh pasture 18-22% DMCorn silage 30-38% DMGrass silage 30-40% DMHaylage / baleage 45-55% DMDry hay 85-90% DMStraw 88-92% DMGrain 85-88% DMDrying hay below 85 percent DM (above 15 percent moisture) is the cutoff for mold-free storage. Above 15 percent moisture, bales heat up over the first week and risk fungal growth and spontaneous combustion in extreme cases. Above 20 percent moisture, hay is essentially unsafe to store as dry hay — wrap it as baleage instead.
Dry matter and silage quality
Silage fermentation depends critically on DM. Wet silage (under 28 percent DM) has too much water for good fermentation — lactic acid bacteria can't drop the pH fast enough, and clostridia (bad bacteria producing butyric acid and ammonia) take over. The result is silage with a sweet-rancid smell that animals refuse and that loses 20 to 30 percent of energy through spoilage.
Dry silage (over 45 percent DM for whole-plant silage) has the opposite problem. There isn't enough free water for fermentation to get going. The pile stays too aerobic, lactic acid is limited, and the silage molds in storage. Yeasts grow, then molds, then heating and DM loss.
The sweet spot for whole-plant corn silage is 32 to 35 percent DM. Cut at the wrong stage (too green, under 30 percent; too dry, over 40 percent) and you lose feed value. Most US dairies harvest corn for silage at 65 to 70 percent moisture (30 to 35 percent DM) — typically when kernels are at half milk-line, which is a visual marker easier than measuring DM directly.
Dry matter intake (DMI) for livestock
Animals are fed by DM, not by as-fed weight. The standard DMI rates: dairy cows 3.0 to 4.5 percent of body weight per day, beef cows 2.0 to 2.5 percent, sheep 3.0 to 3.5 percent, goats 3.5 to 4.0 percent, horses 1.8 to 2.5 percent.
For a 600 kg dairy cow, DMI is 21 kg DM/day. If that comes entirely from corn silage at 33 percent DM, the as-fed weight is 21 ÷ 0.33 = 64 kg of silage. If it comes from dry hay at 88 percent DM, it's 21 ÷ 0.88 = 24 kg of hay. Same DM, vastly different physical weight in the feed bunk.
Body weight, lactation stage, ambient temperature, palatability, and forage quality all affect actual DMI. High-fiber forage limits intake (gut fill). High-energy forage allows higher intake. Cool weather raises intake; heat stress drops it sharply (10 to 30 percent reduction in summer heat).
As-fed vs dry matter basis
Two ways to express any nutrient value: as-fed (the way the feed is delivered, with water in it) or DM basis (after the water is mathematically removed). DM basis is the only fair way to compare different feeds.
Example: corn silage with 8 percent crude protein as-fed at 32 percent DM has 8 ÷ 0.32 = 25 percent protein on DM basis. Alfalfa hay with 18 percent crude protein as-fed at 88 percent DM has 18 ÷ 0.88 = 20.5 percent protein on DM basis. The silage actually has higher protein concentration on DM basis even though the as-fed number looks lower.
Comparing a hay test (88% DM) to a silage test (33% DM) on as-fed basis is meaningless. The silage looks "weaker" because most of it is water. Always convert both to DM basis before judging which is the better feed. Most feed labs report both as-fed and DM columns on the analysis sheet — use the DM column.
Buying feed by dry matter
Feed pricing is often quoted as-fed per ton, but the actual value is per ton of DM. Corn silage at $50 per ton as-fed at 30 percent DM costs $167 per ton DM. Dry hay at $200 per ton as-fed at 88 percent DM costs $227 per ton DM. The silage is the better buy on DM basis, even though it looks more expensive per visible truck-load.
Buying high-moisture feed always has hidden costs: more truck loads to deliver the same DM, more storage volume, more risk of spoilage, lower DM if rain hits before storage. A silage bunker that takes in 32 percent DM material and feeds out at 30 percent DM (after some seepage and surface spoilage) loses about 6 percent of the purchased DM before it gets to the cows.
Common dry matter mistakes
The most common mistake is treating one silage sample as representative of an entire bunker or pile. Bunker silage can vary 5 to 10 percent DM across faces — particularly between fresh, recently exposed face and older inner material. Best practice is to sample weekly during feedout and recalculate the ration.
The second mistake is using one DM value for the whole season. Forage moisture drops over storage time — silage can lose 2 to 5 percent moisture (i.e., DM goes up) over 6 to 9 months. Hay can pick up moisture from humid weather or barn conditions. Re-test every 2 to 3 months for long-stored feed.
The third mistake is microwaving samples without weight checks between rounds. The sample looks dry but is still losing water. Skipping the intermediate weighings overestimates DM by 1 to 3 percent.
- Dry matter formula = (dry weight ÷ fresh weight) × 100
- Moisture = 100 - DM%
- As-fed → DM = nutrient ÷ (DM%/100)
- Pasture DM = 18-22%
- Corn silage target = 32-35%
- Dry hay target = 85-90%
- Dairy cow DMI = 3-4% of body weight per day
- Microwave method accuracy = ±2%
- Lab method accuracy = ±0.5%