Livestock Nutrition Guide

Dry Matter Intake (DMI)

Dry matter intake is the most fundamental concept in livestock nutrition. It represents the total amount of feed an animal consumes after removing all water content. Every feed contains some percentage of moisture: hay is approximately 12% moisture (88% dry matter), fresh pasture is 75% moisture (25% dry matter), and corn silage is 65% moisture (35% dry matter). The dry matter portion contains all the protein, energy, vitamins, and minerals that the animal needs for maintenance, growth, and production.

DMI is expressed as a percentage of body weight and varies by species, physiological state, and environmental conditions. The table below summarizes typical DMI percentages for common livestock species at maintenance level. These values represent the amount of dry matter, not as-fed feed, that the animal will voluntarily consume when given free access to feed of adequate quality.

Several factors affect actual DMI beyond the base percentage. Feed quality is the most important: animals eat more of highly digestible, palatable feeds and less of low-quality, stemmy forages. Forage particle size matters because finely chopped or ground feed passes through the digestive system faster, allowing higher intake but potentially reducing fiber digestion. Environmental temperature affects intake: cold stress increases energy needs and can increase DMI by 10-20%, while heat stress depresses appetite and can reduce DMI by 10-30%.

The conversion from dry matter to as-fed feed is critical for practical feeding. To calculate the as-fed amount, divide the required dry matter by the feed's dry matter percentage (as a decimal). For example, if a cow needs 24 lbs of dry matter and you are feeding hay at 88% DM, the as-fed amount is 24 / 0.88 = 27.3 lbs of hay. If you are supplementing with corn silage at 35% DM, the same 24 lbs of dry matter would require 24 / 0.35 = 68.6 lbs of silage. This conversion explains why animals consuming wet feeds eat much larger volumes than those consuming dry feeds, even though the dry matter intake is the same.

Crude Protein Requirements

Crude protein (CP) is a measure of the total nitrogen content of a feed, multiplied by the factor 6.25 (since protein averages 16% nitrogen). Crude protein includes both true protein (amino acids) and non-protein nitrogen (NPN) compounds such as urea and ammonia. For ruminants, this distinction matters less because rumen microorganisms can convert NPN into microbial protein. For monogastric animals (pigs, poultry), true protein and specific amino acid profiles are more important than crude protein alone.

Protein requirements vary dramatically across species and life stages. The general pattern is that young, growing animals and lactating females have the highest protein requirements, while mature animals at maintenance have the lowest. Turkey poults require 28% CP, the highest of any common livestock species, while donkeys at maintenance need only 6-8% CP. Dairy cows in peak lactation need 16-18% CP to sustain milk protein synthesis, while dry beef cows maintain body condition on 7% CP.

Protein deficiency is one of the most common and costly nutritional problems in livestock production. Symptoms include poor growth rates, reduced milk production, weak immune function, poor reproductive performance, rough hair coat, and loss of body condition. These signs develop gradually and may not be obvious until production losses are significant. Conversely, feeding excess protein above requirements is wasteful because the animal converts the excess nitrogen to urea, which is excreted in urine. This wastes money on expensive protein supplements and increases nitrogen pollution in manure and runoff.

For ruminants, protein nutrition involves two components: ruminally degradable protein (RDP), which is broken down by rumen microbes and used to produce microbial protein, and ruminally undegradable protein (RUP or bypass protein), which passes through the rumen intact and is digested in the small intestine. High-producing dairy cows and fast-growing cattle benefit from feeds with higher RUP content (such as distillers grains, fish meal, and heat-treated soybean meal) because their protein needs exceed what rumen microbes can supply. For most other ruminants at maintenance or moderate production levels, RDP from common forages and grain supplements provides adequate protein nutrition.

Total Digestible Nutrients (TDN)

Total digestible nutrients (TDN) is the traditional measure of feed energy content used in livestock nutrition. TDN is calculated as the sum of digestible crude protein, digestible crude fiber, digestible nitrogen-free extract, and 2.25 times the digestible ether extract (fat). The 2.25 multiplier for fat reflects the higher caloric density of fat (9 kcal/gram) compared to carbohydrates and protein (4 kcal/gram each). TDN is expressed as a percentage of feed dry matter, with most feeds ranging from 45% to 90% TDN.

Feed energy content drives animal productivity. An animal that receives adequate energy will maintain body weight, support milk production, grow at expected rates, and reproduce successfully. Energy deficiency causes weight loss, reduced milk production, poor growth, anestrus (failure to cycle), and increased susceptibility to disease. Energy is typically the first-limiting nutrient in forage-based diets because most forages provide 50-60% TDN, which meets maintenance requirements but falls short of the 65-80% TDN needed for lactation, growth, or high-performance activities.

Alternative energy measures used in modern livestock nutrition include net energy (NE), metabolizable energy (ME), and digestible energy (DE). These measures provide more precise estimates of the energy actually available to the animal for maintenance, growth, and lactation. However, TDN remains the most widely used energy measure in practical livestock feeding because it is straightforward to calculate, widely reported on feed test results, and familiar to producers and feed dealers. The relationship between TDN and other energy measures is approximately: 1 lb TDN = 2,000 kcal DE = 1,640 kcal ME. For dairy nutrition, the NE system is preferred because it accounts for the energy costs of maintenance, lactation, and growth separately.

Matching energy supply to energy demand is the core challenge of livestock ration formulation. When forages alone do not provide adequate TDN, energy supplementation with grains (corn, barley, oats), byproducts (distillers grains, beet pulp), or fat supplements is necessary. The rate of grain supplementation must be balanced against the risk of digestive disturbances in ruminants, where excessive starch from grain reduces rumen pH and can cause subacute or acute ruminal acidosis. A common rule of thumb is that grain should not exceed 50-60% of total diet dry matter for cattle, and any increase should occur gradually over 2-3 weeks.

Mineral Requirements

Minerals are inorganic elements essential for bone formation, enzyme function, fluid balance, nerve transmission, and many other biological processes. Livestock require seven macro-minerals (calcium, phosphorus, magnesium, potassium, sodium, chloride, sulfur) in relatively large amounts and at least nine trace minerals (zinc, copper, manganese, selenium, iodine, cobalt, iron, molybdenum, chromium) in much smaller quantities. Mineral deficiencies and imbalances are among the most common nutritional problems in livestock, yet they are also among the easiest and least expensive to prevent.

Calcium and phosphorus are the most important macro-minerals in livestock nutrition because they are the primary structural components of bone and teeth, and calcium plays critical roles in muscle contraction, blood clotting, and enzyme activation. The calcium-to-phosphorus ratio in the diet should be maintained between 1.2:1 and 2:1 for most species. Laying hens are the exception, requiring a 6:1 or higher Ca:P ratio to support eggshell formation (2 grams of calcium per egg). Phosphorus deficiency causes poor growth, weak bones, reduced fertility, and pica (craving for abnormal substances like soil, bones, or wood).

Trace mineral requirements and tolerances vary dramatically between species. The most important species-specific consideration is copper: sheep are extremely sensitive to copper toxicity and must never receive mineral supplements formulated for cattle or goats, which contain copper levels that are safe for those species but lethal for sheep. Goats, conversely, require higher copper levels than cattle and may develop copper deficiency on cattle mineral supplements, particularly in areas with high molybdenum in the soil which interferes with copper absorption. Selenium requirements and toxicity levels also vary regionally, with selenium-deficient soils in the Pacific Northwest and Great Lakes regions requiring supplementation, while selenium-toxic soils in parts of the Great Plains can cause selenium poisoning.

The most practical approach to mineral supplementation is providing a species-appropriate, free-choice loose mineral supplement year-round. Loose minerals are consumed more consistently than mineral blocks, particularly in cold weather when tongues can freeze to metal or hard salt surfaces. Mineral consumption should be monitored by tracking the amount consumed per head per day. Cattle typically consume 2-4 ounces per day, horses 1-2 ounces, and sheep and goats 0.5-1 ounce. If consumption is significantly above or below these ranges, the mineral formulation, salt content, or palatability should be evaluated.

Water Requirements

Water is the most essential and often most overlooked nutrient in livestock production. Animals can survive much longer without food than without water, and even mild dehydration (2-4% of body weight) significantly reduces feed intake, digestion efficiency, and productivity. Water requirements are directly proportional to dry matter intake, body weight, ambient temperature, and production level. As a general rule, livestock consume 2-4 pounds of water per pound of dry matter consumed, with higher ratios during hot weather and for lactating animals.

Cattle consume 8-20 gallons per day depending on size, temperature, and lactation status. A lactating dairy cow producing 80 lbs of milk daily may drink 30-50 gallons of water. Horses drink 5-15 gallons per day, with hard-working horses in hot weather requiring up to 20 gallons. Sheep and goats drink 1-4 gallons per day. Poultry consume approximately twice as much water by weight as feed. During summer heat with temperatures above 90 degrees F, water consumption increases 50-100% in all species.

Water quality affects both consumption and animal health. Total dissolved solids (TDS) above 3,000 ppm may reduce consumption in cattle, and levels above 7,000 ppm can be toxic. Sulfate levels above 500 ppm cause loose stools and may contribute to polioencephalomalacia, particularly in cattle consuming diets with elevated sulfur from distillers grains. Nitrate contamination from agricultural runoff or shallow wells can cause methemoglobinemia, reducing the blood's ability to carry oxygen. Testing water quality annually is recommended for all livestock operations, particularly those using well water or surface water sources.

Body Condition Scoring

Body condition scoring (BCS) is the most practical tool for evaluating whether a feeding program is meeting an animal's nutritional needs. BCS is a visual and tactile assessment of the fat reserves over specific anatomical landmarks, scored on a numerical scale. Cattle and horses use a 1-9 scale (1 = emaciated, 9 = obese), while sheep, goats, and camelids use a 1-5 scale. The target BCS varies by species and production stage but generally aims for moderate condition with adequate reserves to buffer against stress periods.

For beef cattle, the target BCS is 5-6 at calving, as cows below BCS 4 have significantly reduced rebreeding rates. Each unit of BCS in cattle represents approximately 75-100 lbs of body weight change. Dairy cows should calve at BCS 3.25-3.75 (5-point scale) and lose no more than 0.5 units during early lactation. Horses should maintain BCS 5-6, with ribs easily felt but not visible. Sheep should enter lambing at BCS 3.0-3.5 (5-point scale) to support lamb survival and milk production.

Regular BCS assessment, ideally monthly, provides early warning of nutritional deficiency or excess before production losses occur. A downward trend in BCS indicates that feed quality or quantity is insufficient for the animal's production level and should prompt a ration review, forage testing, or increased supplementation. An upward trend in BCS during periods when weight gain is undesirable (such as late pregnancy in easy keepers) indicates overfeeding and the need to restrict energy intake. Seasonal BCS targets and feeding adjustments are a key component of profitable livestock management.

Forage Testing

Laboratory forage testing is the single most cost-effective investment in livestock nutrition management. A forage test costs $15-$30 per sample and provides accurate measurements of dry matter, crude protein, acid detergent fiber (ADF), neutral detergent fiber (NDF), TDN, net energy values, and mineral content for your specific hay or silage. The book values used in feed calculators (including this one) represent averages that may differ substantially from your actual forage quality.

Hay quality varies enormously based on plant species, maturity at harvest, weather during curing, and storage conditions. Early-cut grass hay may test at 14% CP and 60% TDN, while late-cut hay from the same field may test at 6% CP and 48% TDN. This difference means that a feeding program based on average book values could be either overfeeding protein (wasting money) or underfeeding protein (losing production), depending on the actual hay quality. Only a laboratory test reveals the true nutritional value.

To collect a representative forage sample, use a hay probe or corer to sample at least 12-20 bales from a single lot of hay (same field, same cutting, same storage conditions). Combine the cores into a single sample, mix thoroughly, and send to a certified forage testing laboratory. For silage, collect samples from multiple locations in the bunker or bag face, mix, and submit promptly in a sealed bag to prevent moisture loss. Most state extension services maintain lists of certified labs and provide assistance in interpreting results.

Key values to look for on a forage test report include: crude protein (CP) on a dry matter basis, which directly indicates the protein contribution of the forage; TDN or relative feed value (RFV), which indicates energy content; ADF, which is inversely related to digestibility (lower is better); NDF, which is inversely related to intake potential (lower means higher voluntary intake); and mineral content, particularly calcium, phosphorus, potassium, and magnesium. Armed with these values, you can use the calculator to formulate a precise, cost-effective feeding program that meets your animals' requirements without waste.

Further Reading

Explore feed requirements for specific livestock species or compare nutritional profiles of different feed types to build a balanced feeding program for your operation.