Grass holds a surprisingly full menu: sugars, protein, fats, minerals and vitamins, though most of its weight is water and tough fiber. That menu is only open to animals with the right digestive system, which is why a cow thrives on what a person can’t live on. The same lawn or pasture can also be rich in spring and poor by late summer, so grass is less a fixed food than a moving target.
The Main Building Blocks of Grass
Fresh grass is mostly water, with only about 12 to 30% dry matter.1Animal Feed Science and Technology. Dry matter intake and eating rate of grass by dairy cows is restricted by internal, but not external water That matters at the feed bunk: a cow eats more dry feed as the grass gets drier inside, about 134 g more per percentage point of dry matter.2Animal Feed Science and Technology. Dry matter intake and eating rate of grass by dairy cows is restricted by internal, but not external water The dry part is largely cell walls made of cellulose and related fibers.3PubMed. Fractional and structural characterization of hemicelluloses from perennial ryegrass (Lolium perenne) and cocksfoot grass (Dactylis glomerata) These fibers are built from plant sugars such as xylose and glucose locked into long chains.4PubMed. Fractional and structural characterization of hemicelluloses from perennial ryegrass (Lolium perenne) and cocksfoot grass (Dactylis glomerata)
This is why nutrient figures are usually given per dry weight, not fresh weight. A lab report usually lists each nutrient twice, “as fed” and “dry matter basis,” and only the dry matter column lets you compare two forages fairly. Because fresh grass is so watery, an animal may need to eat a lot of it to match a smaller amount of hay.
Grasses also fall into two families. Cool-season grasses such as ryegrass, timothy, fescue and orchardgrass grow best in spring and fall. Warm-season grasses such as bermudagrass grow best in summer heat and carry more fiber and lignin, the woody material that resists digestion.5PubMed Central. Comparative Effects of C3 and C4 Forages on Growth Performance, Digestibility, and Nitrogen Balance in Korean Crossbred Black Goats So cool-season grasses generally offer more protein, sugars and water and less toughness.6Global Change Biology. C3 grasses have higher nutritional quality than C4 grasses under ambient and elevated atmospheric CO2 Many pastures mix both families, so the nutrition shifts as each takes over.
Sugars and Stored Energy
Cool-season grasses store energy in sugars called fructans, alongside glucose, fructose, sucrose and starch.7Mad Barn USA. Fructans in Horse Forages: Do High-Fructan Grasses Cause Laminitis? In these grasses, fructans make up about 38% of the stored carbohydrates, starch 23% and simple sugars the rest.8Agronomy Journal. Interspecies Variation of Forage Nutritive Value and Nonstructural Carbohydrates in Perennial Cool‐Season Grasses In perennial ryegrass, fructans make up over half the water-soluble sugars.9Crop Science. Seasonal Trends in Nonstructural Carbohydrates in Cool‐ and Warm‐season Grasses Warm-season grasses carry consistently less sugar and fructan.10Crop Science. Seasonal Trends in Nonstructural Carbohydrates in Cool‐ and Warm‐season Grasses
That is why horse owners watch grass sugars. Sugar-rich spring grass is linked with laminitis worries, and ryegrass, with its high fructan share, is the grass most often flagged. Warm-season pasture is often suggested for sugar-sensitive horses, but a horse with a laminitis history should get a vet’s opinion before any change in pasture.
Sugar levels also move through the day and year. Grass builds sugars in sunlight and burns them overnight, so the same field differs between afternoon and dawn. In cool-season grasses, sampling date strongly changes sugar content. Stress such as cold or drought tends to leave the stored sugars in place, because the plant isn’t spending them on growth.
Protein and Fat in Grass
Hay meadow grass ranges from about 110 to 170 g of protein per kg of dry matter, roughly 11 to 17%.11Grass and Forage Science. Long‐term effects of fertilizer on soil nutrient concentration, yield, forage quality and floristic composition of a hay meadow in the Eifel mountains, Germany More nitrogen fertilizer steadily raises grass protein.12Industrial Crops and Products. Green biorefining: Effect of nitrogen fertilization on protein yield, protein extractability and amino acid composition of tall fescue biomass Grass protein is also higher in leucine and lower in glutamic acid than soybean meal.13Industrial Crops and Products. Green biorefining: Effect of nitrogen fertilization on protein yield, protein extractability and amino acid composition of tall fescue biomass
The “crude protein” on a forage test is estimated from nitrogen, not measured directly. Nitrogen from non-protein sources inflates the figure, and standard protein figures can overstate true protein, in one grass by about double.14Botany. Amino acid composition, protein content and accurate nitrogen-to-protein conversion factor for sheepgrass (Leymus chinensis) Use crude protein to compare forages, not as an exact protein count.
Most of the protein sits in leaves. Stemmy grass has less protein per bite, and animals selectively graze leaves when they can, so what they eat is often better than a whole-plant sample suggests.
Grass fat is small in amount, but grasses are among the richest forages in alpha-linolenic acid, an omega-3.15Agronomy. Variation in Fatty Acids Concentration in Grasses, Legumes, and Forbs in the Allegheny Plateau Grasses carry more of it than alfalfa.16Crop Science. Fatty Acid Profiles of Orchardgrass, Tall Fescue, Perennial Ryegrass, and Alfalfa Perennial ryegrass tends to be highest, though omega-3 swings more by season than omega-6.17Agronomy. Variation of Fatty Acids in Cool-Season Grasses
Minerals in Grass and Common Shortfalls
- Zinc: the most often deficient; in a US survey 63% of forages had under 20 ppm and only 2.5% reached the adequate 40 ppm.18Kansas Agricultural Experiment Station Research Reports. National forage survey results: trace mineral and related nutrient levels
- Selenium: 44% of samples were deficient, though some regions have excess.19Kansas Agricultural Experiment Station Research Reports. National forage survey results: trace mineral and related nutrient levels
- Copper: usually 5 to 8 ppm, adequate unless iron or molybdenum block it; 58% had enough molybdenum to do that.20Kansas Agricultural Experiment Station Research Reports. National forage survey results: trace mineral and related nutrient levels
- Manganese: often plentiful, around 85 to 140 ppm in southern grasses.21Journal of Dairy Science. Seasonal Changes of Mineral Content of Southern Forages
Those survey figures show what is common across the US, not what your pasture contains. Zinc, selenium and copper are needed in tiny amounts, yet shortfalls can show up as poor growth, weak fertility or a dull, rough coat, and those signs overlap with many other problems. A forage sample sent to a lab shows what your pasture lacks, and your vet can say whether blood or liver tests make sense.
Free-choice mineral blocks or loose minerals can help cover common gaps. Choose one formulated for your species and region. Sheep tolerate little copper, so avoid cattle minerals for them.
Magnesium is a different kind of problem. Grass tetany is a sometimes fatal drop in blood magnesium in grazing cattle.22European Journal of Agronomy. Potential decrease of grass tetany risk in rangelands combining N and K fertilization with MgO treatments Pastures linked to it carry less magnesium and sodium and more potassium.23The Journal of Agricultural Science. The mineral element content of spring pasture in relation to the occurrence of grass tetany and hypomagnesaemia in dairy cows Risk rises when potassium divided by calcium plus magnesium reaches 2.2 or more.24European Journal of Agronomy. Potential decrease of grass tetany risk in rangelands combining N and K fertilization with MgO treatments Applying nitrogen and potassium fertilizer together pushed grass over that line, while adding magnesium fertilizer did not raise grass magnesium.25European Journal of Agronomy. Potential decrease of grass tetany risk in rangelands combining N and K fertilization with MgO treatments Magnesium supplements for the animals are the usual safeguard on lush spring pasture.
Vitamins and Plant Pigments
Grass holds several milligrams of chlorophyll per gram of dry matter, more on fertile soil.26Forests. Quantification of Chlorophyll and Carotene Pigments Content in Mountain Melick (Melica nutans L.) in Relation to Edaphic Variables It also holds beta-carotene, about 0.6 mg per gram of dry matter in one wild grass.27Forests. Quantification of Chlorophyll and Carotene Pigments Content in Mountain Melick (Melica nutans L.) in Relation to Edaphic Variables Animals turn beta-carotene into vitamin A, and fresh green grass supplies vitamin E as well.
Deep green leaves mean more pigment, while bleached or brown hay has lost much of its carotene. Animals on winter hay may therefore need vitamin A and E support.
How Age and Season Change Grass Nutrition
As grass matures it grows more stem and less leaf protein.28Animal Feed Science and Technology. Effect of maturation and initial harvest dates on the nutritive characteristics of ryegrass (Lolium perenne L.) From May to August, protein and digestibility decline steadily.29Canadian Journal of Plant Science. Comparative trends in forage nutritional quality across the growing season in thirteen grasses In very mature range grass, protein can fall to about 4% as lignin builds up.30Open Journal of Animal Sciences. Effect of Stages of Maturity on Nutritive Value of Some Range Herbage Species in Low-Rainfall Woodland Savanna Southern Darfur, Sudan Warmer temperatures raise fiber further, by about 13% per 1 °C in one report.31PubMed Central. Comparative Effects of C3 and C4 Forages on Growth Performance, Digestibility, and Nitrogen Balance in Korean Crossbred Black Goats
Stems carry far more fiber and lignin than leaves do, so leaves make up a smaller share of each mouthful and average protein drops. Lignin cannot be fermented, so an animal can eat plenty of old grass and still take in little useful energy.
In practice, graze or cut before seed heads form for the best protein. Rotating pastures keeps grass in its leafy stage, and late-summer pasture may need supplemental protein.
Feeding soil beyond what grass needs makes it take up extra nitrogen, phosphorus, potassium and calcium.32Journal of Dairy Science. Effect of Soil Fertility upon the Yield and Nutritive Value of Forages. A Review Even so, fertilizer explains only about a fifth of the year-to-year differences in quality, less than weather and season.33Agriculture, Ecosystems and Environment. Relationships between long-term fertilization management and forage nutritive value in grasslands
Why Animals Digest Grass and People Don’t
Grazing animals rely on an enzyme from gut microbes to free sugars from cellulose.34The Times of India. Why humans can’t digest grass: Understanding the biological limitations Humans lack it, so most grass cellulose passes through undigested.35The Times of India. Why humans can’t digest grass: Understanding the biological limitations Softer forms of cellulose can be partly digested in the human gut,36Biological Reviews. THE RGLE OF CELLULOSE IN NUTRITION but grass works for people only as fiber, not as an energy food.
Animals differ in where the microbes work. Cows, sheep and goats ferment grass in a multi-chambered stomach before it reaches the intestine, while horses ferment it in the hindgut instead. Both depend on their microbes, which is why diet changes must be gradual.
Chewing a few blades is harmless, since they pass through, but it isn’t food. Silica in grass wears teeth, which herbivores constantly regrow or replace. Lawn grass may also carry chemicals, so don’t eat treated lawns.
Natural Toxins and Fungal Risks
Grasses have relatively few built-in toxins.37PubMed. Endogenous toxins and mycotoxins in forage grasses and their effects on livestock Sorghums can form cyanide-releasing compounds, and Phalaris grasses carry alkaloids.38PubMed. Endogenous toxins and mycotoxins in forage grasses and their effects on livestock Fungi living inside tall fescue make ergot alkaloids, and in ryegrass they make tremor-causing toxins.39PubMed. Endogenous toxins and mycotoxins in forage grasses and their effects on livestock When reseeding, ask suppliers for low-toxin or “novel” endophyte varieties.
Ryegrass staggers shows as trembling and an unsteady gait. Fescue problems include poor heat tolerance and lameness. If you see these signs, move the animals off the pasture and call a vet.
Hay and Silage Nutrient Losses
Silage and barn-dried hay preserve far more dry matter and protein than hay dried in the field.40Grass and Forage Science. A REVIEW OF NUTRIENT LOSSES AND EFFICIENCY OF CONSERVING HERBAGE AS SILAGE, BARN‐DRIED HAY AND FIELD‐CURED HAY Once grass is over about 60% dry, each turning shakes off much more leaf.41Grass and Forage Science. Susceptibility to losses during mechanical silage and haymaking operations in relation to grass dry matter content Stored hay also quietly loses carotene and digestible nutrients beyond the visible changes.42Journal of Dairy Science. The Loss of Nutrients in Hay and Meadow Crop Silage during Storage Turn hay while it is still damp, and store it dry and covered.
Green color and a sweet smell suggest good keeping. Dusty, brown or moldy hay has lost value, and a lab test settles any doubts.
Wheatgrass and Barley Grass for People
Wheatgrass and barley grass are the young sprouted leaves of wheat and barley.43PubMed. Assessment of biochemical compounds and antioxidant enzyme activity in barley and wheatgrass under water-deficit condition Dried barley grass is about 30% fiber and 27% protein.44PubMed Central. Preventive and Therapeutic Role of Functional Ingredients of Barley Grass for Chronic Diseases in Human Beings It carries about 252 mg vitamin C and 3,384 mg potassium per 100 g.45PubMed Central. Preventive and Therapeutic Role of Functional Ingredients of Barley Grass for Chronic Diseases in Human Beings Both grasses are rich in plant antioxidants, with comparisons differing on which is higher.46International Journal of Food Properties. Exploring the phytochemical profile of green grasses with special reference to antioxidant properties
Those figures describe the dried powder, and a typical serving is only a few grams, so actual intake is modest; no one eats 100 g at a time. The fiber is mostly the same tough material found in grass for animals, which is why people take it as juice or powder instead of chewing blades.
Health claims rest mostly on lab measures, not proven outcomes. People with kidney disease should check with their doctor first, since the powder is high in potassium.
How Grass Shows Up in Grass-Fed Meat and Milk
Grass-fed beef has more omega-3 and CLA per gram of fat.47PubMed Central. A review of fatty acid profiles and antioxidant content in grass-fed and grain-fed beef It also has more stearic fat and less palmitic and myristic, which raise cholesterol.48PubMed Central. A review of fatty acid profiles and antioxidant content in grass-fed and grain-fed beef Animals grazing varied pastures pass more plant antioxidants into meat and milk,49Frontiers in Sustainable Food Systems. Health-Promoting Phytonutrients Are Higher in Grass-Fed Meat and Milk so herb-rich pastures add more than plain grass.50animal. Milk and meat fatty acids from sheep fed a plantain–chicory mixture or a grass-based permanent sward
In ruminants, gut microbes alter many dietary fats, so the fat in meat is a changed version of what the animal ate. Diet shifts the profile gradually, which is why the length of time an animal spends on grass matters.
For shoppers, the differences are per gram of fat, and lean beef has little fat. “Grass-finished” means the animal ate grass to the end, while “grass-fed” alone may not, and taste can vary.