A blade of grass is mostly water held inside a frame of tough plant fiber. Grass walls are built differently from those of garden flowers and trees, with their own fiber types and cross-links. That makeup explains a lot of everyday puzzles, from why only grazing animals can live on grass to why a lawn curls up in a drought.
Water First, Then Dry Matter
Across many grass species, water makes up roughly 70 to 86 percent of a leaf’s weight.1New Phytologist. Leaf anatomy, specific mass and water content in congeneric annual and perennial grass species A kilogram of freshly cut grass therefore holds only about 150 to 300 grams of solid material. Farmers call that solid remainder dry matter: the fibers, proteins, minerals, sugars and fats left once water is removed.2Germinal Ireland. Nutritional Value of Grass Guide The wetter the grass, the more of it an animal must eat to get the same nutrients.3Germinal Ireland. Nutritional Value of Grass Guide Spring grass is often wetter than late-summer grass, so the same field can hold very different amounts of solid material across the season. Water content swings with weather, time of day and season, so feed comparisons are usually made on a dry-matter basis.
The dry part is mostly cell wall fiber, which is the largest share. Protein, sugars and starches make up a smaller share. Minerals, called ash, and fats are the smallest parts. A square meter of grass leaf weighs only about 23 to 50 grams of dry material,4New Phytologist. Leaf anatomy, specific mass and water content in congeneric annual and perennial grass species because grass leaves have fewer cell layers and a narrower shape than most broad-leaved plants.5PubMed. Allometries of cell and tissue anatomy and photosynthetic rate across leaves of C(3) and C(4) grasses
A nutrient figure quoted “as fed” includes the water, while the same figure on a dry-matter basis does not, so numbers from the two cannot be compared directly.
The Fiber Frame Cellulose Hemicellulose and Lignin
Grass cell walls are mostly cellulose, hemicellulose and lignin.6University of Pretoria. Estimation of indigestible neutral detergent fibre in forages from cell wall components Cellulose is long chains of glucose sugar bundled into strong fibers. Lignin is a woody, water-resistant compound that stiffens the wall.
Cellulose fibers act like rebar, and hemicellulose fills the space between them. Lignin and another compound, ferulic acid, add extra bonding that waterproofs the wall and locks it together. The more lignin, the stiffer the wall and the harder it is to digest.
Grass uses its own kind of filler. About 120 million years ago, grass ancestors switched to a filler built from xylose-based chains instead of the pectin-rich walls of most flowering plants.7PubMed Central. Evolution of the Cell Wall Gene Families of Grasses Grasses also make a wall fiber largely special to their family.8PubMed Central. Evolution of the Cell Wall Gene Families of Grasses Grass xylans come in two patterns, one with evenly spaced side branches and one with clustered ones,9PubMed Central. Grass xylan structural variation suggests functional specialization and distinctive interaction with cellulose and lignin so the walls are tuned for different jobs. Even branched xylan chains sit within a few billionths of a meter of cellulose fibers, partly binding them and partly filling the gaps.10PubMed. Structure and dynamics of Brachypodium primary cell wall polysaccharides from two-dimensional (13)C solid-state nuclear magnetic resonance spectroscopy Ferulic acid links lignin, wall sugars and wall proteins together,11PubMed. Ferulic acid: a key component in grass lignocellulose recalcitrance to hydrolysis so grass walls resist being broken apart, whether by microbes or by industrial processes.12PubMed. Ferulic acid: a key component in grass lignocellulose recalcitrance to hydrolysis
The wall changes with age. As grass matures, its stems build more lignin, and old stalks feel tough and stringy. Young leaves have thinner, softer walls.
Most animals and people lack the enzymes to cut the bonds linking the wall’s sugars, so that sugar passes through unused. Microbes that carry the right enzymes can open those bonds.
Glass Inside the Leaf Silica
Grasses take up dissolved silica from soil water and harden it into microscopic glass bodies called phytoliths. In sorghum, a specific protein controls this silica building inside dedicated silica cells.13PubMed Central. Phytolith Formation in Plants: From Soil to Cell In a giant bamboo, 11 shapes of silica body were found, most inside small short cells of the leaf surface.14PubMed Central. Exploring silica accumulation in bamboo leaves: a study on phytolith morphology and epidermal patterning in the tropical giant bamboo Dendrocalamus copelandii Silica builds from the leaf tip toward the base as leaves develop.15PubMed Central. Exploring silica accumulation in bamboo leaves: a study on phytolith morphology and epidermal patterning in the tropical giant bamboo Dendrocalamus copelandii
These silica bodies are why some grass edges feel rough or even cut skin. Over time they wear down the teeth of grazing animals. They also add stiffness. After the plant rots, the silica bodies can survive in soil for thousands of years.
When grass is burned, silica stays in the ash, and feed reports list it within the ash figure. High ash means less digestible material per bite. Silica is a cheap material compared with building more organic fiber, and tough, gritty leaves are less pleasant for insects and grazers to chew, so it acts partly as a defense.
The Waxy Skin and the Hinge Cells
A layer of wax on the leaf surface helps grass resist drought, cold and pests.16PubMed. Leaf Cuticular Waxes of Bermudagrass Response to Environment-Driven Adaptations of Climate Effect Inferred from Latitude and Longitude Gradient in China Creeping bentgrass carries about 2.5 times more wax than annual bluegrass and keeps more through winter,17Journal of Agronomy and Crop Science. Differential Leaf Cuticular Wax Responses to Cold Acclimation in Annual Bluegrass (Poa annua L.) and Creeping Bentgrass (Agrostis stolonifera L.) so more wax may help some turf survive winter.18Journal of Agronomy and Crop Science. Differential Leaf Cuticular Wax Responses to Cold Acclimation in Annual Bluegrass (Poa annua L.) and Creeping Bentgrass (Agrostis stolonifera L.) In ryegrass, the underside lacks a wax compound that powdery mildew spores need to start growing.19PubMed. Two sides of a leaf blade: Blumeria graminis needs chemical cues in cuticular waxes of Lolium perenne for germination and differentiation
Hinge cells are large cells mostly on a leaf’s upper surface.20PubMed. Bulliform cells: anatomical, physiological, genetic, and computational perspectives on plants’ drought adaptation When the plant is well supplied, they hold a lot of water, which keeps the leaf flat and open. In drought they shrink more than neighbouring cells,21PubMed Central. Structure‐function analysis of the maize bulliform cell cuticle and its potential role in dehydration and leaf rolling pulling the two edges of the blade toward each other. The leaf rolls up, exposing less surface to sun and wind and losing less water.22PubMed. Bulliform cells: anatomical, physiological, genetic, and computational perspectives on plants’ drought adaptation Plants with more or bigger hinge cells roll more strongly.23PubMed Central. Rolling Leaf 2 Controls Leaf Rolling by Regulating Adaxial-Side Bulliform Cell Number and Size in Rice
You can read this on a lawn. Blades folding into thin needles signal the grass is short of water, and footprints that stay pressed down are another sign. Watering usually lets leaves unroll within hours. The curling is protection, not death.
Protein Sugar and Fat in Grass
The main protein in grass leaves is Rubisco, the enzyme that captures carbon dioxide. Protein pulled from ryegrass is mostly Rubisco and contains all the essential amino acids people need.24Food Hydrocolloids. Impact of the extraction method on the composition and solubility of leaf protein concentrates from perennial ryegrass (Lolium perenne L.) In some warm-season grasses, about 30 percent of leaf nitrogen sits in the light-capturing membranes.25PubMed Central. Faster Rubisco Is the Key to Superior Nitrogen-Use Efficiency in NADP-Malic Enzyme Relative to NAD-Malic Enzyme C4 Grasses A growing leaf takes most of its nitrogen at its base, where new cells form,26PubMed Central. Nitrogen Use within the Growing Leaf Blade of Tall Fescue and older parts then reuse that nitrogen to build the main leaf protein.27PubMed Central. Nitrogen Use within the Growing Leaf Blade of Tall Fescue
Rubisco works rather slowly, so leaves hold a great deal of it. That is why young, leafy grass has a higher protein content than stemmy mature grass.
People can’t live on grass protein, though. It is locked in tough walls, and our gut cannot break enough wall to release it. Extraction is needed to make it usable.
Grass stores energy in different forms depending on type. Cool-season grasses, like ryegrass and fescue, grow in spring and autumn and store energy mainly as fructans, chains of fructose sugar.28PubMed Central. Effects of water-soluble carbohydrates from grasses on the nutrition of grazing ruminants Warm-season grasses, like Bermuda grass, thrive in summer heat and store mostly starch instead.29PubMed Central. Effects of water-soluble carbohydrates from grasses on the nutrition of grazing ruminants Tropical grasses mainly hold free glucose and starch.30Grass and Forage Science. Diversity of non‐structural carbohydrates in grasses (Poaceae) from Brazil Horse owners watch spring pasture because cool-season grass then can be rich in fructans.
Grass also carries fat. Grass chloroplasts are rich in alpha-linolenic acid, an omega-3 fat,31University of Nottingham Repository. Nutrient composition and digestibility of chloroplast rich fractions from green leaf materials which helps explain the omega-3 in grass-fed milk and meat.
Why Cows Can Digest Grass and People Cannot
Cows and sheep host microbes in their stomach that break down cellulose. These animals have a large first stomach chamber where these microbes ferment plant fiber while the feed stays for many hours. The microbes produce acids from the fiber, and the animal absorbs these and uses them as its main energy source. Further along, the animal digests the microbes themselves, which gives it extra protein.
Even so, part of the fiber is indigestible for those microbes,32University of Pretoria. Estimation of indigestible neutral detergent fibre in forages from cell wall components which limits how much energy the animal gets.33University of Pretoria. Estimation of indigestible neutral detergent fibre in forages from cell wall components How lignin links to other wall parts affects how fast fiber digests.34PubMed. Effect of lignin linkages with other plant cell wall components on in vitro and in vivo neutral detergent fiber digestibility and rate of digestion of grass forages
A farm feed test shows this. Reports list dry matter, crude protein, ash and fiber. NDF measures total wall fiber, while ADF and lignin show the toughest parts. Lower fiber usually means more energy. Hay cut late feeds animals less, because mature grass has more stem and lignin.
Why Grass Turns Purple
Cold air with bright sun in autumn, late winter and early spring can turn turf leaves purple.35turf.umn.edu. What Is Grass Made Of? From Structure to Molecules The leaf builds anthocyanins, purple pigments,36turf.umn.edu. What Is Grass Made Of? From Structure to Molecules which absorb excess light and protect photosynthesis.37PubMed Central. Combination of abiotic stresses accelerates photoprotection in exceptionally chilling-tolerant C4 grass by fine-tuning the content of multiple pigments Some bentgrass varieties purple strongly at 50°F under bright light while others barely change.38turf.umn.edu. What Is Grass Made Of? From Structure to Molecules
In cold weather photosynthesis slows, so the leaf takes in more light than it can use, and the plant builds pigment to absorb part of it. The pigment is made from the sugars the leaf has on hand.
Patchy purple on frosty sunny mornings is usually harmless, and the colour fades as weather warms. Purple in warm weather can have other causes, including a nutrient shortage, and may be worth a closer look.
Alkaloids From Hidden Fungi
Some grasses carry fungi living inside their tissues.39PubMed. Contribution of fungal loline alkaloids to protection from aphids in a grass-endophyte mutualism These fungi make alkaloids that protect the grass from insects like aphids.40PubMed. Contribution of fungal loline alkaloids to protection from aphids in a grass-endophyte mutualism
Some turf seed is sold as endophyte-enhanced for pest resistance. The fungus is invisible and does not harm the grass. Some endophyte chemicals can affect grazing livestock, so pasture seed may be sold as endophyte-free or as a safe type.
What Happens When Grass Breaks Down
Grass leaf litter breaks down faster than roots, which decay up to several times more slowly.41PubMed. Intrinsic effects of species on leaf litter and root decomposition: a comparison of temperate grasses from North and South America Roots contain tough suberin and lignin.42AoB Plants. Warming and elevated CO2 alter the suberin chemistry in roots of photosynthetically divergent grass species Material with much lignin and little nitrogen rots most slowly,43Soil Biology and Biochemistry. The initial lignin:nitrogen ratio of litter from above and below ground sources strongly and negatively influenced decay rates of slowly decomposing litter carbon pools so tough old grass stores carbon in soil longer.
Soil microbes feed on the easy parts first, such as sugars and protein, so fresh clippings lose mass quickly. Lignin lasts longest and becomes stable organic matter in the soil.
In a garden, fresh green clippings are nitrogen-rich and break down quickly, while dry brown stalks rot slowly. Mixing them balances a compost pile. Leaving clippings on the lawn returns nutrients.
Grass is also a raw material. Its cellulose and xylose can be turned into bioplastics, with ferulic acid and lignin separated as products.