By Robert Fears
Successful beef cattle producers realize that the ranch is only as good as the forage base, primarily grass. They often state that they are grass farmers first and cattle producers second. The art of grass-farming requires a deep understanding of how grass grows and how it is affected by grazing. It has been a long time since most of us have sat through a botany class, so maybe it is time for a review.
Information in the following review was primarily taken from the paper “How Pasture Plants Grow.” The paper was prepared by Stephen Barnhart, emeritus, Iowa State University Extension, and addresses the growth of perennial grass.
Grass structure is similar among many species and in bunch grasses, it is primarily a collection of tillers or shoots that grow from buds at the base of the plant. The term “shoots” is often confused with “stems.”
Shoots are the new fresh growth composed of a series of repeating units consisting of a leaf, stem node and stem internode. Each leaf is attached to the stem at a node.
Early in the development of a grass plant, the internodes are very short, and the stem remains compact at the plant base. The stem elongates, and new leaves are formed from its tip where the growing point is located. As long as the growing point remains intact, the plant is capable of producing new leaves.
Later, in tiller development, the growing point stops producing leaves and begins to form a seedhead. Once this transition occurs, some of the upper internodes start to elongate and eventually raise the seedhead to the top of the tiller. New tillers emerge from the plant crown to form regrowth.
Grass develops through a sequence of three primary growth stages — vegetative, elongation and reproductive. Producers need to have the ability to recognize these stages to manage grazing properly.
During the vegetative period, leaf growth occurs. The stem and its growing point remain near the soil line. Once a critical number of leaves has formed on a tiller, the older and lowermost leaves will generally die at approximately the same rate of new leaf growth. This results in the number of leaves on a tiller remaining relatively constant.
Elongation is the stage during which stem internodes lengthen and for this reason, it is often called the jointing stage. The elongation stage is initiated in response to changes in day length. During this stage, only the upper internodes elongate while the lower internodes remain at the plant base. The lower nodes, internodes, dormant buds, and related tillers form the plant crown.
When the developing seedhead begins to push through the uppermost leaf sheath, the plant is in the boot stage, which signals the end of elongation. The reproductive stage is the period when the seedhead develops, pollination occurs, and seed is formed.
Manage grazing to promote plant growth
In grazing management, it is important to know which plants are warm-season grasses and which are cool-season grasses.
The growth rates of warm-season grasses are greatest during the warmer days of the growing season, whereas cool-season grasses grow more during the cooler days, normally in early spring and late fall.
Examples of warm-season grasses are switchgrass, big and little bluestem, indiangrass and Sudan grass.
Cool-season plants include Texas wintergrass, annual ryegrass, cereal grasses, and annual brome. Even though grass has somewhat predictive growth habits, its production is governed throughout the season by the type of plant, weather, and soil conditions.
In the spring, grass leaves grow from an active growing point near the soil surface. Proper grazing will remove only leaf tips without interfering with growing point activity. When changes in day length and temperature cause seed stem elongation, the growing point is elevated and is subject to removal by grazing or harvest. If the active growing point is removed, new tiller leaf growth develops from dormant basal buds.
Plants absorb solar energy from sunlight through their leaves and convert it to usable carbohydrates in a process called photosynthesis. If grazing animals remove only a small amount of the active green leaf area, photosynthesis can proceed, and the grass plant replenishes carbohydrate reserves while top and root growth continues.
If grazing animals remove most of the leaf area every few days, the plant allocates almost all of its energy to new leaf growth. The root system diminishes and less energy (carbohydrates) is stored in the root system. Frequent leaf removal without adequate time for the plant to restore its vigor is the physiological basis of overgrazing. Overgrazed pastures produce far below their potential because of a low stand density and poor vigor.
Warm-season grasses grow very slowly during the cool months of spring and fall but recover quickly following four to six weeks of rest during their ideal summer growing period. It is possible to shorten the rest or recovery period by leaving a taller leaf area following grazing. More residual leaf area can contribute photosynthesis energy quickly, supplementing stored carbohydrate reserves to aid in a faster recovery. Recommendations are to leave about four to eight inches of leaf area on warm season grasses following grazing.
The amount of rest required by a grazed plant to recover its vigor and regrow an effective leaf area is influenced by the period of use during the growing season and amount of remaining active leaf area.
A cool-season grass can recover in two to three weeks during its ideal spring and fall growing periods but may require six weeks or more to recover during the more stressful months of July and August. Cool-season grasses and legumes should have three to four inches of residual leaf area for rapid recovery from grazing.
Like bunch grasses, new plants of sod grasses develop from buds in the crown. They also have strong, creeping rhizomes that extend through the soil or stolons that grow laterally along the top of the ground. Rhizomes and stolons are actually stems with nodes and internodes. They form roots at the nodes and shoots at their tips. Examples of sod grasses are bermudagrass, smooth bromegrass and dallisgrass.
Sod grasses cover a pasture with a good stand much more quickly than is possible for bunch grasses. Empty spaces exist between bunch grasses which creates greater opportunities for weed infestations. Bunch grasses, on the other hand, are normally more productive than sod formers because they put all their energies into above-ground growth which is available for grazing or harvest. Examples of bunch grasses are annual and perennial ryegrass, big and little bluestem, sideoats grama and switchgrass.
Grazing management is not a simple, easy-to-understand task. It takes practice and experience to master the job. The place to start is in understanding how grass grows.
How Does Grass Grow? is excerpted from the November 2018 issue of The Cattleman magazine.
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