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Grass Growth and Response to Grazing

By   /  October 12, 2015  /  Comments Off on Grass Growth and Response to Grazing

Knowing your forage can help you better manage it. Here’s some background on how grasses grow, photoshythesize and store energy – all important things if they’re going to grow and feed livestock!

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Grasses are the dominant plants in most forage-based enterprises throughout the world. Whether livestock graze native rangeland or tame pastures, grasses usually are the basis of the energy and nutrients for animal growth and maintenance. Grazing livestock should harvest only part of the perennial forage crop to maintain the health and vigor of grasses.

Quick Grass FactsEnergy and nutrients from forage-based diets produce approximately 80 percent of the red meat products consumed in the United States. Animal gains from forage-based programs usually are less expensive than from any other current program. Animal products come from lands that usually are not suited for production of other food or fiber for human consumption. These lands include rangelands that usually are not capable of being cropped and pasturelands that are not suited for long-term intensive crop production because of low productivity, high erosion risk or other problems. Manage these lands to sustain perennial grass production.

Growth and Development

Figure 1: A vegetative grass tiller. Leaf 1 is oldest and leaf 8 is just being exerted. The enlarged area of the crown shows the apical meristem that produces the leaves.

Figure 1: A vegetative grass tiller. Leaf 1 is oldest and leaf 8 is just being exerted. The enlarged area of the crown shows the apical meristem that produces the leaves.

A grass plant is a collection of plant parts, like a tree or shrub, made up of growth units called tillers. Each tiller produces roots and leaves. Vegetative tillers consist primarily of leaves (Figure 1), whereas reproductive tillers produce a stem, seedhead, roots and leaves (Figure 2). The basal area of the stem, where roots often arise, is the crown.

The crown usually has a number of buds (growing points) that produce new tillers and roots. New tillers are anatomically and physiologically connected to older tillers. Therefore, several connected tillers may all live and share water, carbohydrates and nutrients. If one tiller dies, an adjacent tiller with established roots and leaves usually lives.

Some tillers stay vegetative, while others become reproductive and produce seedheads. Whether a tiller becomes reproductive depends on environment and hormones produced in the plant.

For example, a reproductive tiller may remain vegetative if the growing point (terminal meristem) is removed by grazing. Vegetative growth, therefore, is favored by some grazing, which reduces the number of seedheads produced and may stimulate the formation of new tillers. Vegetative tillers usually are less stemmy and more nutritious than reproductive tillers.

Seed production may be valuable, however, if the operator wishes to harvest a seed crop or if there is a need for seed to produce new seedlings in the stand. Seed production is not always essential for stand maintenance, as many grasses reproduce by vegetative means such as tillering or production of new stems from underground rhizomes.

Vegetative Growth

Figure 2: A reproductive grass tiller. This tiller has a stem (or culm) and seedhead that differs from the tiller in Figure 1. Intercalary meristematic tissue at the base of the leaf blade, near the ligule (insert), allows for leaf expansion.

Figure 2: A reproductive grass tiller. This tiller has a stem (or culm) and seedhead that differs from the tiller in Figure 1. Intercalary meristematic tissue at the base of the leaf blade, near the ligule (insert), allows for leaf expansion.

An apical meristem (expanded portion of Figure 1) is responsible for leaf formation. The intercalary meristems at the base of leaf blades and sheaths are responsible for leaf expansion (insert in Figure 2). Each leaf is rolled into a tube-like form in its lower portion and unfurls as the blade extends. Subsequent leaves follow the same pattern.

As new leaves push up from the center of the rolled tube portion of the first leaf, the growth is similar to extension of a telescope. In Figure 1, leaf 1 is the oldest; leaf 8, the youngest, is emerging. In this example, the growing point (apical meristem) is at or near the soil surface and is protected from large grazing animals. Grazing, therefore, removes leaf tissue but, in most cases, will not harm the growing point that produces the leaves.

Grass growth, for either cool- or warm-season species, begins in spring when the soil warms. As the first grass leaf is exerted, it extends in length or height through formation and growth of new cells at the base of each leaf blade. This growth area (intercalary meristem) is at the base of the leaf blade adjacent to the sheath (insert in Figure 2).

Chlorophyll, which develops rapidly in young leaves, gives plants the ability to carry on photosynthesis. Photosynthesis uses energy from sunlight and carbon dioxide from the air to produce carbohydrates. However, photosynthesis may not meet the energy demands of the rapidly growing new leaf. Production of the first one to three leaves requires a substantial amount of energy in the form of carbohydrates stored in the crown of the plant. However, as these first leaves fully extend, rapid rates of photosynthesis supply sufficient carbohydrates for growth of other leaves and roots. When severe defoliation occurs, carbohydrates stored in the roots and crowns may be needed to initiate new growth.

Leaves have a definite life span, as do tillers. The first spring leaf normally dies in the summer. Leaves are most photo-synthetically active when they reach full expansion. As they age, their capacity for photosynthesis declines. The excess carbohydrate produced through photosynthesis helps produce additional leaves, reproductive organs or roots. Thus, photosynthate produced by the plant is used efficiently in growth and maintenance. Once a leaf can no longer produce enough carbohydrates through photosynthesis for its own needs, it dies.

Reproductive Growth

Grasses often begin a transition from vegetative to reproductive growth when most of the vegetative growth is produced for that year. Plant hormones and physiology control the transition from the vegetative to the reproductive state.

Reproductive meristems are stimulated to begin growth, which results in development of stems, a few leaves, and reproductive structures. These reproductive structures often grow rapidly, with little production of leaf area, but rapid expansion of the flower stalk (culm) and seedhead (inflorescence) (Figure 2).

In grasses, most of the reproductive structure contains chlorophyll and is capable of photosynthesis. Thus, little, if any, carbohydrate reserve in crowns or roots is used for production of grass seed.

The apical meristem elevates during growth of reproductive structures (Figure 2). This is different from the vegetative meristem, where leaves form at the base of the plant and the apical meristem remains at or near the soil surface (Figure 1). Grazing can remove the reproductive apical meristem and halt seedhead production. For seed production, avoid grazing during this period. However, you can manage grazing to reduce the seed crop and stimulate future tiller production.

Carbohydrate Reserves

Grasses commonly store carbohydrates when most leaf growth is complete. Even though leaves still have a high photosynthetic capacity and sufficient leaf area for photosynthesis, there are few demands for new growth. Therefore, carbohydrates accumulate in roots and crowns and serve as storage organs for growth the next spring. These carbohydrate reserves also are necessary for plant respiration during winter dormancy when photosynthesis is not possible but crowns and roots remain alive. This means that you need to protect these from overgrazing even in the winter.

Next Week: How Your Grass Grows Tells You How to Graze It.

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