Ken Moore
Department of Agronomy
Iowa State University
Structure of the Grass Plant
How Grasses Develop
Development and Plant Growth
Energy and Grass Growth
Grass Regrowth Following Grazing
Forage Quality and Grass Development
Keying Grazing Management Decisions to Grass Development
Additional Reading
Copyright
You may not find the idea of watching grass grow very appealing, but it is the key to successful grazing management. In MIG timing is everything and the clock to watch is the grass. In MIG systems it is not merely sufficient to rotate livestock among several pastures -they must be rotated at the right time and for the right reasons. The purpose of this article is to give you enough background on the growth and development of grasses to be able to make informed pasture management decisions.
The structure of the grass plant is remarkably simple and similar among the many species of grasses. A grass plant is a collection of tillers or shoots that arise from buds at the base of the plant which is called a crown. Each tiller is composed of a series of repeating units consisting of a leaf, stem node, stem internode, and a bud. Each leaf is attached to the stem at a node and the bud develops at this point as well. Early in the development of a grass plant the distance between nodes (internodes) is very short and the stem remains compact at the base of the plant. At the top or apex of the stem is the growing point where new leaves and stems are originated. As long as this growing point remains intact it is capable of initiating new leaves. Later in the development of the tiller the growing point will undergo a change and cease initiating leaves and begin developing the inflorescence or reproductive structure of the plant. After this point in time, the growing point is no longer capable of initiating any more new leaves and its removal has no impact on further leaf development. Once this transition occurs, some of the upper internodes will begin to elongate eventually raising the inflorescence to the top of the tiller.
The development of grass tillers proceeds through a sequence of developmental stages that are relatively common among grasses. There are three primary growth stages in grasses that you need to be able to recognize for grazing managment: 1) vegetative; 2) elongation; and 3) reproductive. The vegetative growth period is characterized by the development of leaves. Once a critical number of leaves has developed on a tiller, the older, and lowermost leaves generally die off at a rate that is about equal to the rate of development of new leaves. So once this has occurred the number of leaves present on a tiller becomes relatively constant. Elongation is the period during which stem internodes elongate and is often referred to as jointing. When the tiller begins to elongate, usually in response to changing daylength, the uppermost internodes elongate. The lowermost internodes do not elongate and remain at the base of the plant. These lower nodes and internodes together with those of related tillers constitute the crown of the plant. The elongation period is sometimes referred to as transition because it represents the transition between vegetative and reproductive growth. As a result of elongation, the developing inflorescence pushes through the uppermost leaf sheath to form what is commonly referred to as boot stage. The reproductive stage is the period during which the developing inflorescence emerges and pollination occurs.
There is a distinction between plant growth and development. As just described, development refers to changes in the structure or form of the plant. Growth on the other hand, is an irreversible increase in the weight or size of a plant. Generally the two are related; as a plant grows it also develops (Fig. 1). Grass growth is initially slow, but then goes through a very rapid period, followed by a slow down as the plant matures. This general pattern of growth is the same for new growth in the spring and growth following defoliation. However, the rate of growth will vary greatly depending on the season. The extent of development also varies with season. Many temperate grasses will flower only once during the growing season. The regrowth of these species is limited to vegetative development although some will produce sterile stems.
The productivity of grasses varies with species and season. Cool-season grasses produce most of their growth in the spring and early summer (Fig. 2). Warm-season grasses on the other hand produce most of their growth during the warm summer months. This seasonal variation in grass productivity is one of the major constraints in grazing management. The nutritional needs of livestock increase at a constant rate over the grazing season. Because the ability of a pasture to support livestock production varies throughout the season, it is difficult to balance the nutritional needs of livestock with nutrients available from the pasture.
Pasture plants, like all green plants, convert solar energy into chemical energy through the process of photosynthesis. The plant uses this energy to carry out various physiological processes which allow it to grow and ultimately reproduce. In the spring, the energy required for the plant to initially grow comes from energy reserves stored the previous growing season in the form of carbohydrates. The site of energy storage varies among grass species, but typically it is in the roots and lower portions of stems. As the plant grows it uses this energy to produce new leaves. Once a critical number of leaves has developed the plant is able to produce enough energy to support further growth and begins to store energy for the next growth cycle (Fig. 3). It is extremely important that the plant not be grazed until it has had enough time to reestablish its energy reserves. If the plant is defoliated before this occurs, regrowth will be stunted and the stand may be weakened. The time required for a grass plant to store an adequate amount of energy for regrowth varies with the season and intensity of defoliation. It is shortest when the plant is growing under optimum conditions. As a rule of thumb, cool-season grasses require a rest period of two to three weeks during the spring and fall, and five to six weeks during the summer. Conversely, warm-season grasses require a three-week rest period during the summer, but at least six weeks of rest in the spring and fall.
The amount of the grass plant that has been removed by grazing also has an impact on the amount of time required for regrowth to occur. The less growth that is removed, the shorter the recovery period will be because the residual growth will be able to photosynthesize and contribute to the recovery. For this reason, care should be taken not to graze cool-season grasses below a height of three to four inches and warm-season grasses below four to eight inches. Overgrazing of grasses will lower the regrowth rate and increase the period of time required to recover energy reserves.
The way grass pastures are managed in the fall will have an impact on how well they grow the following spring. Since the initial growth in the spring is dependent on stored energy, it is important that pastures have an adequate recovery period between the last summer grazing period and the first frost. Generally, it is preferred not to graze a grass pasture three to four weeks prior to frost. This is obviously not possible in many cases, but it should be realized that pastures grazed during this period will require a longer period of time to reach their maximum growth rate in the spring. It is a good management practice to defer grazing on these pastures until late in the rotation the following spring.
The stage of development has a major impact on how grasses respond to defoliation by grazing. As long as the growing point at the apex of the stem is not removed by grazing it can continue to develop new leaves. Developing leaves that are only partially defoliated by grazing can also continue to grow, however, their growth is limited. Once the growing point has become reproductive or has been removed by defoliation new regrowth must occur from buds located at the base of the plant. In general, removal of the growing point is required to break the dormancy of buds associated with that tiller and therefore has a beneficial effect on plant growth. In most grasses, stem elongation does not occur until after the growing point becomes reproductive. Since the growing point is located at or below ground level, these species are resistant to close grazing. In some grasses, however, the growing point is elevated early in their development (prior to the reproductive phase) and is, therefore, vulnerable to removal by grazing. Smooth bromegrass and switchgrass are especially susceptible to defoliation during this period because their basal buds are not yet ready to develop into new tillers and energy reserves to support new growth are low.
The nutritive value of grasses for grazing livestock decreases at a steady rate as the plant grows and develops (Fig. 4). It is highest during the vegetative stage of development and for all practical purposes is too low to support livestock production beyond the reproductive stage. One of the most common mistakes made in grazing management is to delay the initiation of grazing until after most of the quality of the pasture is lost. A grass pasture should never be allowed to enter into the later reproductive stages of development before either being grazed or cut for hay. Even though quality is highest during the vegetative stage, caution must be used to prevent grazing too early so that adequate energy can be stored to support regrowth (Fig. 3). As a general rule, the optimum time to graze a pasture is during the period of rapid growth preceding the reproductive stage. This timing provides the optimum compromise between yield and quality, allows sufficient time for storage of energy reserves, and assures the removal of the growing point which will stimulate tillering.
Based upon the foregoing discussion, it is possible to make some generalizations about timing grazing management decisions based upon grass development. Grass pastures are ready to graze in the spring when most of the tillers have between three to four fully expanded leaves. At this time the plants have begun to store energy reserves for the next growth cycle and have excellent forage quality. This is not the optimum time to graze all paddocks for reasons described above, but it is the best time to get started. Remember you are shooting at a moving target and if you delay the initiation of grazing too long you will not be able to graze later pastures in the rotation before they become reproductive. Pastures that reach the reproductive stage should be cut for hay and allowed a recovery period before grazing. There is just not enough quality left in these pastures and livestock performance will suffer if they are grazed. Rest periods need to be adjusted according to the season of growth. Most grasses will maintain three to five leaves on a vegetative tiller. This is the optimum number for photosynthesis and regrowth should not be grazed until after this has been achieved. In conclusion, by observing the development of your grass pastures you can learn to manage them for optimum productivity which will in turn result in improved livestock production.
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Adapted from a paper presented by the author at the Management Intensive Grazing Symposium, 24 January 1995, Des Moines, IA.
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Ken Moore
Department of Agronomy
Iowa State University
Copyright © 1995 - All rights reserved