Morphological (external structural) changes occur in the grass plant throughout its development. These changes are easy to see and coincide with certain physiological (internal chemical) changes that are impossible to see. Morphological characteristics can be very useful when making grazing management decisions.
THE GRASS TILLER
Vegetative reproduction of perennial grasses takes place through the production of new shoots or tillers. Each grass tiller has a growing point which may turn into seedhead, stem, leaves, roots, joints or nodes, internodes, and dormant buds.
The growing point is where new cells are developed. Trees, shrubs, and forbs have growing points at the outer tips of their branches. The growing points of grasses are found just above the last completed joint or node of each stem. In young grasses and in new growth of perennial grasses, the joints are crowded together near the soil surface. A leaf arises from each of these joints. There may be 7 to 15 joints in the first inch of stem depending on species.
All grass tillers begin growth with a growing point developing from a dormant bud. As long as the tiller is vegetative, it has the potential to produce an indefinite number of leaves. Once the tiller is triggered to become reproductive, there is no further potential for new leaves to form.
As the growing season progresses, hormonal changes in the tiller cause the stem to push upward. The distance between the joints increases, or in plant talk, the internodes elongate. At first the tiller appears to be vegetative since no seedhead is evident. In many situations, elongation is a transitional period between the vegetative and reproductive stages of plant growth. In some grasses, tillers remain elongated vegetative tillers because the seedhead either never develops, or aborts at a very young stage.*(See NOTE below.) If a tiller has become reproductive, a seedhead will emerge and the tiller no longer has the capability of producing leaves.
This does not mean the grass plant stops growing. Associated with each tiller are dormant buds. These buds are located at the nodes at the base of the tiller (basal buds), on the stem (aerial buds), and at the nodes on the stolons or rhizomes. Dormant or inactive buds have the potential to produce a new tiller with a new growing point. Basal, rhizome, and stolon buds are generally the most rapid source of growth. Aerial buds, although active on some grasses such as reed canary grass, are the least productive of the new tillers.
Removal of the growing point of an elongated tiller breaks the dormancy of the buds associated with that tiller. Growth of new leaves from the activated growing point can then occur.
Buds not only account for new tillers during the growing season; those that remain dormant must survive the winter and produce next year's tillers. A spring tiller developing from a dormant bud can be compared to an annual plant developing from seed. Perennial grasses must be allowed to manufacture and store sufficient energy to develop buds that are vigourous enough to survive the winter and begin spring growth. Energy reserves increase in crowns during the latter part of the growing season and buds for next year's tillers develop. Consequently, severe grazing near the end of the growing season will reduce the production of crown tissue and cause a decline in forage production the following year.
|*NOTE: Studies have shown that in most perennial grasses, seed production is typically low and often erratic. Thus, vegetative reproduction (tillering) plays the major role in perennial grass survival. New tillers have support from the original plant until their own root systems develop, minimizing adverse environmental affects. BC research has shown that bunchgrasses such as bluebunch wheatgrass have a 40 year lifespan. Frequent reproduction from seed is unnecessary. Range improvement practices should encourage vegetative reproduction with secondary consideration for reproduction from seed.|
GRAZING INFLUENCES TILLER GROWTH
Because a bud developing into a new tiller has no leaves, it must depend on energy provided by remaining leaf material or storage organs. After severe leaf removal, the only source of energy for new tillers is stored carbohydrates or carbohydrates produced by other tillers.
A developing tiller draws down stored reserves. If sufficient time is allowed for new leaf material to develop and replenish the carbohydrate reserves, no harm is done. However, if severe grazing is repeated after an insufficient rest period, the plant starts to sacrifice root growth to produce leaves, and the plant is weakened.
When grazing is not severe and sufficient leaf material is left, new tiller growth can receive energy manufactured by the remaining leaf material and there is no depletion of carbohydrate reserves. In fact, removal of an elevated growing point, under proper grazing management and good growing conditions can stimulate new vegetative growth, maintaining the plant in a healthy vigourous state.
TIMING OF ELONGATION
The timing of internode elongation is important. Kentucky bluegrass, junegrass, green needlegrass, red top, and many other grasses do not raise their growing points until near the time the plant enters the reproductive phase. The growing point is maintained close to or below ground level for most of the growing season and is not exposed to removal by the grazing animal. These grasses are relatively resistant to continuous, close grazing.
Grasses such as smooth brome, western wheatgrass and Canada wildrye elevate their growing points early in their development. In fact, smooth brome, reed canarygrass, and western wheatgrass may have elevated growing points without seedhead development at certain times of the year. These species are productive, yet less resistant to grazing due to their elevated growing points.
The effect of defoliation (leaf removal) can be detrimental or beneficial to the plant. It depends on the severity of defoliation as determined by height, frequency and duration of grazing and rest periods. The grass plants of British Columbia evolved under the grazing pressure of wild ungulates; however, they did not evolve under seasonlong, continuous grazing.
Proper defoliation can increase total production under suitable growing conditions. If a grass is allowed to "head out" and is only grazed once in the growing season, the total yield will be less and quality will be lower than if it were grazed less heavily several times during the growing season. If grazing is done with consideration of the plant's requirements, the forage can be maintained in an active growth and tillering phase longer than if it were allowed to mature naturally. It must be remembered however, that good growing conditions must exist and production will always decline as the plant nears dormancy.