Many of the desirable forage species for livestock and wildlife are higher seral herbaceous species. Therefore, their ability to compete as an individual plant against neighboring plants is important. Most of the reproduction of perennial herbaceous species in North America is vegetative, that is, from stolons, rhizomes, or an increase in the number of stems. Seed recruitment is generally rare in most existing stands of native, perennial vegetation. Therefore, it is important that we maintain the vigor and reproductive capacity of vegetative parts of preferred species as much as possible by leaving these reproductive structure mostly intact and leaving enough leaf material for the plant to support these new plant parts that take root.
This does not mean that allowing plants to make seed periodically is unnecessary, or that seedling recruitment is unimportant. What it does mean is that vegetative reproduction is normally the most rapid means of increasing the percentage of total production represented by preferred species. But, in low seral communities, seedling recruitment is often the only means high seral species have available to propagate themselves. Likewise, allowing a plant to set seed is one way to ensure that you have provided enough recovery following defoliation for the plant to maintain its competitive capacity. Plants should be allowed to set seed at least often enough to ensure that vigor remains high from year to year and that a viable seed bank is maintained in the soil.
The harmful effects of defoliation to an individual plant include a decrease in photosynthetic capacity because of a decrease in leaf surface. If defoliation is too severe (common wisdom says 50% of leaf surface, but this figure may range from about 30% to about 60%), it also temporarily stops root growth. The plant must then reallocate energy among plant parts to survive until enough leaf grows to allow the plant to produce more energy than is being used. This interruption of root growth and decrease in photosynthetic activity puts that plant at a disadvantage compared to its neighbors, and allows its competitors to capture more of the limited nutrients, water, or sunlight available. This may or may not decrease the total production of a given community, because the roots of other plants may simply occupy that space in the soil. However, these other plants are often less productive, less palatable plants like threeawn and many of the weedy forbs and brush, which would cause livestock productivity to decrease.
The other way in which a plant can be harmed by severe grazing is when the growing point at the tip of the stern is removed. When a grass plant is in the early stages of growth, the growing point is near or below the soil surface. As the plant matures, the growing point elevates, generally just prior to producing a seed stalk. At this time, the plant is storing energy in the seedhead, rather than the crown and roots. If the growing point is removed at this time, growth must begin again from secondary buds, usually near the base of the plant. This may increase the number of tillers making up the plant, given adequate time and conditions for regrowth. However, the plant must survive until this growth begins again, which may weaken it if enough time with good growing conditions is not available for the plant to restore these energy reserves from regrowth. It may, therefore, actually decrease the number of stems produced by a plant when poor growing conditions prevail. This is another reason why some species disappear while others become more abundant under heavy grazing pressure.
Chronic heavy grazing also creates a selection pressure within a species against plants that grow more erect and for those that grow closer to the ground, have larger numbers of small tillers, and reduced leaf numbers and leaf areas. The latter type of plant is more competitive compared to its erect, leafy cousins in situations where heavy levels of grazing are common. Taller growth habits with high leaf areas give the plants possessing these characteristics an advantage in ungrazed to moderately grazed situations.
Possible positive effects of defoliation include removal of dead growth, opening of the canopy to allow earlier soil warming in the spring, decreased moisture losses from transpiration , removal of some older leaves that may be inefficient and intercept significant amounts of rainfall where low volume rainfall events are common, and allowing secondary buds to produce new stems.
Figure 1 will be helpful in discussing plant-animal interactions associated with the grazing process. The solid line represents a growth curve for any living thing. Early in development growth is relatively slow, if measured on a weight per unit time basis. This is the area designated as I on the left of the diagram, a period we arbitrarily refer to as Phase I. This is followed by a rapid growth spurt, designated as Phase II or II in the diagram. After the rapid growth phase, a plant starts to slow its growth rate before setting seed and going dormant or dying. We will refer to this period as phase III. It is labeled III on the diagram. In humans, phase I would be referred to as infancy and childhood; phase II would be similar to adolescence and early adulthood, and phase III would be middle- into old-age. In grasses, phase I is the seedling, or in the case of perennial grasses early shoot growth from the crown. Phase II is the period of rapid growth when leaf area is sufficient for high rates of photosynthesis and weather is optimal. Generally this is when individual stems contain four to six vigorous leaves. Phase III begins about the time the seedhead is forming and continues until dormancy or death.
Anything that increases the rate of growth makes this curve steeper, and the conditions that increase growth rate vary with plant species. Cool season plants generally grow best when light and moisture are adequate, and temperatures are about 75°F. Temperatures of 85-90°F are usually considered optimum for warm season plants.
Both phases I and III are energy inefficient. In phase I, insufficient leaf material is available to manufacture the energy necessary to support growth and respiration. In phase III, the buildup of aging tissue and damage to the waxy covering of older leaves may cause them to lose more water than would be the case if they were younger and undamaged. They also may be less efficient in producing energy and even be a net energy drain to the plant. The quantity and vigor of leaves in phase II make this the most efficient phase of growth.
The time when a single defoliation has the most detrimental effect on a grass plant is when the growth point is elevating (the “boot” stage). At this time the plant already has committed large amounts of energy to seed production and reserves may be low. Also, leaf efficiency is decreasing and a high percentage of this material is likely to be removed at the same time. If we consider that this is also probably near the end of the period when good growth conditions can be expected, the plant has only a limited opportunity to regrow. This will set it back to phase I with little chance to recover before dormancy or death.
If the plant is grazed in phase I, it will cause an energy drain and set the plant’s rate of maturity back, but since this period is usually early in the season, when conditions are, or are likely to become relatively good, a single defoliation at this time is not as critical. However, since the regrowth on this plant later in the season will be younger, more palatable and nutritious than leaves from ungrazed plants of the same species in that pasture, the real danger is that long graze periods during the early growth stages will increase the chances that the plant will be grazed again before it recovers and will set it back into phase I again. This would prevent it from attaining a sufficient level of photosynthesis to maintain vigor, and if continued, would cause the plant to die.
If the plant is in phase II, the amount of energy produced by the leaves is greater than the amount being used by the plant. If excessive amounts of leaf material are not removed, it could theoretically stay in phase II. Grazing animals, unfortunately, have not read many books on proper grazing use and often graze palatable plants excessively, setting them back to phase I. Even if this occurs, however, the plant can resume growth and recover with little effect on plant vigor and production if an adequate period of recovery is allowed during a time when growing conditions are suitable. Therefore, our goal as a manager, from a vegetation standpoint, should be to keep plants in phase II as much as possible, but while still allowing enough times when the plant sets seed to maintain a viable seedbank in the soil.