by Don Ball
Grazing animals have a profound
effect on pastures with regard to stand density, botanical composition, plant vigor, and stand persistence. Forage crops are impacted by most factors that affect other types of crops, but the influences that grazing animals have are unique to forages. A basic understanding of these influences is essential to obtaining and maintaining highly productive pastures of desirable species.
The removal of vegetative material from plants normally has more influence on pastures than any other factor relating to grazing animals. Removal of leaf tissue reduces the photosynthetic area and thus the ability of the plant to manufacture food. If overgrazing occurs, regrowth depletes food reserves and weakens plants, some plants being more sensitive to this than others.
Perennial plants are more tolerant of defoliation after they are well-established than they are during the establishment year. Defoliation during the establishment year slows or stops root growth, thus making the plants more susceptible to subsequent stresses such as severe drought. Once plants have a good root system and have accumulated some stored food reserves, they are much more tolerant of defoliation.
It is particularly important to recognize that animals do not necessarily evenly graze the various species of plants that are present within a pasture. Rather, they selectively graze those plants or plant parts that they prefer. The result is that the defoliation stress on some plants is much greater than others, thus putting them at a competitive disadvantage.
Treading effects include physical damage to plants, which is especially detrimental to young plants, and compaction of the soil. The extent of treading damage depends on many factors including soil type, amount of moisture in the soil, number of animals/unit area, and sod density.
The greatest treading damage occurs when plants are small and the soil is loose. Therefore, new pastures planted into a prepared seedbed are especially vulnerable. It is particularly important to avoid heavy grazing of new pastures when the soil is wet. Both compaction and damage to young plants is greatest under wet conditions. New pastures should not be grazed until the ground is settled and firm, and the plants have a good enough root system that they will not be pulled up when animals graze.
Some nutrients are removed from pastures when animals are sold off pasture, but this is a relatively minor loss. However, the recycling and redistribution of nutrients is another matter. Recycling of nutrients is much more uneven with continuous than in rotational grazing systems, particularly at low stocking rates. Grazing systems which allow the animals to spend a great deal of time in loafing areas result in high quantities of recycled nutrients in these areas and little recycling in other parts of the pasture.
The ability of animals to distribute seed of both forage plants and weeds is often underestimated. Some weed seed cling to the bodies of grazing animals, and are thus transported by them. Also important is the fact that some seed can be consumed by the animals, then pass through their bodies still capable of germinating. Bahiagrass, serica lespedeza, tall fescue, and many weeds are known to have this ability. In such cases, not only are the seed deposited into a spot which maybe far from the location of the plant which produced them, but they also have a ready source of nutrients deposited along with them.
Grazing animals provide some of the strongest and most unique influences on pastures. In fact, the botanical composition of pastures is largely determined by grazing animals, which
in turn affects forage yield, forage quality, and availability of pasture forage at various times throughout the year. Of course it is important to realize that actually, it is the grazing management imposed by the pasture manager that largely determines the types and extent of influences grazing animals have on pastures.
Don Ball is an Extension Forage Crop Agronomist with Auburn University.