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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 |
