Agriculture Reference
In-Depth Information
such as wheat or barley [27]. The effective rooting depth of perennial for-
age legumes may be greater (e.g., alfalfa) or less (e.g., white clover) than
that of grasses, and legumes often store carbohydrates and proteins for
regrowth underground, in the tap root and crown (the overwintering stem
base).
Pasture management can have a profound effect on root function as
well as on the botanical composition of pastures: In grasses, the fi brous
root systems are comprised of many relatively thin roots, and grasses have
a relatively shallow rooting depth. Overgrazing grasses will deplete both
the aboveground shoot material as well as the carbohydrate storage of the
grass plant, which is located in the stem or leaf bases near the soil surface
(stubble), while well-managed grazing of grazing-tolerant forage species
can enhance the soil by encouraging the turnover of fi brous roots, increas-
ing soil organic matter. Excessive removal of photosynthetic tissue slows
the rate of regrowth of the shoot, which is largely dependent on current
photosynthesis to support growth. Overgrazing also results in self-pruning
of the root system, because the root has no capacity for photosynthesis
and is therefore entirely dependent on current or stored photosynthesis
to support the maintenance of existing roots, much less growth. Under
continuous grazing, where ruminants have access to a large area of pasture
or rangeland for months at a time, the most desirable forage species will
be grazed fi rst, and will be re-grazed as soon as they begin regrowth. This
undermines the ability of the shoot to compete for sunlight and of the root
to compete for water and soil nutrients. Over time, the botanical composi-
tion of grazingland will shift away from the most desirable plant species to
the least desirable species, which were left ungrazed and able to success-
fully compete for resources, reproduce vegetatively by tillering, stolons or
rhizomes, and complete their life cycle by maturing and distributing seed.
4.3 THE EFFECT OF GRAZING ON RUMINANTS
Ruminants that acquire the majority of their feed from forages, as hay or
by grazing, may be more affected by variations in forage nutrient quality
and availability than those on diets that contain higher proportions of sup-
plemental feeds. Further, ruminants may selectively absorb or passively
 
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