Plants have both positive and negative responses to grazing. Proper grazing management can stimulate plant growth, maintain optimal leaf area, enhance nutritive value, remove excessive litter, accelerate nutrient cycling and manipulate botanical composition.
Rangeland plants have evolved with a long history of grazing and browsing, although there are considerable differences in resistance to grazing among the species. Early studies indicated that degree and season of defoliation were the primary indicators that determined the level of damage. Plants thus were considered to be most vulnerable to grazing damage when carbohydrates were at their lowest and reserves were not sufficient enough to initiate regrowth. Although there are many exceptions to the main factors that have been found to increase grazing resistance in grasses, forbs and shrubs are as follows:
Grasses are more resistant to grazing when they have a higher proportion of culmless (stemless) shoots with low resistance, there is a greater delay in elevation of the apical buds, they sprout more freely from basal buds after defoliation and when they have a higher ratio of vegetative to reproductive stems.
Forbs are more resistant to grazing when they produce a large number of viable seeds, they have a delayed elevation of growing points and when they have antiquality components and chemical compounds present that reduce palatability.
Shrubs are most resistant to grazing when spines and thorns are present to discourage browsing, volatile oils and tannins are present to reduce palatability, when branches are present (thus making removal of inner leaves difficult) and when the apical meristem is removed stimulating axillary bud development.
Grazing effects are numerous and can be positive or negative. Heavy grazing negatively impacts range plant physiology by causing decreased photosynthesis, reduced carbohydrate storage, reduced root growth, reduced seed production, reduced ability to compete with ungrazed plants and reduced mulch accumulation. On the other hand, positive impacts are also possible and are typically associated with light to moderate grazing. These include increased photosynthesis, increased tillering, reduced shading, reduced transpiration losses, inoculation of plant parts with growth promoting substances and reduction of excessive mulch accumulations that may physically and chemically inhibit vegetative growth. Excessive mulch can provide habitat for pathogens and insects that can damage forage plants.
Plants that are highly resistant to grazing are generally less productive and palatable than those with low grazing resistance. This is because photosynthetic products that are used for physical and chemical protection from grazing could otherwise be used for plant growth and reproduction.
Photo: Elaine Stovin (Hereford grazing in grassland)
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