Herbivory, or grazing, is a powerful ecological process that can influence the amount and kinds of vegetation present on the landscape. This occurs primarily because herbivores graze selectively, choosing some plants or parts of plants over others, which alters the competitive ability of plants. Continued heavy grazing can decrease palatable, preferred plants while allowing unpalatable, poisonous, or invasive plants to increase in the community. By controlling the species of herbivore and the timing and intensity of grazing, managers can eventually shift a forb-dominated system to a grass-dominated system and vice versa. "Targeted grazing" is the use of grazing animals to accomplish specific vegetation management goals through strict control of the species of grazing animal, timing of grazing, and intensity or frequency of grazing.
The absence of herbivory can also be a valuable vegetation management tool. The absence of herbivory, whether through delayed grazing or complete rest periods, is designed to improve the forage stand. Nongrazing periods can be assigned to specific pastures or worked into a planned rotation system. The benefits of planned, nongrazing depend upon the time of year it is implemented:
- Early spring — enhance leaf production by plants
- Spring — enhance plant regrowth when conditions are optimal
- Summer — allow for seed production
- Autumn — improve carbohydrate production and storage, particularly if summer dormant grasses had fall regrowth
- Yearlong — enable seedlings to establish, increase vigor of preferred species, accumulate fine fuels for prescribed burning.
A regularly scheduled or occasional deferment can help range types such as midgrass, semidesert bunchgrass, sagebrush-grass, and mountain grasslands to increase forage plant vigor, plant reproduction, carbohydrate root reserves, and general range condition.
The term "rest" is used in grazing management to denote a full year (12 months) of no grazing. This allows the plants to undergo a complete growth cycle without being grazed. The benefits of rest are best realized in special management situations such as:
- severe drought
- following reseeding
- providing fuel for prescribed burns
- when critical site rehabilitation is required.