Written by Rachel Frost and Jeff Mosley, Montana State University
While strategies to cope with grazing vary greatly between plant species, plants in general either avoid grazing or tolerate grazing. Plants avoid grazing by diminishing their accessibility or palatability to herbivores. The presence of thorns, hairs, or secondary compounds in some plants are examples of avoidance mechanisms. Plants that tolerate grazing have effective mechanisms to facilitate growth following defoliation. Grasses are usually considered to be the plant group with the highest grazing tolerance.
Plants that withstand grazing generally have one or more of the following characteristics:
- Growing points are low, or elevation is delayed.
- Plants have a high ratio of vegetative-to-reproductive shoots.
- Apical meristems are activated, and new root growth is initiated following defoliation.
Range plants can be classified by how they respond to grazing pressure. The amount of grazing pressure that a plant sustains depends on how much it is preferred by grazing animals. The species of grazing animal may also influence how a plant responds to grazing because different species have different dietary preferences and eating behaviors.
Decreaser plants are the first plants to die out under continued heavy grazing. These plants decrease because they are either quite palatable and sought out by grazing animals or they lack physiological attributes that help them recover from grazing. The more preferred plants are the first to be grazed, and animals may continue to defoliate these plants throughout the growing season.
Increaser plants generally increase their numbers as decreaser plants are reduced. Many increaser plants avoid grazing damage because they grow close to the ground or are less palatable than decreasers. Increasers often also possess physiological mechanisms that help them recover from grazing. These plants should be monitored because they are a sign of high grazing pressure and can increase in number and abundance beyond what is desirable. If overuse continues, even increaser plants may decline in the community.
Invader plants are commonly weedy plants that become established because more desirable plants have been diminished by excessive grazing. A high proportion of invader plants in a community is usually a sign of overgrazing. However, some noxious rangeland weeds, such as leafy spurge and spotted knapweed, are capable of invading healthy rangeland plant communities.