Grazing Response Index
The Grazing Response Index is a simple way to measure the effects of grazing on rangelands. Three components factor into the Grazing Response Index (GRI) rating.
Frequency of Defoliation
The number of times a plant is defoliated (eaten) during a grazing season. In general, plants need 7-10 after being defoliated to grow enough leaf material to be eaten again. Overgrazing occurs when select plants are grazed over and over again due to their high palatability.
Intensity of Defoliation
How much leaf material is removed during the growing season. In general, if less than 50% of the leaf material is removed, it will not inhibit plant growth. Hence why Mark always says take half, leave half. Intensity is measured as Light (<40% utilization), Moderate (41-55% utilization), or Heavy (56% or more utilization).
Opportunity
Amount of time plants have to grow before being grazed, or the amount of time they have to regrow between grazing events. It is essential that plants get periods of rest so they can recover and maintain plant vigor. Opportunity is determined by whether the plants have had a full season, most of the season, some chance, little chance, or no chance to rest and recover.
When the values from all three factors are summed, the result is your Grazing Response Index (GRI). GRI can be used to determine the duration of use, stocking rate, and timing of grazing.
To read more on GRI, see the links below.