The grazing response index (GRI) is a simple do-it-yourself way to evaluate the impact of grazing pastures, and it doesn’t cost anything but your time.
“The GRI gives quick feedback each year, but doesn’t replace comprehensive assessments every five years or so to gather the full set of information for monitoring long-term trends,” explains Dr. Wendy Gardner, assistant professor of Natural Resource Sciences, Thompson Rivers University, Kamloops, B.C. She has been instrumental in introducing the GRI in British Columbia, where ranchers give it great reviews for its simplicity and user-friendly approach.
Her confidence in the GRI traces to a three-year study she worked on with Agriculture and Agri-Food Canada (AFFC) rangeland specialists Kerry LaForge at Swift Current and Mae Elsinger at Brandon to examine 33 years of historic grazing information to confirm that the index developed by Dr. Roy Roath, Floyd Reed and David Bradford of Colorado State University would work in Western Canada.
“We wanted to know if there is a good basis for the relationship between range condition, comprehensive assessments and GRI scores and there is. If the comprehensive assessments showed that the range had improved over 20 years, we generally saw the GRI improve as well,” Gardner says.
AAFC has facilitated workshops to prepare rangeland and forage specialists to introduce the GRI to producers across the Prairies and partnered with Ducks Unlimited, Saskatchewan Agriculture and the Saskatchewan Forage Association in a three-year demonstration project to evaluate the potential of the index in assessing the impact of grazing on tame-forage pastures.
Research tells us native forages need seven to 10 days to regrow after grazing but that doesn’t leave the plant time to replenish the energy lost through photosynthesis. So plants bitten every week eventually become less vigorous as root reserves are drawn down.
To score frequency, divide the total number of days in the grazing period by seven or 10 days of rest during the grazing period. Seven would be appropriate early in the grazing season with favourable weather for quick regrowth. If not, use 10 days.
An answer of 1.0 or less gives a frequency score of +1 because grazing plants only once has a positive effect on plant vigour.
Between 1.0 and 2.0 score 0 for frequency because double nipping in itself has little effect on plant vigour.
When the answer is greater than 2.0, the frequency score slips to -1 because plants grazed three or more times in succession start to lose vigour.
For example, if a grazing period is 21 days and the rest period is 10 days, cattle could potentially choose to graze the same plants three times (days one, 10 and 21). Dividing 21 by 10 gives you 2.1, therefore, the frequency score would be -1.
When there is more than one grazing period on the same pasture during the growing season, think about whether the plants had full opportunity to recover between grazing periods. If not, add the grazing days together to score all grazing periods as one.
Research shows leaving at least half the leaf intact ensures native plants will regenerate by photosynthesis without dipping too far into root reserves.
What you’re after is an assessment of the volume removed by grazing, that reflects the energy cost to the plant. Grazing exclusion cages provide a helpful idea of what the forage would look like without grazing.
Grazing intensity is rated as light (score +1) if utilization is less than 40 per cent; moderate (score 0) if utilization is 41 to 55 per cent; and heavy (score -1) if greater than 56 per cent.
Opportunity is evaluated at the end of the grazing season to account for the impact of grazing. It should honestly reflect the opportunity plants had to grow from before turnout, during rest intervals, and after the cattle were removed to the end of the growing season.
Regrowth depends on soil moisture, air temperature and leaf area but even if your rotation allows enough time for those factors, it also has to account for actions beyond your control like grasshoppers, wildlife grazing, drought or summer semi-dormancy.
Score +2 if the plants had a full season to regrow, or didn’t appear to be grazed. Score -2 for a continuously grazed pasture since the plants had no opportunity to fully regrow. Between those two score +1 if plants had most of the season to regrow, 0 if they had some chance to regrow, and -1 if there was little chance to regrow.
4. Overall Score
The overall score is the sum of the frequency, intensity and opportunity for each site. A positive score means your management practices are benefiting plant health. Zero is neutral and a negative score suggests you will harm the plants in the long term if you don’t change anything.
For more detail read the Grazing Response Index AAFC fact sheet or contact Wendy Gardner in B.C. at 250-371-5570 ([email protected]), Kerry LaForge in Saskatchewan at 306-770-4495 ([email protected]) or Mae Elsinger in Manitoba at 204-578-6634 ([email protected]).