Range Ruminations: Is “Take Half, Leave Half” a Good Way to Calculate Stocking Rate?
By Dr. Jeff Mosley, MSU Extension Range Management Specialist
Former eastern Montana rancher and Past President of the Society for Range Management Dan Fulton once wrote, “Range, and particularly Great Plains range, cannot be rationally managed on the basis of range carrying capacity surveys. The only way to know for sure how many cows can run on it is by grazing cows on it.” Fulton’s statement reminds us that grazing management is a circle. We begin by observing the land and animals, and then make a plan based on what we have observed. Next we implement the plan and observe how well the plan is working. Based on what we observe, we make adjustments, observe again, make more adjustments, and so on. Grazing capacity surveys and stocking rate calculations can provide helpful estimates of an area’s sustainable stocking rate, and these ballpark estimates can be used to develop a plan, but they are not the final answer.
Stocking rate calculations include an estimate of proper utilization, and historically in Montana, the concept of “take half, leave half” (i.e., 50% utilization) has often been used. This approach recognizes that to remain healthy, grass plants must replace about 1/3 of their roots each year, and grass plants stop growing roots when more than 50% of their current year’s foliage is removed during the growing season. I believe 50% utilization is an appropriate threshold for growing-season grazing wherever most forage plants have similar palatability that results in relatively homogeneous levels of grazing among plants. However, I believe stocking rate calculations using 50% utilization provide stocking rates that are unnecessarily conservative in some situations and stocking rates that are unsustainable in other circumstances.
Situation #1. Grass plant response to grazing definitely depends on how much foliage is removed during the growing season, but plant response also depends on when the grazing occurs. Plants grazed later in the growing season can sustain 60-70% utilization if they have already had sufficient time earlier in the growing season to grow foliage, grow roots, and replenish energy reserves. Plants can sustain 80% utilization if grazing occurs when foliage and roots are not growing during winter plant dormancy.
Situation #2. Late spring/early summer is generally the time when grasses are most vulnerable to defoliation, but plant response to grazing also depends on how often grazing occurs. Grass plant health declines when 50% defoliation during late spring/early summer occurs for more than two successive years. Two years in a row is okay, but more than two consecutive years of moderate grazing during late spring/early summer is not sustainable. Some form of rotational grazing is necessary when moderate use occurs during late spring/early summer. A moderately stocked rotational grazing system that provides rest during late spring/early summer once every three years averages 33% utilization across the three years (50% use in Year 1 + 50% in Year 2 + 0% in Year 3 / 3 years = 33%). In this situation, stocking rate calculations should be based on 33% rather than 50% utilization.
Situation #3. Typically when utilization averages 50% on bunchgrass rangeland, many or most bunchgrasses have received heavy use (e.g., 70% utilization), a few bunchgrasses have received light use (e.g., 30% utilization), and a few bunchgrasses have remained ungrazed. Preferred bunchgrasses grazed during the growing season at 70% utilization are unable to grow roots and replenish energy reserves. Thus, some form of rotational grazing is necessary on bunchgrass rangeland grazed moderately during the growing season.
A 2-pasture rotational grazing system that provides growing season rest every other year averages 35% utilization for the preferred bunchgrasses over the 2-year period (70% use in Year 1 + 0% use in Year 2 = 35% utilization), which is safely below the 50% threshold. In this 2-pasture system, stocking rate calculations should be based on 25% utilization rather than 50% utilization (i.e., 50% in Year 1 + 0% in Year 2 / 2 years = 25% utilization).
Similarly, a 3-pasture rotational grazing system that provides growing season rest one year out of every three averages 47% utilization during the growing season for the preferred bunchgrasses over the 3-year period (70% use in Year 1 + 70% use in Year 2 + 0% utilization in Year 3 / 3 years = 47% utilization), again safely below the 50% utilization threshold. In this 3-pasture system, stocking rate calculations should be based on 33% rather than 50% utilization (i.e., 50% in Years 1 and 2 and 0% in Year 3, averaging 33% across the three years). This example also illustrates the incentive for 3-pasture versus 2-pasture rotational grazing systems. The 3-pasture system can be sustainably stocked 32% heavier than the 2-pasture system (i.e., 33% utilization – 25% utilization / 25% utilization = 32% increase).
In summary, stocking rate calculations can help inform grazing management planning and decision-making, but an area’s sustainable stocking rate can only be determined by making your best guess, implementing your plan, and then making adjustments as needed based on how the land and animals respond. Going forward, I suggest that initial stocking rate estimates will be more accurate and helpful when stocking rate calculations adjust the take half, leave half approach (i.e., 50% utilization) when needed, accounting for the time of year that grazing occurs, how often grazing occurs among years, and how much utilization levels vary among individual forage plants. Happy ruminating.
