Science Says Post-Fire Grazing on Rangelands Is Not Harmful

Current federal regulations recommending that rangelands be rested from grazing for two seasons after fire are not supported by science. In fact, the research shows that healthy prairie ecosystems require grazing and fire. When tested on the ground, evidence shows that moderate post-fire grazing causes few impacts on northern mixed-grass prairie vegetation.

That’s the conclusion of Emily Gates of Montana State University working with researchers at the USDA-Agricultural Research Service’s Fort Keogh Livestock and Range Research Laboratory. In their paper, “Reconsidering rest following fire: Northern mixed-grass prairie is resilient to grazing following spring wildfire” the authors review the existing science surrounding the effects of fire and grazing on prairie health, and provide the results of a two-year study comparing grazed and ungrazed plots in three burned pastures in South Dakota.

The federal recommendations are based on several assumptions:

1. Fire weakens plants and makes them less able to recover if grazed.
2. Fire kills plants so recovery will depend on new seedlings.
3. Increased bare ground after a fire increases soil erosion risks, and could increase with grazing.

The authors first reviewed past research to find that these assumptions are not supported by science. First, plants do not necessarily die after fire, so new seeding is not required. In addition, erosion due to bare ground in these ecoregions is not a great concern because fire can actually increase germination and seedlings of native species. New seedlings that might otherwise have been smothered by an accumulation of dead plant material and litter, have a better chance to grow and survive. In fact, the conclusion of scientists studying prairie ecosystems is that without fire or grazing, a build up of litter in the tallgrass prairie can actually decrease plant productivity and system health.

Next, the researchers designed a study to determine if grazing the first growing season following a spring wildfire would harm rangelands. Following a wildfire in northwestern South Dakota, they built exclosures in three burned pastures to compare post-fire rest with post-fire grazing. In 2014, at the end of the second growing season after the fire, they measured productivity and plant community composition.

Productivity was greater for loamy than sandy ecological sites with loamy sites producing 2,640 lbs per acre and sandy sites producing 2,096 pounds per acres.  Grazed and ungrazed sites were similar. Ungrazed sites produced 2,275 pounds/acre and grazed sites produced 2,281 pounds per acre. Soils made a big difference in what species grew on a site. Loamy sites consistently contained more western wheatgrass (30% vs 0 for sandy sites), blue grama (18% vs 8%) and threadleaf sedge (4% vs 1%). The only two grasses affected by post-fire rest were needle-and-thread grass and the non-native crested wheatgrass. Rested sites had 22% needle-and-thread grass vs 15% on grazed sites. Crested wheatgrass was reduced by grazing on sandy sites (40 vs 63%), but was not affected on loamy sites (2 vs 1%).

Here are the conclusions of the researchers from the Discussion section of their paper:

“The results of this study support the hypothesis that grazing the first growing season following fire will not affect the basal composition of the burned community. Neither litter, bare ground nor any other basal cover metric with the exception of fecal cover, differed between grazed or rested sites by the second growing season following the fire, indicating that grazing did not adversely affect the recovery of ground cover.

“By the second growing season following fire, ecological site had a greater impact than grazing treatment on productivity, community composition on an individual species basis and basal cover composition than did grazing treatment.

“The results of this study indicate that moderate post-fire grazing rendered few impacts on northern mixed-grass prairie vegetation and was positive with regard to diversity. Although post-fire rest from grazing may be desirable in some situations, the results of this study and growing evidence from other research (Bates et al., 2009; Vermeire et al., 2014) indicate that rest is not required to maintain plant productivity or ground cover.”

The results do not support the recommendation that a two growing season rest period following fire is required in the northern mixed-grass prairie.

BUT

Scientists know that there is always more to know so they tend to be cautious when making pronouncements. In this case, the authors note that their results are for northern mixed-grass prairie. Additional research has shown that caespitose grasses, those that grow as singular plants in clumps, tufts, hummocks, or bunches, rather than forming a sod, are more sensitive to fire. So rangelands dominated by these species “may require more consideration for post-fire grazing management.”

These recommendations work best on rangelands dominated by western wheatgrass, needle and threadgrass, blue grama, prairie junegrass, threadleaf sedge, and Sandberg bluegrass.

They also note that after a fire there will be less forage available so stocking rates will need to be adjusted accordingly.

What Can You Do With This Information?

Prairie fires are becoming more and more common. Long considered an area at low risk of wildfire, a new USDA Forest Service study shows that fires in the grasslands of the Great Plains have increased from 33 per year in the 1985-1994 period to 117 per year in the 2005-2014 period. So fire could be coming to a pasture near you. If you’re grazing federally managed lands, you can use this paper to begin a conversation with your range conservationist or local land manager to discuss how grazing might be handled in the event of a fire. Turning the ship of state is like trying to turn an oil tanker on the ocean, so you’ll want to give everyone time to consider the science, talk about the specifics of your sites, and how you can all work together to come up with a plan that will preserve natural resources and your business.

If you’re on staff at a federal agency managing grazing, we’d love to hear from you about the effect of studies like this on the work that you do and how we can all work together to come up with good solutions. You can respond in the comments below, or drop Kathy Voth a line if that works better.