Final Report for LNC07-277
Patch-burn grazing, continuous grazing and rotational grazing treatments were applied in spring 2008 and 2009 on five participating ranches in eastern South Dakota. Vegetation data was collected to evaluate forage quality, diversity, and structure. Analyses validated the ability of patch burn-grazing to create more heterogeneity than rotational or continuous grazing. Concern was noted by producers regarding forbs (specifically goldenrod species) was increasing under patch-burn grazing. This concern was raised by producers on field tours. Calf weights were measured between patch-burn grazing and continuous grazing, and no differences were found. Patch-burn grazing may need to be conducted at moderate stocking rates to ensure appropriate fuel loading for next year’s burn compared to rotational or continuous grazing. Heavier stocking rates on pastures that are continuous or rotationally grazed would produce more beef per acre than patch-burn grazing. Tours were held on several research sites and were attended by professional grassland managers represented by state, federal, and non-governmental agency personnel as well as rancher participants. We also had additional tours for ranchers and grassland managers demonstrating the pluses and minuses of patch-burn grazing. Keen interest exists by ranchers to conduct prescribed fires. A rancher workshop was held on how to conduct prescribed burns in August 2010. Patch-burn grazing in the north central U.S. works to create more structural heterogeneity than traditional grazing systems but it does come at higher cost to the producer.
During the last 30 years, scientific scrutiny of standard grazing practices designed to optimize livestock production on grasslands of the Great Plains has led to observations that other ecosystem services that grasslands provide are constrained. In addition to livestock production, grasslands provide habitat for wildlife, sequester carbon, improve water quality, and are a refuge for maintaining native plant diversity. Strong profit motives are likely responsible for decisions to apply high stocking rates that often result in a landscape dominated by shorter, more grazing resistant plant communities. It’s easy to place blame on this practice because of the striking impacts overstocking has on wildlife, water quality, and plant diversity. Yet, some more sophisticated techniques of rotational grazing often result in a uniform vegetation structure across the landscape, albeit not as short as heavy, continuous grazing.
Recent investigations suggest that age-old management tools aimed at improving grazing distribution have resulted in a predictable habitat structure that may actually reduce plant and animal diversity across a managed landscape. Grassland bird diversity has become the focal illustration to prove this point. Proponents of a multiple-use concept, meaning grasslands could be managed for both ecosystem goods (livestock production) and services (habitat, carbon sequestration, diversity, etc.), argue for a change in how we manage livestock grazing. Historically and still to this date, rest-rotation and deferred rotation grazing strategies have been effective tools at providing contrasting vegetation structure that have benefited wildlife. In the last 10 years, a new strategy called “patch-burn grazing” has been proposed as an alternative to conventional rotational grazing techniques (Fig. 1).
The concept behind patch-burn grazing is to reintroduce an ecosystem processes that once dominated the Great Plains. Historical evidence suggests that fire and large ungulate grazing co-existed for a long time in the Great Plains such that these ecosystem processes created a shifting mosaic of vegetation structure across the landscape. This mosaic of vegetation structure created a tight evolutionary bond between plants and animals and maximized regional diversity. Patch-burn grazing offers several intriguing benefits to the landscape and the land manager. First and most obvious, patch-burn grazing brings back fire as a management tool to the landscape. Fire has been suppressed in the Great Plains for the last 100+ years. There are unique places such as the Flint Hills of Kansas or the Osage Hills of Oklahoma where fire is still a consistent part of the land management process. However, for the most part fire is quite rare, especially in the northern Great Plains states of Nebraska, South Dakota, and North Dakota. Secondly, no rest period is required after fire. Cattle can be placed in the pasture immediately after the burn or can even be in the pasture when the burn occurs. Thirdly, no fences or additional water developments are needed. The desired outcome of patch-burn grazing is to increase the structural heterogeneity across the landscape and hopefully result in increases in plant and animal diversity; “if you build it, it will come”. Several recent publications from the southern Great Plains have provided evidence that plant and animal diversity is increased through patch-burn grazing. Traditional grassland management has shown fewer forbs, insect species, and grassland birds compared to patch-burn grazing.
This patch burn-graze strategy has been tested in the tallgrass prairie ecoregion of Oklahoma and Kansas where the predominant native grass species are warm-season and late-spring burns are effective in controlling exotic cool-season species. In addition, these experiments have been carried out on large tracts (>4,000 acres) in a region where grassland fragmentation is less severe. In the northern tallgrass prairie region of the US, landscapes are highly fragmented, tract sizes are smaller, and dominant plant species are quite different which makes them vulnerable to invasive species, and reduces environmental quality.
1. Knowledge of patch burn-graze impacts on change of plant community structure
2. Scientific community informed about vegetation change of patch burn-grazing
3. 100 ranchers informed about patch burn-grazing
4. 6 cooperators measure vegetation diversity
5. Improved diversity and structural characteristics of pastures for the 6 cooperating ranchers
6. 25 regional ranchers adopt patch burn-graze management strategies
We used a completely randomized design with five replications of the continuous grazing treatment, three replications of the rotational grazing treatment, and four replications of the patch-burn grazing treatment. Controlled burns on the patch-burn graze treatment were conducted by The Nature Conservancy (TNC) personnel each spring. The patch-burn graze treatment was divided into four equally sized sections and one of these sections, determined by fuel load, was burned each year, thus receiving a fire return interval of once every four years. Each grazing treatments was partitioned into four subunits. Within each subunit, a sampling area was permanently marked by a flag or t-posts and global positioning system (GPS). These sample locations were used for the length of the study. The Robel et al. (1970) pole method was used to measure visual obstruction at each of the four sampling locations within the treatment areas. Dimensions were taken at each of the four cardinal directions from the Robel pole with each dimension averaged to achieve one number. Dimensions are marked at 2.5 cm increments with visual obstruction increments averaged to the nearest 2.5 cm. Five measurements within each unit area of treatment location were recorded at the four cardinal directions around the permanently marked site to achieve 20 dimensions per sampling area, and 80 samples per treatment every June and September in 2007, 2008, and 2009. Cover estimates were made using a 0.25 m2 frame at each sampling site. Five measurements were taken about 20 paces apart at each of the four cardinal directions around the sampling site, totaling 20 measurements per sampling area and 80 samples per treatment. Percent cover of litter, bare ground, and plant functional groups; native grass, introduced grass, native forbs, introduced forbs, and shrubs were estimated by ocular estimates. Measurements were taken late June/early July and late August in 2007, 2008, and 2009.
At one of the producer sites, calf weights were measured on the two grazing systems in 2008 and 2009 and gain per acre was calculated. The stocking rate for the patch-burn was 0.88 and 0.91 AUM/acre in 2008 and 2009, respectively. The stocking rate for the continuous season-long system was 1.79 and 1.69 AUM/acre in 2008 and 2009, respectively.
Forage quality was measured in 2009 from taking two samples at each of the four sampling locations within the grazing treatments in mid-June, mid-July, and mid-August. Samples were analyzed for crude protein, acid detergent fiber, and neutral detergent fiber.
Runoff and sediment yield was measured in late June 2008 from the grazing treatments using the Cornell sprinkler infiltrometer. Two samples were collected at each of the four sampling locations within each treatment.
Cover, visual obstruction, forage quality, runoff and sediment yield were all calculated and analyzed using proc mixed (SAS 2006). Calculations were obtained individually for each year. The calculations included location, treatment and location x treatment as fixed effects and site and site x treatment as random effects. Means were separated using the least squares PDIFF option when the F-test of the main effects was significant (p-value < 0.10). Principal component analysis (PCA) was also used to analyze visual obstruction, litter cover, and cover of plant functional groups taken in August 2009 using PROC PRINCOMP (SAS 2006). Areas were analyzed independently since all treatment areas have vegetation composition differences. Principal component analysis was used in order to show changes of locations in heterogeneity and vegetation associated with fire.
Results of patch-burn grazing studies conducted in the northern tallgrass prairie of eastern South Dakota indicate similar ecosystem processes of burning and grazing have resulted in increased spatial structural heterogeneity across the landscape. We used a multivariate technique called principal component analysis to compare continuous seasonal grazing to patch-burn grazing in vegetation structural components across the landscape. As shown in Figure 2, the first axis, which accounts for 29% of the variation, separates the sites based on native forbs and shrubs versus introduced grass and visual obstruction. The second axis, which accounts for 24% of the variation, separates the sites based of native grass, litter, and visual obstruction versus introduced forbs.
Sampling locations within treatments were separated based on principal component scores for axis 1 and axis 2 (Figure 3). Continuous grazing resulted in sampling locations being more similar as determined by the area outlining the four sampling locations (Fig. 3). Patch-burn grazing sampling locations were spread farther apart based on principal component scores. The sampling locations in the rotational grazing treatment were also more uniform as evident by the smaller perimeter in Fig. 3. These data provide evidence that structural heterogeneity is increased through patch-burn grazing compared to traditional continuous grazing or rotational grazing in native northern tallgrass prairie.
The determining factors that are responsible for causing structural heterogeneity (visual obstruction and native forbs) in the central and southern tallgrass prairies of the Great Plains were not as prominent in the northern tallgrass prairie. Previous research has shown that livestock will spend 80% of their grazing time on recently burned areas, thus giving rise to the structural differences between burned and unburned areas within a pasture. Although, we did not measure grazing time spent on the burned areas, we measured forage quality throughout the pasture during three times throughout the grazing season. Crude protein was greater and acid detergent fiber (inversely related to digestibility) was lower in June for patch-burn grazing compared to continuous season-long grazing (Table 1). This was most likely caused by the recent burn (May) that attracted livestock to that area. The fresh regrowth would likely have been more palatable and thus would have been grazed heavier. However, in July and August there were no differences in forage quality between the grazing systems (Table 1). This reinforces that in the northern tallgrass prairie, there is abundant green vegetation later in the growing season (July and August) compared with the central southern tallgrass prairie.
This even distribution of forage quality throughout the grazing season suggests that cattle were more likely to graze the entire pasture more evenly. Thus cattle likely did not spend the majority of their grazing time on the burned patch.
We have not yet collected other ecosystem service measurements, such as invertebrate or grassland bird diversity. Further research is needed to confirm the response to patch-burn grazing by other ecosystem structural components. However, we did measure water runoff and sediment yield from continuous season-long grazing and patch-burn grazing in 2008. We used the Cornell sprinkler infliltrometer to measure runoff and sediment yield from four locations within the continuous season-long and patch burn-grazing systems. Areas that were burned had greater runoff and sediment yield than non-burned areas in the patch-burn grazing system (Table 2). This data suggests that runoff and erosion from recently burned areas could be a concern during a period before the vegetative canopy reforms if burned patches are located near critical riparian zones. Another concern could be an increase in soil bulk density from cattle grazing burned-patches during wet periods. Our results showed that soil bulk density was 6% greater on continuous season-long (1.07 g/cm3) compared with patch-burn (1.14 g/cm3) grazing.
Since the stocking rate was higher for the continuous season-long system and there were no differences in average daily gain, gain per acre was almost twice as high for the continuous season-long grazing system than patch-burn grazing (Table 3).
This data suggests that the stocking rate to allow for patch-burn grazing to continue each year cannot be as high as continuous grazing. So producers that graze on the moderate to heavy side (which is reasonable from a beef production objective) may have to reduce their stocking rate to incorporate patch burn grazing. This data set helps us understand that patch-burn grazing on private lands may produce less beef per acre compared to traditional management.
- Fig. 2. Eigenvector loadings on principal component axes for vegetation components measured in August 2009 from continuous grazing, patch-burn grazing, and rotational grazing in eastern South Dakota native prairie. First principal component (Eigenvector 1) and second principal component (Eigenvector 2) accounts for 29% and 24% of the variation respectively. VO= visual obstruction, IG=introduced grass cover, L=litter cover, NG=native grass cover, S=shrub cover, NF=native forb cover, and IF=introduced forb cover.
- Fig. 3. Principal component scores of vegetation measurements conducted in August 2009 from sampling locations within continuous grazing (closed markers), patch-burn grazing (open diamond, triangle, and square), and rotational grazing (open circle) in eastern South Dakota native prairie.
- Table 2. Runoff (cm/min) and sediment yield (kg/ha) from continuous season-long and patch-burn grazing systems using a Cornell sprinkler infiltrometer measured in late June 2008 in eastern South Dakota native prairie.
- Table 3. Beef gain per acre (lbs/acre) for continuous season-long and patch-burn grazing at one producer site in eastern South Dakota.
- Table 1. Crude protein, acid detergent fiber, and neutral detergent fiber measured in mid-June, mi-July, and mid-August from continuous season-long and patch-burn grazing systems in eastern South Dakota native tallgrass prairie in 2009.
Knowledge of patch-burn grazing has led to a better understanding of how to create structural heterogeneity across the landscape using fire and grazing. Research and tours validate the environmental benefits of grasslands managed by fire and grazing. However concerns by some ranchers and grassland managers, resulting in increased forbs and heavier focused grazing, has resulted in reservation of endorsing patch-burn grazing. Additionally, moderate stocking rates are necessary to perpetuate the continual process of annual fuel buildup to successfully burn each year. Producers in this region tend to graze between moderate to heavy and may not be able to maintain enough fuel to sustain patch-burn grazing on an annual basis. Beef production per acre also was lower on patch-burn grazing because lower stocking rates were required to maintain adequate fuel loadings. This is a major concern for producers whose main goal is beef production.
A simple economic analysis showed that producers can make approximately $50/acre more in revenue compared to patch-burn grazing because of the higher stocking rates. In addition, the burns were applied by The Nature Conservancy who was reimbursed from this grant. In 2010, the NRCS cost shared prescribed fire at $36/acre in South Dakota for mid contract management of CRP. Using this figure, it is reasonable to equate the cost of burning to equal the cost share rate the NRCS uses. Given these constraints producers likely would be reluctant to adopt patch-burn grazing in the north central U.S. If society values the benefits of increased structural heterogeneity produced by patch-burn grazing, then this economic analysis could be used to value an economic incentive payment.
This study is likely to see very limited farmer adoption in the north central US because of concerns with weeds and changes in carrying capacity. Most likely adoptors will do it for wildlife benefits. These may include non-profit groups such as The Nature Conservancy or state and federal conservation agencies like state Game Fish and Pards departments or the US Fish and Wildlife Service.
Educational & Outreach Activities
Smart. A. 2008. Managing grasslands for structural heterogeniety. Center for Grassland Studies. Fall Issue 14(Vol. 4) pp 1 and 5.
Areas needing additional study
Further research is needed to assess animal diversity in this highly fragmented region of the northern Great Plains.