Baseline productivity

This section just reviews forage productivity and recommended stocking rates for pastures along the southern boundary of Fort Keogh.

Area (acreage) of candidate patch-burning pastures.
pasture Acres
Upper Lignite Creek 12416
Upper Coal Creek 10267
3 Pasture 7653
2a North 5468
Moon Creek 4662
2a South 4366
N 4 2485
Sandridge North 1463
S 4 1463
Sandridge South 1207

Productivity data from SSURGO Rangeland Productivity:

Stocking rate calculations follow the MT NRCS stocking guide.

\[ 1~AUM = 915~lbs = 30~lb~d^{-1}~\times30~d \]

Then multiplied by an efficiency factor, 0.25.

Available AUMs by candidate patch-burn pasture under three precipitation scenarios.
pasture high low norm
Upper Lignite Creek 3418 1847 2641
Upper Coal Creek 3311 1742 2523
3 Pasture 2790 1437 2094
2a North 2098 1084 1575
Moon Creek 1858 931.9 1396
2a South 1642 855.4 1246
N 4 711.7 388.3 549
Sandridge North 625.5 315 472.1
Sandridge South 488.8 245.7 362.7
S 4 450.2 247.4 348.6

So if a grazing season is to last 5 mo, the following number of cows would be assigned to each pasture: First, script to calculate stocking rate

pbg_prod %>%
    mutate(acres = as.numeric(st_area(.) * 0.0002471054), # sq. m. to acres
         across(c(low, norm, high), ~ . * acres) ) %>%    # per-acre production x acreage
  pivot_longer(names_to = "scenario", 
               values_to = "t_ac", 
               cols = c('low', 'norm', 'high')) %>%
  filter(!is.na(t_ac)) %>% 
  group_by(scenario, pasture) %>%
  summarize(productivity = sum(t_ac),           # total productivity for each unit
            .groups = 'drop') %>%
  mutate(pairs = ((productivity*eff)/915)/5,    # stocking for 5 mo; eff = efficiency factor
         pairs = round(pairs, 0)) %>%
  as_tibble() %>%
  select(-geometry, -productivity) %>%
  pivot_wider(names_from = scenario, 
              values_from = pairs) %>%
  arrange(desc(norm)) %>%
  pander::pander("Number of cow-calf pairs to stock for 5 mo at 50% forage consumption under three precipitation scenarios.")
Number of cow-calf pairs to stock for 5 mo at 50% forage consumption under three precipitation scenarios.
pasture high low norm
Upper Lignite Creek 684 369 528
Upper Coal Creek 662 348 505
3 Pasture 558 287 419
2a North 420 217 315
Moon Creek 372 186 279
2a South 328 171 249
N 4 142 78 110
Sandridge North 125 63 94
Sandridge South 98 49 73
S 4 90 49 70

Potential study designs

The proposed patch-burning study will compare two different fire regimes:

Grazing within each is discussed below.

Given the variability of pasture sizes in the candidate set and non-study needs for grazing availability, the candidate patch-burning pastures are classified into two categories:

Study pastures

8 smaller to mid-size pastures selected for research in what could formally be called a patch-burn grazing study, in that fire and grazing regimes will both be experimentally manipulated.

In addition to the two fire regimes (4-yr and 8-yr fire return intervals), two grazing regimes will be applied:

  • Annual grazing in which each pasture will be stocked in every year of the study
  • Biennial grazing in which each pasture will only be grazed every other year

Management pastures

These pastures ought to have a fire management plan but likely cannot have grazing regulated enough to be included in a formal experiment. They are also huge (an order of magnitude larger area than study pastures). As such, I propose the same fire regimes (4-yr and 8-yr return intervals) on 1/4 of the total area, but divided between two fire seasons (1/8 in fall, 1/8 in spring).