The stakes could hardly be higher. The eastern migratory monarch population has declined by roughly 80 percent over the past thirty years in the United States, a collapse driven by the loss of host and nectar plants, land use change, and pesticide exposure. Milkweed, the obligate host plant on which monarch females lay their eggs and on which monarch caterpillars feed exclusively, has fared especially poorly in agricultural landscapes. Researchers estimate that milkweed in the midwestern United States declined by about 40 percent between 1999 and 2014, a loss of approximately 860 million stems. Because an estimated 38 percent of monarchs overwintering in Mexico originate from the Midwest, that loss has been directly correlated with declines in the overwintering population.

Conservation planners have proposed adding 1.6 billion milkweed stems across the Midwest to stabilize the overwintering population at its six-hectare target. Much of the effort to date has focused on croplands, roadside rights-of-way, and urban plantings. But rangelands, the unplowed expanses of native grasses and forbs that cover nearly twice as much land as croplands in the United States, remain an understudied frontier. Prior estimates suggest rangelands already host some 4.1 billion milkweed stems across 13.2 million hectares. The question the new study set out to answer was whether ranchers could push those numbers higher, not by taking land out of production, but simply by changing how cattle are distributed across the landscape in space and time.

The research team, led by Ellysa Johnson of North Dakota State University, conducted their experiment at the Central Grasslands Research Extension Center near Streeter, North Dakota, between June and August of 2021 and 2022. The site sits in mixed-grass prairie on the western edge of the eastern monarch’s breeding range, and it supports eight native milkweed species, although five species dominated the counts: common milkweed made up 72 percent of stems, showy milkweed 20 percent, oval-leaf milkweed 8.4 percent, with swamp and green comet milkweed trailing far behind. Over two summers, the researchers walked 288 transects, each 150 meters long, three times per season, tallying every milkweed stem, every flowering forb, and every monarch egg, caterpillar, and adult they encountered.

The experimental design was the heart of the study. The team compared three grazing strategies that had been implemented in 2017, each replicated across four 64-hectare pastures stocked at similar rates with cow-calf pairs. The first, season-long grazing, is the most common approach in the Northern Great Plains: cattle roam the entire pasture continuously through the growing season with no internal fencing. The second, patch-burn grazing, exploits a phenomenon called pyric herbivory, in which cattle are drawn to lush, nutritious regrowth on recently burned patches, leaving older unburned areas comparatively lightly grazed. A quarter of each patch-burn pasture is burned annually on a four-year rotation, creating a mosaic of vegetation structure. The third strategy, a modified twice-over rest-rotation system, builds heterogeneity without fire by dividing each pasture into four fenced paddocks: one fully rested for the year, one moderately grazed, one fully grazed, and one heavily grazed, with paddock roles rotating between years so that each section experiences a shifting disturbance regime.

The results were striking. Across both years, the team counted 33,893 milkweed stems, and abundance differed sharply among grazing strategies. The modified twice-over rest-rotation system supported significantly more milkweed than either patch-burn grazing or season-long grazing. The researchers attribute this to the rested paddock: because cattle will eat milkweed when it is available, completely removing grazers from a portion of the pasture for a full growing season gives the plants a reprieve. In both patch-burn and season-long pastures, cattle have continuous access to milkweed across the entire area, and even though patch-burn cattle concentrate most of their foraging on burned patches, they still target flowering plants throughout the pasture. Notably, milkweed abundance did not decline during the severe drought that gripped the site in 2021, likely because the dominant species, common and showy milkweed, are clonal plants whose underground networks make them resistant to disturbance.

Flowering forbs told a more complicated story. The team recorded 185,375 flowering forbs from 117 species, and their abundance depended on an interaction between grazing strategy and year. The rest-rotation system hosted the most flowers overall, but the advantage was significant only in 2021, the drought year. That year, patch-burn pastures suffered a dramatic floral crash, a consequence of burning early in the season followed immediately by extreme drought and grazing pressure. Yet the recovery was remarkable: flowering forbs in the patch-burn treatment rebounded fourfold in 2022, matching the other strategies. The rested paddocks of the rest-rotation system, which held 40 percent of the treatment’s flowering forbs in the drought year compared to 21 percent in 2022, appear to have functioned as refugia, sheltering the floral community from concentrated grazing when drought reduced forage production across the landscape.

The monarch counts tracked the vegetation. Over two summers, the researchers detected 182 adult monarchs and 165 juveniles, all of the eggs and caterpillars found on either showy or common milkweed. Statistical models revealed that both adult and juvenile monarch abundance were positively and significantly correlated with milkweed abundance, meaning that as milkweed stems increased, monarchs of every life stage followed. What surprised the team was what did not matter: flowering forb abundance showed no significant relationship with monarch counts, for either adults or juveniles, despite the fact that adult monarchs depend on nectar to fuel their flights. The authors caution that the survey design could not fully disentangle the two resources, because plots with abundant milkweed rarely also had abundant flowers, but the takeaway is clear: in these rangelands, milkweed is the resource that best predicts monarch presence and reproduction. Altogether, the rest-rotation pastures supported 192 monarchs, versus 76 in patch-burn and 79 in season-long grazing.

The milkweed densities the team measured, between 445 and 634 stems per hectare, fall below figures from more eastern states such as Minnesota and Iowa, where densities approach 1,500 stems per hectare, but they are consistent with averages from protected grasslands across the eastern monarch range, around 399 to 543 plants per hectare, and far above densities reported from Nebraska. For a region north of the 45th parallel, on the edge of the breeding range, those are respectable numbers, and they demonstrate that even actively grazed rangelands can produce high milkweed densities. The monarch is currently proposed for listing as threatened under the Endangered Species Act, and the findings suggest that rangelands, particularly in the Prairie Pothole Region, can serve as working landscapes that support both livestock production and monarch conservation simultaneously, without requiring land to be retired from ranching.

The broader lesson is about heterogeneity, the patchwork of different vegetation structures and disturbance intensities that fire and grazing historically created across the Great Plains. Both the patch-burn and rest-rotation strategies are explicitly designed to recreate that mosaic, and both supported more monarchs than uniform season-long grazing. The magnitude of the benefit, however, depends on how disturbance is distributed in space and time, and on how weather interacts with management, as the drought-driven floral collapse in the patch-burn pastures made plain. The researchers recommend adaptive management, particularly adjusting grazing in pastures scheduled to burn ahead of severe drought, and call for future work on how the timing of grazing and milkweed species identity shape oviposition preference, predator pressure, and monarch success. What is already certain is that billions of milkweed stems are waiting on rangelands, and with the right grazing playbook, ranchers may become some of the monarch’s most important allies.

Subject of Research: Effects of livestock grazing strategies on milkweed availability and monarch butterfly abundance in Great Plains rangelands

Article Title: Novel grazing strategies increase milkweed (Asclepias spp.) availability and monarch (Danaus plexippus) abundance in rangelands

Article References: Novel grazing strategies increase milkweed (Asclepias spp.) availability and monarch (Danaus plexippus) abundance in rangelands. (n.d.). https://doi.org/10.3389/fevo.2026.1914674

Image Credits: AI Generated

DOI: 10.3389/fevo.2026.1914674

Keywords: monarch butterfly, milkweed, rangelands, grazing management, patch-burn grazing, rest-rotation grazing, conservation, pyric herbivory, drought, prairie pothole region, pollinators, working landscapes