Cattle foraging on milkweed: Intra‐seasonal herbivory shifts and implications for monarch butterfly conservation

Managing milkweed ( Asclepias spp.) is of high conservation concern due to its association with monarch butterflies ( Danaus plexippus L.), which are proposed for listing under the Endangered Species Act. To counteract monarch declines, conservation goals aim to establish ≥1.6 billion nonagricultural milkweed stems within the Midwestern United States. Rangelands offer a unique opportunity to promote milkweed within the eastern migratory monarch range, but in grazed rangelands, little is known about the influence of herbivory on milkweed. To learn more about cattle−milkweed interactions, we measured the abundances of ungrazed and grazed milkweed stems in North Dakota mixed‐grass prairie rangelands throughout the growing seasons of 2021 and 2022. We counted stems in three different grazing management strategies: (1) modified twice‐over rest‐rotation grazing (MTORG), (2) patch‐burn grazing (PBG), and (3) season‐long continuous grazing (SLG). We found that total available (ungrazed plus grazed) and grazed oval‐leaf milkweed ( Asclepias ovalifolia Dcne.) were much rarer than other milkweeds, which included common ( A. syriaca L.), showy ( A. speciosa Torr.), and their hybrids (common/showy milkweed). Cattle grazed only 2% of oval‐leaf but 13% of common/showy milkweed stems across all grazing strategies. However, the amount of grazing increased as the growing season progressed, and common/showy milkweed went from 3% of stems being grazed to upwards of 30% of stems being grazed at the end of the season. Oval‐leaf and common/showy milkweed total availability was impacted by grazing strategy, with MTORG and PBG generally having greater abundances than SLG. Only the number of grazed common/showy milkweed was impacted by grazing strategy, with MTORG having more grazed stems compared to PBG and SLG. Our results indicate that consumption was not solely a function of stem availability, as herbivory of common/showy milkweed increased throughout the grazing season despite availability not changing. Overall, we found that cattle herbivory of milkweed differed by milkweed species and through the growing season, with some changes due to management. Reducing grazing pressure later in the summer where common/showy milkweed is abundant will improve monarch butterfly conservation in rangelands by moderating cattle−milkweed interactions.