Targeted grazing, conceptualized as the strategic deployment of specific livestock types at designated times, durations, and intensities to achieve desired vegetation or landscape outcomes (1), is an increasingly important tool for grazing land managers. Targeted grazing takes advantage of complex plant-herbivore interactions that influence plant communities to achieve desired results such as control of individual plant species and removal of fine fuels to mitigate fire risk. It is possible to achieve such goals because livestock can be managed to selectively defoliate the available herbage (2), including undesirable plant species. Targeted grazing provides the integrated advantage of food and fiber production along with control of undesirable plant species (3), distinguishing it from alternative control methods such as herbicides and mechanical removal. However, the efficacy of these strategies is often influenced by environmental conditions, particularly drought, which can alter plant availability and nutritional quality.

Targeted grazing is typically applied to invasive species like leafy spurge (Euphorbia esula L.) and spotted knapweed (Centaurea stoebe L.), yet its applications extend beyond invasive plant control to encompass various contexts, including the removal of toxic plant species. Larkspurs (Delphinium spp.) are a serious toxic plant concern for cattle grazing on rangelands of the western U.S. as they contain numerous diterpenoid alkaloids that block nicotinic acetylcholine (nAch) receptors at the muscular synapse (4, 5). This can result in muscular paralysis, bloat, and death of cattle (6). Sheep exhibit significantly higher resistance to larkspur alkaloids compared to cattle, with a demonstrated tolerance four to six times greater (7, 8).

Sheep at times may preferentially select forbs over grasses due to their distinct prehensile mouth structure and digestive physiology compared to cattle (9, 10). In addition to differences between cattle and sheep on preferential intake of plant functional groups (11), grazing behaviors and dietary preferences differ among sheep breeds. This is particularly evident between hair sheep breeds and fine and medium wool breeds. For example, Dorper sheep exhibit a preference for shrubs (36%) over grasses (64%), in contrast to grass dominated diets of Merinos (86%; 12). Moreover, the U.S. sheep industry exhibits substantial genetic diversity across various breeds, and recent advancements in this domain have enabled producers to enhance productivity and achieve a range of industry objectives (13–15). Given the genetic variability and observed differences in grazing behavior among these breeds, further elucidating these distinctions would be instrumental in optimizing precision-targeted grazing systems.

Although broad trends appear between different species and breeds, grazing is complex and involves a variety of factors beyond inter-animal comparisons. Botanical diversity and composition, past experiences of the animal, recent diet, and post ingestive feedback to plant secondary compounds can influence dietary selection and preference (16–18). These challenges intensify under drought conditions, where climatic extremes further restrict forage availability and quality, requiring more precise management strategies for successful targeted grazing. Precision grazing management integrates precision livestock farming (PLF) techniques to enhance resource efficiency and animal monitoring, with the goal of improving animal performance, nutritional status, welfare, health, and forage utilization (19, 20). In the context of targeted grazing for plains larkspur control, precision grazing management would entail removal of larkspur while ensuring optimal forage availability and protecting both sheep and cattle welfare. Thus, the need to quantify diet selection and preference of grazing animals is important in targeted grazing, especially when co-managing both livestock species with sheep grazing preceding cattle grazing.

A changing climate coupled with land use conversion and changing plant communities increases the need for precision multispecies grazing management for control of noxious plant species (21, 22), and bite counts and fecal DNA barcoding (f.DNA) in tandem are emerging tools that can aid grazing land managers. Integrating multiple grazing species can enhance rangeland conditions by optimizing forage resource utilization, bolstering carrying capacity, and fortifying ecosystem resilience (10, 23). However, managing multispecies grazing under drought conditions presents challenges for targeted grazing strategies. These challenges arise from the need to align plant species and phenological stages with the dietary preferences of both sheep and cattle, a task further complicated by the variable precipitation typical of northern mixed-grass prairie ecosystems. This is particularly evident when the objective of targeted grazing is controlling poisonous plants such as plains larkspur without compromising forage availability for subsequent cattle grazing. Thus, underlying knowledge of forage preferences and dietary selection of sheep targeted grazing is key to designing precision grazing management strategies. Here, our objectives were to investigate the effects of (1) stocking density and (2) breed and age on sheep foraging behavior and dietary preferences for plains larkspur using bite-count and f.DNA methodologies in a northern mixed-grass prairie under drought conditions.

Under drought conditions neither sheep breed nor age influenced bite count proportions or f.DNA diet proportions of grasses, forbs, and plains larkspur. The lack of breed and age differences for selection of plains larkspur concurs with low palatability of this species during early growth stages (34–36). The timing of targeted grazing in the current study (late May to June) coincided with the early vegetative phenological stage of plains larkspur. Moreover, desirable cool-season (C3) grasses expressed their growth during this period which further reduced desirability of plains larkspur. Pfister et al. (37) recommended that sheep could be herded onto discreet patches of larkspur to reduce larkspur abundance (i.e., targeted grazing), while acknowledging that when larkspur is more uniformly distributed across a pasture, a herding strategy for larkspur reduction is more limited.

Due to their purported generalist grazing tendencies (12), Dorper sheep were initially hypothesized to exhibit less avoidance for plains larkspur and greater forb components in their diets, although their prior experiences and whether they had been exposed to larkspur previously was not known. Drought conditions and lack of biomass across all plant functional groups likely constrained expression of divergent grazing behavior across breeds. Periodic drought has been shown to decrease species richness and above-ground plant biomass in grasslands (38–40), and in the current study where precipitation was 46% of the historic average, biomass was 1,215 kg/ha compared to 1,547 kg/ha under normal conditions (36). Under ideal experimental conditions characterized by high plant biomass and forb plant components, differences in dietary preferences might be more pronounced (12). Thus, it’s likely that under drought conditions where biomass is limited that breed differences matter less than timing, duration, and stock density. Nonetheless, this study offers insights into grazing dietary selection in drought conditions common to the U.S. high plains region, which are becoming increasingly common in a climate change scenario. Future research on sheep breed dietary preference merits further investigation, especially under more conducive grazing conditions of adequate biomass and plant species diversity.

Previous research at this site observed significant reductions in larkspur density (plants/m²) at moderate and high stocking densities but not light stocking density with sheep grazing. Stock density was more influential than breed or age of sheep in the current study, which is similar to grazing behavior differences (e.g., straighter lines of grazing; 41) and lower weight gains (42) seen with cattle and high stock density; however, standing crop estimates also indicated that sheep grazing could cause significant reductions in available biomass, especially during a drought year (36). In arid and semi-arid rangelands, adaptive grazing to ensure adequate recovery of range condition through plant regrowth is easier achieved when grazing pressure is reduced (43–45). Therefore, a lighter stocking rate may be more effective in a multispecies grazing scenario. The lack of differences in observed bite counts of grasses and forbs between the high and light stock density was unexpected, as we initially hypothesized that sheep would select for more forbs and fewer grasses at a light animal density. Though this was partially true because we saw differences between the high density and d 1 of the moderate density treatments, we attributed the lack of differences between high and light densities to the stocking rate of our high and moderate density treatments. Both treatments were grazed at the same stocking rate (40 Animal Unit Days (AUD)/ha), even though stocking density differed between the two treatments. Hunger resulting from multiple days of high grazing pressure and inadequate forage availability likely constrained the selectivity of these animals when entering the light density treatment. A crucial aspect of targeted sheep grazing is that it will require high capacity in adaptive decision making to respond to changes in climate and forage availability within and across years, when applied in general as well as for control of larkspur. For example, utilizing predictive grazing tools such as Grass Cast and U.S. Drought Monitor combined with ground-based assessments of animal utilization and range condition can assist managers in making adaptive decisions (46).

Added complexity for those wanting to use sheep targeted grazing for plains larkspur control is the context associated with ensuring leaving sufficient forage for subsequent cattle grazing. This requires the sheep to actively select for larkspur while leaving sufficient grass biomass for cattle. Optimizing both sheep and cattle grazing on the same area of land will require substantial management acumen for land managers, principally knowledge of ecological sites, plant diversity, and forage preferences of grazers. For example, conflicts in outcomes with cattle and sheep grazing the same area depends on which species grazes first as well as the extent of dietary overlap. Cattle grazing followed by sheep in grass-dominated subirrigated pastures was beneficial by increasing available grazing days by >50% because it facilitated grass regrowth during the peak growing season (47). Conversely, in mixed perennial ryegrass (Lolium perenne)/white clover (Trifolium repens) swards, when sheep grazed first, more reproductive grass stems and lower white clover abundance occurred (48). In the context of poisonous plants for cattle, grazing sheep first is necessary for an optimal sequence in these rangelands which contain larkspur. Grazing is a complex interaction between herbivores and plant communities, depending on more factors than species and breed differences alone. Botanical diversity of the plant community and physiological status of the animal have been shown to influence foraging behavior, which were not addressed in the current study (16, 18). Furthermore, it is not known what plant species this cohort of sheep had dietary exposure to prior to the study, which could be important for targeted grazing (17). Subsequent research could further delineate the selectivity of cattle and sheep in this system based on plant diversity, physiological status, and recent environments of the animals being employed which were not accounted for in the study design.

Knowledge of the species consumed by grazing animals with targeted grazing is fundamental in determining the efficacy of this grazing management strategy (49, 50) and has important implications to precision livestock management in extensive systems. Bite counts are easy to conduct and require minimal equipment but are often conducted on a small subset of the grazing animals (50, 51). The bite count methodology tends to overestimate grasses and underestimate forbs (31). In contrast, f.DNA methods can be used on a much larger set of grazing animals through fecal sampling; however, local knowledge of the plant species for comparative analyses of the laboratory-derived information is needed. The methodology has limitations with accuracy of data from fed-diet studies, including misdetection of species and underrepresentation of warm-season (C4) plants in the diets compared to known fed proportions (52). Despite the limitations, the methodology has been successfully used in the detection of poisonous plants (53, this study).

Neither bite-count nor f.DNA proportions revealed discernable differences between sheep breed or age at the plant functional group level, but the ability to observe specific plant species as well as plant richness and diversity using f.DNA revealed additional insights. Dorper lambs demonstrated significantly higher proportions of short-statured, fine-leaved grasses (blue grama and cheatgrass) compared to Dorper ewes. Due to their small mouth and body size, the lambs will select shorter plants over taller ones (54) and plants with a high leaf:stem ratio (55) for higher dietary quality. The greater botanical diversity selected by Dorper lambs compared to ewes in phase 2 indicates that the lambs grazed more species. It is important to recognize that the Dorper sheep came from the same source flock, so it’s possible they would have been exposed to similar grazing lands with similar plant compositions prior to the study, suggesting there was a naivety component. That is, they had not attained the same experiential knowledge of grazing–and were therefore more exploratory (56).

Diet selection and composition of Rambouillet and Dorper ewes differed little with targeted grazing within a northern mixed-grass prairie during a drought period in late spring/early summer 2022. Minor differences were observed between Dorper ewes and lambs for several plant species and diet diversity according to f.DNA. Drought-induced reductions in plant richness, diversity, and aboveground biomass likely constrained the expression of breed and age selectivity for plains larkspur. High stock density was more influential under these drought conditions, although implementing high-intensity, short-duration grazing of smaller paddocks (e.g., 0.25 to 0.75 ha) may be impractical for managers on a large scale. Herding, both traditional and evolving virtual herding systems (i.e., virtual fencing), might provide alternative ways to increase stock density, but tradeoffs with impacting non-target plant species and available forage for cattle grazing are important to consider. Future research should aim to understand the mechanisms driving animal diet selection and overlap across species, particularly under more favorable grazing conditions with greater precipitation, to improve precision sheep-targeted grazing strategies.