Final Report for GNC15-207
Cattle grazing operations are an important industry throughout the world and a vital component of the economy of the Northern Great Plains. Rangeland management is important to ensure that cattle grazing remains not only profitable but also environmentally sustainable. Conventionally managed rangeland systems that practice continuous grazing and repeated applications of chemicals such as avermectins pose a risk to the continuing productivity of rangelands. These practices have ecological consequences, primarily to the arthropod community that inhabits cattle dung pats. This diverse community works together to recycle dung pats and make the nutrients in dung accessible to the surrounding plant community, an essential role in the functionality of rangelands. The research within this thesis describes the influence of management systems on the dung arthropod community across eastern South Dakota. This community is represented by 172 morphospecies represented by 14 orders of arthropods that inhabit dung pats. We have gained an understanding of the community’s response to dung pat age and their contribution to the degradation of dung within pastures. A dung arthropod community with lower abundance, species richness and diversity will result in a dung pat takings 33-38 days longer to breakdown than pats accessed by an intact arthropod community. The management system strongly influenced the arthropod community and use of continuous grazing and avermectin parasiticides resulted in a disruption of arthropods living in dung. These conventional practices resulted in a lower number of beneficial predators, higher abundances of pest maggot species, and most notably; a decreased in abundance and diversity of dung beetles relative to management systems that used high density, rotational grazing and no avermectins. Dung beetles are keystone species within the dung fauna and their colonization of dung pats is essential for a complete community structure to occur within a pasture’s dung pats. Future research should continue to implement system level changes to a pasture over multiple grazing seasons and observe the changes in the dung arthropod community as well as economics of the management changes.
Accumulated cattle dung reduces ranch profitability by suppressing plant growth, polluting pastures and providing a habitat for pests and parasites of cattle. When a dung pat is deposited on a pasture, all of the available forage around the pat is unused by grazing cattle until the pat is broken apart and incorporated into the soil. A large number of dung arthropods inhabit cattle dung and provide a crucial role in breaking apart dung and increasing the nutrient cycling in rangelands. While a majority of arthropods are beneficial, there are pest species found within the community. Flies (Diptera) are one of the most harmful arthropod pest groups to cattle, affecting many aspects of livestock’s health; their bites irritate cattle and cause stress, weight loss, and transmitting diseases that reduce ranch profitability. Filth flies lay eggs in dung pats where maggots consume dung and mature into adult flies, and thus management decisions that affect dung have important implications for fly management. To manage these pests and reduce economic losses, ranchers apply pesticides to their animals, the most common of which are avermectins. Avermectins are applied in a variety of ways (injections, topical sprays, slow-release bolus capsules, etc.), but their use can result in environmental problems. Avermectins are often excreted in the dung of the treated animal; 80-90% of the active ingredient applied to the animals is found in the animal’s dung. Non-target effects lead to a decrease in dung degradation rate, leaving ranchers with a new set of problems of pasture fouling and reduced nutrient cycling. An alternative to chemical control of flies may lie in regenerative management of cattle herds. Ranchers that practice a more frequent rotation program with high stocking rates can provide a more biologically diverse system that provides resistance to maggot proliferation through competition and predation.
The present report summarizes several studies that observed the dung arthropod community structure, quantified benefits to arthropods presence in cattle dung, and how management practices will affect the community in eastern South Dakota pastures.
Study 1 uses an exclusion/inclusion cage system to evaluate the role of dung arthropods on dung degradation over time in eastern South Dakota. Exclusion cages help to isolate the arthropod community’s contribution to dung pat degradation. We pair exclusion cages with a comprehensive description of invertebrate communities within the dung both early and late in the summer, to understand how elements of this community affect degradation over the season. Identifying the impact of the dung arthropod community on degradation will provide ranchers a greater understanding of the benefits to conserving this poorly understood community.
Study 2 observes the effect of the combination stocking density, rotation frequency and endectocide use on arthropod community. This experiment will observe how the management practices typically used in eastern South Dakota will alter the dung arthropod community. If ranchers in the area are choosing more regenerative management practices, then there could be a correlation to a more complete dung arthropod community structure.
Study 3 will finally use the earlier mentioned arthropod community on ranches representing a series of management systems, with specific attention paid to maggot populations. A systems level approach was used to see whether regenerative rangeland management practices influence the relationship between pest maggot species and their predators and competitors relative to more conventional rangeland systems. The effects of avermectin use on maggot populations are described. An exclusion cage study accompanied these community observations to document maggot population growth in the absence of most competitors and predators. The results demonstrate that ecologically intensive management systems for cattle offer superior maggot suppression relative to systems reliant on avermectins.
Exculsion/inclusion study was conducted on a ranch in eastern South Dakota in the summer of 2016. After collecting cattle dung free of any insecticides, the dung was frozen to remove all arthropods that may be living in the fresh pats. After thawing and weighing the pats were placed into pasture to act as the sentinel pats. Each sentinel pat was placed on top of mesh with 2.5 cm square holes to allow for ease of pat removal and randomly assigned to one of three treatments. The treatments were inclusion: left completely exposed with no covering, exclusion: surrounded by cylinder and covered with mesh screen, and open: again surrounded by a cylinder but with holes cut in the sides and large wire top. This third treatment was added to test whether the exclusion cage had direct effects on dung degradation rates. The experiment was repeated twice throughout the grazing season once beginning on 10-June and once on 28-July. Randomly selected pats were removed 2, 4, 7, 14, 28, and 42 days after the sentinel pats were placed. Once removed, dung degradation and arthropod community were measured in each pat..
The multi-ranch dung community analysis used ranches (n = 16; 10 in 2015 and six in 2016) represented a variety of cattle management practices in eastern South Dakota. The systems were ranked as regenerative, intermediate, or conventional based on cattle management system consisting of several practices. In our study, the most regenerative ranches do not use avermectins, and rotate cattle herds frequently. Conventional systems involved cattle given multiple avermectin treatments along with lower stocking densities in continuously grazed pastures or those with infrequent rotation. Each of the ranches was sampled monthly from May to September. Two- to 5- days old dung pats (n = 10 per ranch) were randomly selected from each ranch and all arthropods were collected from the pats using Berlese funnel system. All extracted arthropods were identified microscopically and cataloged; each unique specimen was identified to the lowest taxonomic level possible representing a functional morphospecies. The amount of avermectin in dung pats was quantified using a direct, competitive enzyme-linked immunosorbent assay (ELISA) test.
A total of 109 morphospecies were found in the sentinel dung pats that were made up by nearly 87,000 arthropod specimen. Dung pats in the pasture had different arthropod communities when arthropods were excluded and as the dung pat aged. Arthropod biomass inside the exclusion cages was 10% of the biomass found in the inclusion pats early in the season and exclusion pats had 13% of the biomass of the inclusion later in the summer. The biomasses of arthropods were significantly greater on younger pats (2, 4 and 7 days old) versus older pats (14, 28, 42 days old) both early and late in the season. Likewise, arthropod abundance was significantly affected by exclusion and time both early and late in the season. In both early and late seasons, the 14, 28, and 42 days old dung pats all had significantly fewer arthropods compared to younger pats. Exclusion dung pats only averaged 52% and 57% of the arthropod specimens that were found in the inclusion dung pats in early and late seasons, respectively.
Arthropod exclusion and time had significant effects on dung pat wet weight in both the early and late season. Dung pats colonized by arthropods weighed less than the exclusion pats, with greatest losses coming in the first 4 days when arthropod abundance and biomass was highest. The ash-free organic matter (AFOM) of the dung pats was also significantly affected by exclusion and time in the early and late seasons. Throughout the season the uncaged dung pats had significantly less AFOM than the caged pats. Based on our calculated AFOM rate of loss, early season dung pats with arthropods are estimated to completely degrade before 70.76 days. This duration increases to 103.85 days when arthropods are excluded from the pats. Late season dung pats achieve complete breakdown at a faster rate with estimates of 61.49 days and 99.82 days in inclusion and exclusion pats respectively. Although they represented 1.5-3% of the arthropod community recovered, abundance of dung beetles was significantly and positively correlated with total arthropod biomass, arthropod abundance, and total species richness in both the early and late seasons. This shows that dung beetles being found in the pat helps drive the colonization of the dung pat by other arthropods within the community.
In sum, 116,244 arthropod specimens were identified representing 172 morphospecies. Arthropod communities in dung changed over the summer and were different among the range management systems. The number of arthropods differed among months of the season, but abundance was not affected by our range management systems. Arthropod abundance started low in May the remaining months were indistinguishable from each other, but had significantly more arthropods than were found in May. Species richness and diversity were found to be significantly lower in pastures with continuous rotation, lower stocking densities and high avermectin use compared to the more regenerative systems. Pastures managed conventionally had significantly fewer predators than the more regenerative ranches. Regenerative pastures, with frequent rotations, high stocking densities and no avermectin use, had significantly more dung beetles and a greater diversity of dung beetles.
The amount of avermectin was found to be significantly affected by the month sampled and the management system. Avermectin in May was significantly higher than all other months except September, and avermectin quantity in September similar to that found in every month but May. The most conventionally managed systems had the highest avermectin contamination, with intermediately managed systems averaging nearly half the amount per dung pat. The regeneratively managed pastures consonantly averaged lower levels of avermectin, often with undectectable amounts. Avermectin in dung pats influenced many aspects of the arthropod community. While the entire community abundance was not correlated to avermectin, there was a negative correlation of species richness and species diversity along with abundances of preditor species and dung beetles.
Management system affected the complexity of the dung arthropod community, altering maggot abundance as a result. Dung in regenerative systems had significantly fewer maggots than conventionally managed pastures. The amount of avermectin found in dung was significantly and positively correlated to maggot abundance. Management system had a significant effect on the ratio of maggots to predator arthropods per ranch. The ratio of maggots to coprophagous arthropod (competing with maggots for the dung resource) abundance was similarly found to be significantly affected by pasture quality. There were more maggots per predator and competitor on the ranches with low stocking density, low rotation frequency, and high avermectin use relative to the regenerative ranches. .
Using the inclusion/exclusion cage system for Study 1, the abundance of maggots and predator species were measured to compare between the caged and uncaged pats. Predator abundance in the early and late season was significantly higher in pats with complete arthropod colonization than the exclusion pats where the colonization was restricted. Inversely, maggot abundance was significantly higher in caged dung pats and while uncaged pats remained statistically similar the 4 d and 7 d old caged dung pats had significantly highest abundances across all pats sampled.
Educational & Outreach Activities
This research was conducted as part of work towards a Master of Science in Biological Science degree through the Natural Resource Management department at South Dakota State University with support provided by the Ecdysis Foundation as well as this NCR-SARE student grant. In addition to the thesis document the individual studies described in this document will be submitted for publication in various rangeland and entomology journals for sharing within the scientific community. Once published, this information allow understanding of the dung arthropod community to grow and allow for similar studies to be conducted in other areas where cattle production is an important economic industry.
I hope to make the information available to not only the scientific community but the general population as well. Ranchers and farmers in the area are arguably those that can most benefit from the distribution of the research contained within this study. With this in mind we hope to create easily accessible extension materials that package the information from these studies in an easily understood message. This will be supplemented with outreach events where I hope to speak to audiences of ranchers to share the findings and answer questions that they may have about the research.
The results from this research provide further evidence that dung beetles contribute multiple ecosystem functions to rangelands by the dung arthropod community. Arthropod communities are major contributors to dung degradation, and dung beetles influence many of the characteristics of this community (abundance, richness, and diversity). Dung beetles colonize fresh dung pats and feed on the liquid portion of the dung; they leave when water becomes limited. Dung beetles can also alter the dung pat and the arthropods that will colonize it. Through their tunneling and bioturbation of the dung pat, they allow air to reach the center of the pat and cause it to degrade faster by converting it into forms accessible to plant roots and microbes. Dung beetles’ robust bodies also provide a “highway system” that other arthropods such as predatory beetles or spiders can use to search for prey such as pest maggots. These tunnels also open up the pat’s interior to the micro-coprophage community that lack the ability to burrow through the pat; further increasing their effect on pat degradation. Their impact can be seen in the large amount of OM lost in the first days of arthropod colonization. The management of cattle can decide the extent of these benefits from the dung arthropod community.
Cattle management system significantly influenced the dung arthropod communities found in rangelands. Regenerative systems had 19% more species and 16% greater diversity than the conventionally managed pastures. These losses in community structure may limit the ability of the community to provide ecosystem services. One of the factors associated with the regenerative designation of the pasture was the reduced reliance on avermectin in the system. Avermectins are touted for their ability to control arthropod pests, however this efficacy comes with risks to non-target species. Avermectin levels collected from conventionally managed ranches were often within the range where lethal or sub-lethal effects could be experienced by dung beetle larvae. The toxicity of avermectin, coupled with its insect-repellent properties, likely contributed to the disruption of the arthropod community. Dung beetles were an important driver of the relative abundances of other dung taxa in these different management systems. In systems with the most conventionally managed pastures and highest avermectin use, there were 66% fewer dung beetles than the regenerative pastures. This lower abundance not only represents a decrease of an important coprophagous group, but dung beetles’ multifunctionality and role as a keystone species means that fewer numbers may have knock on effects on the remaining arthropod community.
Components of healthy arthropod communities that can help to reduce maggot abundance in dung include predators and coprophagous competitors. Regenerative ranches had significantly more maggots per predator and coprophagous competitor relative to conventional ranches. Predator communities can suppress the level of maggot pests found in dung pats. Competition between maggots and large coprophagous species such as dung beetles can limit maggot populations by eliminating the dung resource. The reduction of many predator and competitor species in the caged dung pats was associated with significantly greater maggot populations in both the early and late seasons. We found only 26% of the maggot levels when they were raised on pats with a complete community of predators and coprophagous competitors. Indeed, predators and competitors were only 38% as abundant in the caged compared to uncaged pats.
For ranchers in eastern South Dakota, the implications of this study can influence future grazing decisions. With the relatively short grazing period in the region, rapid elimination of dung pats from the soil surface increases the available forage for grazing as well as limit pest abundance within dung. These services contributes to the profitability of ranches through increased grazing efficiency. Regenerative management practices in cattle operations result in a complex dung arthropod community structure capable of providing faster degradation and increased pest suppression services to the rancher. If they conserve dung arthropods, especially dung beetles, with management choices (e.g., limiting insecticide use) they can accelerate the breakdown of dung pats in their pasture. If a management system reduces dung beetles, then the ecosystem services that they provide could be reduced as well, furthering a reliance on chemical products that come at an ecological and economic cost to the rancher. By implementing regenerative practices, a rancher can provide a food source and habitat for thousands of arthropods that increase the profitability and natural resource base of a ranching operation.
Areas needing additional study
The information gathered in these experiments open the door for further exploration of the impact that cattle management has on the dung arthropod community as well as arthropod community's effect on ranch profitability. This research has shown that there beneficial services of having a diverse arthropod community on rangelands but quantifying these benefits within the different systems, while challenging, will further prove the benefits of arthropod conservation. This next step would involve the examination of the financial costs associated with different South Dakota cattle management systems and comparing this to the benefits of accelerated dung pat degradation and pest suppression. This would allow for ranchers to examine their own operation and more effectively decide what management practices to implement within their system.