Case Study of Switchgrass Bedding and Bioenergy Production

Introduction

In the pursuit of innovative and sustainable farming practices, a leading Pennsylvania dairy farm recently partnered with Penn State Extension and the C-Change Grass2Gas project to explore the integration of switchgrass into their operations. This farm conducted a two-month trial to assess switchgrass's potential as both dairy cattle bedding and a feedstock for their on-farm anaerobic digester. This case study describes the findings of that trial and delves into the dual-purpose use of switchgrass, examining its impact on bedding performance and biogas production.

For further context on the broader applications of perennial grasses, our related article, "Integrating Perennial Grasses into Dairy Farm Operations," explores various ways these grasses can be effectively incorporated into dairy farming beyond the scope of this trial. Through this trial, we share insights into the logistical, economic, and environmental implications of replacing conventional sawdust bedding with switchgrass, shedding light on its viability and benefits in real-world dairy farming.

Background

The case study dairy farm, located in central Pennsylvania, has been integrating dairy farming with advanced waste management practices for over a decade. The farm process manure and locally sourced food waste to produce biogas. This biogas is not only used to power the farm but also sold back to the grid, with the waste heat being utilized for various on-farm activities.

The farm's waste management system is as follows: Manure is collected daily from the barns and combined with food waste in the digesters. The processed material leaving the anaerobic digester called digestate, is then separated into liquid and solid fractions. The liquid part is either used as a nutrient-rich fertilizer or stored for future use. The solid fraction is mixed with corn stover and lime to create bedding material for the cows. When corn stover is not available, the farm purchases sawdust as a substitute.

In this trial, the farm is exploring the use of switchgrass as a replacement for purchased sawdust. By incorporating switchgrass into their bedding mix, the farm aims to boost sustainability, enhance bedding quality, and reduce their dependence on outside materials.

Figure 1. Process diagram of the on-farm anaerobic digester system. This diagram illustrates the integration of manure and food waste for biogas production, which generates heat and power. The digestate is separated into liquid and solid fractions, with the liquid used as fertilizer and the solid as dairy bedding. The highlighted green section emphasizes the trial component where switchgrass is incorporated into the bedding mix before being reintroduced into the digester.

The Potential of Switchgrass

Switchgrass is a deep-rooted, warm-season perennial grass that thrives even on marginal lands, making it an excellent choice for farms looking to optimize less fertile areas (for more information see NewBio Energy Crop Profile: Switchgrass, Alternate Markets for Dedicated Grass Energy Crops, and Warm-Season Grasses and Wildlife). Known for its impressive biomass yield and minimal cultivation needs, switchgrass stands out for its environmental and economic benefits.

Environmental Benefits: Switchgrass is more than just a high-yield crop. Its deep, extensive root system enhances soil health by improving soil structure and preventing erosion. This makes it particularly valuable for rehabilitating degraded lands and controlling soil erosion in riparian zones. Its ability to grow well on less fertile soils also reduces the pressure on more productive agricultural lands.

Economic Advantages: From an economic perspective, switchgrass offers several advantages. It can significantly reduce reliance on imported bedding materials like sawdust, which can be costly or difficult to obtain. By growing switchgrass, farms may be able to cut costs associated with purchasing bedding and can also leverage the grass for other purposes. Additionally, switchgrass presents opportunities for generating revenue through bioenergy production. It has potential to be used as a heating fuel or as feedstock in anaerobic digesters to produce biogas, which can then be converted into electricity or heat. Moreover, switchgrass has potential in carbon trading markets due to its role in carbon sequestration, providing an additional revenue stream for farms engaged in environmental stewardship.

Compatibility with Anaerobic Digestion: Switchgrass could be a feedstock for the anaerobic digestion processes. Its fibrous structure and high carbon content makes it compatible with manure and organic waste and can help to balance the carbon-to-nitrogen ratio of some digesters. While the biogas yield of switchgrass (especially for winter harvested switchgrass) is not high, researchers are working to develop new ways to improve its digestibility.

Practical integration of switchgrass into dairy operations extends beyond its environmental benefits. In our article "Integrating Perennial Grasses into Dairy Farm Operations," we delve into various ways perennial grasses, including switchgrass, can be seamlessly incorporated into farm activities. This includes using switchgrass for bedding, forage, and erosion control, among other applications.

Trial Objectives and Methodology

In this on-farm trial, switchgrass was tested as both bedding material and anaerobic digestion feedstock. This hands-on trial aimed to explore how switchgrass can be integrated into dairy farming operations, offering a practical look at its potential benefits and efficiencies.

The trial objectives were:

• Biogas Production: To determine if switchgrass could produce the same amount of biogas and electricity as traditional bedding materials like sawdust. This involves comparing the energy outputs from a digester using switchgrass versus using sawdust.
• Dairy Bedding Efficiency: To evaluate how well switchgrass performs as bedding material for cows. How comfortable and healthy are the cattle when switchgrass is used as bedding, compared to traditional options? This includes evaluating factors like moisture absorption, odor control, and overall cow comfort.
• Economic Impact: To assess whether switchgrass is cost-effective compared to traditional bedding materials.

The switchgrass used in the trial was purchased from an external supplier. To make it suitable for use as bedding, the switchgrass needed to be ground into smaller particles. This preprocessing was similar to the method used for preparing corn stover-digestate bedding, so no new equipment was necessary. For the trial, switchgrass was mixed with digestate solids to create bedding material for the cows. After about one week of use, this bedding was incorporated into the anaerobic digester along with other organic waste for processing. This setup allowed us to assess switchgrass's effectiveness both as bedding and as a feedstock for biogas production. The trial spanned two months, during which we monitored several factors including biogas production levels, cow health and comfort, and the overall costs associated with switchgrass compared to traditional bedding materials. The total amount of switchgrass bedding added to the digester was relatively small, compared to the other feedstocks. Thus, the switchgrass was not likely to boost biogas production by a noticeable amount. However negative impacts, in terms of system clogging or biogas inhibition, were a risk.

Operational Insights and Laboratory Analysis

Throughout the trial, biogas and electricity outputs were monitored to assess the impact of switchgrass on the farm's energy production. The results showed that switchgrass bedding did not hinder biogas production or electricity generation. The amount of energy produced was on par with what was previously achieved using sawdust, demonstrating that switchgrass can effectively serve as an alternative energy source. Furthermore, the digester operated smoothly during the trial, indicating that switchgrass integrated effectively into the digester system. This is an important finding, indicating that switchgrass is not likely to create problems in a digester. Laboratory tests of manure, bedding, and digestate confirmed these results. Key metrics like pH levels, nutrient content (including nitrogen, phosphorus, and potassium), and solids content remained stable and similar to those observed with sawdust bedding. Additionally, the switchgrass bedding appeared to help reduce moisture levels, which in turn lowered bacterial growth. This improvement in bedding quality was linked to better udder health, as indicated by favorable somatic cell counts in the milk. Cow health remained high during the trial, with no noticeable issues from the animals. On the negative side, dust was initially an issue when spreading the ground switchgrass in the barn, and so a coarser grind (or perhaps unground material) may be more desirable.

Economic and Environmental Benefits

Although the initial costs for switchgrass were higher, including material and transportation expenses, the farmer expressed interest in its use despite these higher costs. This interest highlights the perceived value of switchgrass as an alternative to traditional bedding materials like sawdust. In the long run, switchgrass could offer financial benefits through healthier cows, reduced veterinary costs, and potential savings from on-site energy production. Also, if grown on site instead of being purchased, the cost of the switchgrass for bedding may be more economical.

Environmentally, switchgrass has the potential to transform underutilized lands into productive areas. Its cultivation supports essential ecosystem services, such as creating wildlife habitats, sequestering carbon, and enhancing biodiversity, making it a valuable addition to sustainable farming practices

Conclusion

The two-month trial at this Pennsylvania dairy farm highlights the promising potential of switchgrass as both a bedding material and a feedstock for anaerobic digestion. This real-world application demonstrates that switchgrass can be effectively integrated into dairy farming operations, offering comparable biogas production and beneficial impacts on cattle health. Despite higher initial costs, the trial indicates that switchgrass could provide long-term financial and environmental benefits. By reducing reliance on traditional materials and enhancing sustainability, switchgrass has potential to be a valuable addition to modern dairy farming practices. Future exploration and development of switchgrass integration may further optimize its role in improving farm efficiency and environmental stewardship.