Integrating tannin-rich legumes into tall fescue pastures to strengthen the sustainability of beef cattle production in the southern United States

Commodities

Practices

Tall fescue (Schedonorus arundinaceus [Schreb.] Dumort., nom. Cons.; TF) is a dominant, perennial cool-season forage that covers approximately 15 million hectares of the southern United States and is commonly  infected with the endophytic fungus Epichloë coenophialum ⁽¹⁾. Due to this fungus, animals that graze on TF pastures usually display several symptoms associated with reduced animal performance, collectively known as fescue toxicosis ⁽²⁾. However, despite its toxicity, TF remains a dependable and desirable forage for many ruminant livestock producers, since it is easy to establish, highly persistent, and adaptable to diverse climatic conditions with limited management ⁽³⁾. Therefore, reducing the effects of TF toxicosis has been a topic widely investigated. One of the possible alternatives to attenuate the effects of fescue toxicosis would be to use compounds that would inhibit its action. Condensed tannins (CT) possess a strong binding affinity with steroidal and protein-like alkaloids that are produced by the endophytic fungus in TF, which might potentially reduce the post-ingestion effects of fescue toxicosis ^(4;5). Further, CT is effective in decreasing greenhouse gas (GHG) emissions, increasing N use efficiency, and shifting N loss through the urine to more stable N complexes in the feces⁽⁴⁾. However, in most investigations, CT is used in its isolated form or in grazing systems with plants of hard establishment ⁽⁶⁾, both of which are generally impractical for beef cattle producers. In forest systems, tannins in plant litter have been shown to slow rates of nutrient cycling by supplying more recalcitrant C substrate, binding with proteins in the soil, inhibiting N mineralization, and enhancing organic matter persistence in the soil ^{(7; 8)}. Due to high affinity with some minerals, such as Al ⁽⁹⁾ and Fe⁽¹⁰⁾, tannins might also increase availability of other minerals to the plant ⁽¹¹⁾.  

Although many studies have investigated the effects of CT on the animal, little is known regarding its dynamics in the soil of pasture systems. Therefore, we propose a systems-based applied research and education approach to incorporate CT into a TF grazing scheme as a strategy to enhance the sustainability of southern beef production systems. We will evaluate the effects of two different CT-rich legumes (birdsfoot trefoil [BFT; Lotus corniculatus L.] and crownvetch [CV; Securigera varia {L.} Lassen]) seeded into TF in a grazing trial focusing on the interaction of plants, soil, and animals. At the animal level, we will evaluate performance, toxicosis incidence, methane emission, nitrogen utilization efficiency, changes on rumen microbiota, and overall health of the animals. At the plant level, we will be evaluating herbage responses, biological nitrogen fixation, and yield. Finally, at the soil level, we will evaluate soil microbial communities, nitrogen utilization efficiency, GHG emissions, and soil organic matter. Research information from this proposal will then be delivered to producers as a management tool in sustainable beef cattle systems in the region through workshops, online courses, web-based content, handouts, and farmer demonstrations on field days. 

Our long-term goal is to ensure economically and environmentally sustainability of beef production in the southeastern U.S. through an integrated system that considers soil, plant, and animal altogether. Our overall objective is to evaluate the incorporation of CT-rich plants into a TF grazing scheme as a strategy to reduce farmers inputs, as well as any detrimental effects from grazing systems. Our central hypothesis is that the inclusion of CT-rich plants will reduce TF toxicosis, production of enteric methane, and nutrient input into the soil while augmenting productivity of beef cattle, soil health, and nitrogen utilization efficiency. To accomplish this, we propose four specific objectives:  

Specific Objective 1: Evaluate the effect of perennial, cool-season, CT-rich legumes incorporated in TF pastures on herbage responses and beef cattle performance.  

When animals graze a diverse vegetation containing plant secondary metabolites, the different chemicals ingested in the process improve animal performance while increasing the resilience of agroecosystems (72). Therefore, the goal for the research under this specific objective is to evaluate how the inclusion of CT-rich legumes incorporated into TF pastures can affect grazing beef cattle performance and herbage responses. Our hypothesis is that the inclusion of CT-rich legumes will increase the intake of TF, and consequently, gain of animals. Simultaneously, we hypothesize that herbage responses will also be improved due to better biological nutrient fixation attributed to the inclusion of CT-rich legumes. Our approach will use the incorporation of BFT and CV into TF pastures in a grazing study. Our rationale for this objective is that CT present on BFT and CV may potentially boost performance in beef cattle production, while still allowing a higher fixation of nitrogen by those legumes. Our expected outcome at the end for this objective is that the inclusion of CT-rich plants will increase carrying capacity, increase average daily gain, increase herbage mass and quality, and increase gain per unit area. With that, our overall outcome is to provide a more productive grazing system for beef cattle producers while still reducing their inputs.  

Specific Objective 2: Evaluate the role of CT-rich legumes in mitigating TF toxicosis and CH4 emissions, and nitrogen utilization efficiency in beef cattle. 

Due to its toxicity, TF can be detrimental to beef cattle. CT possess a strong binding affinity to the toxic component in TF, which might potentially reduce effects of fescue toxicosis (4; 5). Meanwhile, CT also can decrease GHG emissions and increase N use efficiency (4), which can all lead to a higher production of beef cattle. With that, the goal for this specific objective is to evaluate how the inclusion of CT-rich legumes can affect TF toxicosis, carbon loss through methane, and nutrient utilization dynamics of beef cattle. Our hypothesis is that the inclusion of CT-rich legumes will help lessen the effects of TF toxicosis, while decreasing CH4 enteric emissions and increasing N use efficiency. Our approach for this objective will use a replicated grazing study to assess animal dynamics. The rationale is that CT will bind to the alkaloids present in TF and reduce post-ingestion effects of TF toxicosis, while also reducing methane synthesis and N excretion due to inhibition of methanogens and binding to proteins, respectively. Altogether, our overall outcome at completion of this objective is to develop grazing management recommendations for TF pastures in association with CT-rich plants that will provide both economically and environmentally sustainable practices.   

Specific Objective 3: Evaluate soil microbial communities, nitrogen utilization efficiency, GHG emissions, and soil organic matter of TF mixed with cool-season, CT-rich legumes. 

Tannins in plant litter have been shown to slow rates of nutrient cycling by supplying more recalcitrant C substrate, binding with proteins in the soil, inhibiting N mineralization, and enhancing organic matter persistence in the soil (7; 8). Also, the affinity of CT to alkaloids can shift the N metabolism site, reducing the N excreted in the urine, and improving the amount of N excreted by feces. This process may improve the N cycling in the soil, mitigating the N leaching. With that, the goal for this specific objective is to evaluate how the inclusion of CT-rich legumes can affect soil in organic matter, GHG emission, and nutrient utilization dynamics by soil microorganism. Our hypothesis is that the inclusion of CT-rich legumes will help the reduction of N excretion by urine, decreasing GHG soil emissions and increasing N use efficiency. Our approach for this objective will use static chamber to evaluate GHG emission and soil measurements in TF and legume mixtures grazing pasture. The rationale is that like animal rumen, CT will bind to the alkaloids present in the soil and facilitate the metabolism by microorganism. Thus, our overall outcome is with inclusion of CT-rich legumes into TF pasture will improve the microorganism performance into soil via excreta, facilitating the N cycling and mitigating GHG. 

Specific Objective 4: Create and extend educational products on managing TF pastures with CT-rich legumes as a practice to reduce GHG emissions and increase soil health.  

All information obtained from the previous objectives is only relevant if transmitted to forage and livestock farmers. Therefore, the goal of this objective is to translate the research results obtained from Specific Objectives 1, 2 and 3 into accessible educational materials and training for beef cattle producers which will enhance environmental and economical sustainability. Our hypothesis is that the dissemination of educational resources to producers will enhance the potential for adoption of the sustainable practices explored in this project. We will approach this objective using educational products, farmer demonstrations, field days, and online resources to disseminate the importance of using CT-rich legumes on TF pastures to strengthen the sustainability of beef cattle production.  The rationale is that those techniques can only be put into practice if we enhance the awareness and knowledge of the end users on this matter. Therefore, our expected outcome is that farmers will start to implement the practices on this project. Upon completion, our overall outcome is to provide a more accessible resource for sustainable beef cattle producers.