Reducing the Environmental Impact of Beef Production Through the Use of Naturally Occurring Secondary Plant Metabolites in Southeastern Forage Systems

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Commodities

• Animals: bovine

Practices

• Animal Production: feed/forage
• Energy: methane reduction, greenhouse gas emissions

Sustainability in beef production systems is vital to the environmental and economic health of livestock production in the southeastern United States. In this region, two principal forage systems predominate: 1) warm-season perennial grass pastures with interseeded cool-season annual forages ⁽¹⁾, and 2) cool-season perennial grass pastures. Bermudagrass (Cynodon dactylon [L.] Pers.), the predominant warm-season perennial grass, accounts for approximately 14 million ha in the United States ⁽²⁾, while tall fescue (Schedonorus arundinaceus [Schreb.] Dumort., nom. cons.) accounts for approximately 15 million ha ⁽³⁾. These are economically viable systems supporting nearly year-round grazing but come with their own limitations. Enteric methane is one of the leading greenhouse gasses (GHG) linked to animal agriculture, and most methane production is linked to the period in life when cattle are grazing forages high in cell wall components. In order for this system to continue to provide animal-sourced protein for a growing human population, strategies must be identified to address environmental concerns without sacrificing economic viability.

Plant secondary metabolites (PSM) are chemical compounds produced by plants that do not take part in biological function but are often produced in response to stress as a means of defense ⁽⁴⁾. Plant secondary metabolites have been implicated in modulation of animal health ⁽⁵⁾, especially enteric methane production in ruminant animals ⁽⁶⁾. However, in most investigations, these PSM have been used in their isolated form. For a fully viable system, these PSM need to be present in forage-based beef cow-calf systems as grazed or conserved forage. The use of biculture grass and forb pastures, both in cool- ^{(7; 8)} and warm-season ⁽⁸⁾ systems, has been identified as a viable solution for increased forage yield and nutritive value in support of beef cattle production. In addition to their augmentation of the pasture environment, legumes are noted for their increased expression of plant secondary metabolites ⁽⁵⁾.

Therefore, we propose a systems-based applied research and education approach to incorporating PSM into a grass/forb forage system for environmentally and economically sustainable beef production. We will evaluate the effects of species and location on the expression of PSM from forage forbs grown in a small-plot experiment. These plots will be used in vitro to determine methane production potential of grass/forb biculture systems. Forbs showing the most promise for methane reduction will then be used in a digestion and metabolism experiment to examine the effect of plant secondary metabolites on ruminal fermentation. We will then verify that these identified forage systems are capable of supporting growing beef cattle through the use of a grazing experiment. Research information from this proposal will be incorporated into the final objective which is to update current Extension resources and deliver information to stakeholders, including an emphasis on management to reduce GHG in forage-based beef cattle systems in the region.

Our long-term goal is to support economically and environmentally sustainable beef production in southeastern pasture-based systems through efficient nutritional and nutrient management. Our overall objective is to identify forb species that can be economically incorporated into existing southeastern grass pastures to reduce enteric methane production in beef production systems without sacrificing production performance. The central hypothesis is that forb inclusion in grass pastures will reduce enteric methane production while augmenting productivity of beef cattle. For this SARE Research and Education Proposal, we propose four specific objectives:

Specific Objective 1. Determine effects of species and location on yield potential, nutritive value, and plant secondary metabolite expression of forage forb species. Plant secondary metabolite expression varies widely across species ⁽¹⁶⁾. In addition to intraspecific variation, however, environmental influences (e.g., location as a representation of weather, soil, and management) can play a major role in the determination of both degree of expression and type of secondary metabolite produced ⁽³⁴⁾. The goal for the research under this specific objective is to evaluate environmental influences (i.e., location effects) on yield and secondary metabolite expression of commercially available and native forbs species. Our hypothesis is that environmental stressors, such as soil characteristics, rainfall, temperature, and humidity, will alter plant secondary metabolite expression. Our approach will use locations across the southeastern United States to produce forb species for assessment of plant secondary metabolite expression and in vitro methane reduction potential. Our rationale for this objective is plant secondary metabolites may potentially reduce enteric methane emissions in beef cattle production. The expected outcome is that select forb species will show methane reduction potential and promise for evaluation in larger scale trials. Upon completion of this objective, our expected overall outcome is the development of a list of methane-reducing forbs available for environmentally sustainable as well as productive grazing systems.

Specific Objective 2. Determine digestive and metabolic parameters in beef cattle exposed to grass/forb systems including naturally occurring plant secondary metabolites. While plant secondary metabolites may reduce enteric methane production ⁽¹⁷⁾, they have also been implicated in detrimental alteration of ruminal metabolism and reduction in digestibility potential ⁽²³⁾. The goal for the research under this specific objective is to determine the effect of forbs on nutrient utilization dynamics and carbon loss through methane. Our hypothesis is that supplementing secondary metabolite-producing forbs in the diet of beef cattle will result in increased carbon capture through productive means and less loss through methane. Our approach will use ruminally-fistulated steers in a replicated metabolism trial to assess digestive and metabolic parameters of feeding. Our rationale for this objective is plant secondary metabolite inclusion in a forage-based ration will result in an altered ruminal environment that will favor carbon capture as volatile fatty acids (VFA) rather than loss as methane. The expected outcome is that supplementation of grass-based diets with secondary metabolite-producing forbs will decrease enteric methane production and increase energetic efficiency of fermentation. Upon completion of this objective, our expected overall outcome is development of forb inclusion recommendations that support energetic efficiency in feeding beef cattle.

Specific Objective 3. Determine beef cattle performance using grass/forb systems including naturally occurring plant secondary metabolites. Although supplementation of grass-based cattle diets with forbs may reduce enteric methane production, such a system is only viable if it can be incorporated into grazing management. Previous research has shown that voluntary intake may decrease with increasing inclusion of plant secondary metabolites ^{(35; 36)}. The goal for the research under this specific objective is to determine how secondary metabolite-producing forb inclusion in pasture grazing influences voluntary intake and animal performance. Our hypothesis is that forb inclusion in grass pastures will result in increased land carrying capacity and cattle body weight gains due to enhanced energetic efficiency of ruminal fermentation. Our approach will use a replicated grazing study to assess animal and plant dynamics. Our rationale for this objective is decreased methane emissions will results in carbon being shuttled to VFA production, thereby increasing animal performance through carbon capture. The expected outcome is that inclusion of secondary metabolite-producing forbs in grass pastures will increase carrying capacity, increase gain per unit area, and increase average daily gain of grazing cattle. Upon completion of this objective, our expected overall outcome is development of grazing management recommendations for secondary metabolite-producing forb inclusion in perennial grass pastures.

Specific Objective 4. Create and extend educational products on forage management practices to reduce greenhouse gas emissions in forage-based livestock systems. Enteric fermentation from grazing livestock is implicated in the production of 27% of all methane emissions in the United States ⁽¹⁰⁾. The predominant source of enteric methane production in beef cattle is consumption by cattle of feedstocks dense in cell wall material (e.g., when cattle are grazing)⁽¹¹⁾.The goal for the research under this specific objective is to develop and deliver programs to educate beef cattle producers on forage management practices that will enhance environmental sustainability based on research in Specific Objectives 1, 2, and 3. Our hypothesis is that access to and dissemination of educational resources will enhance awareness, knowledge, and potential for adoption of sustainable practices in beef production. Our approach will use farmer demonstrations, field days, and online resources to disseminate information on sustainable, economically beneficial practices. Our rationale for this objective is that lack of access to scientifically valid information and visual demonstrations of application of these practices is a barrier to adoption of techniques. The expected outcome is that education will result in increased adoption of sustainable forage management practices. Upon completion of this objective, our expected overall outcome is accessible recommendations and resources for sustainable beef cattle production in the Southeast.