Toward an Optimum Legume Proportion in Legume-grass Pastures: From radiation use efficiency to animal performance

Progress report for GS23-279

Project Type: Graduate Student
Funds awarded in 2023: $15,029.00
Projected End Date: 08/31/2025
Grant Recipient: University of Florida
Region: Southern
State: Florida
Graduate Student:
Major Professor:
Dr. Lynn Sollenberger
University of Florida
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Project Information


Forage legumes are overwhelmingly successful and widely used in temperate climates, but lack of competitiveness and poor persistence have limited their adoption in warm-climate grasslands. Our data show that rhizoma peanut (Arachis glabrata Benth) is a long-lived, grazing tolerant legume that can compete effectively when grown in pasture mixtures with bahiagrass (Paspalum notatum Flügge) in the Gulf Coast region of the southern USA. A knowledge gap in use of rhizoma peanut-grass mixtures is the optimal proportion of legume in the mixture. Achieving a greater legume proportion is more costly and not always advantageous, so understanding the effects of legume proportion on pasture and animal responses is critical. We hypothesize that legume proportion in the pasture affects canopy bulk density and the canopy height at which most incident light is intercepted, thus affecting radiation use efficiency, pasture production, and optimal grazing management. In addition, the realized proportion of legume likely affects biological nitrogen (N) fixation rate as well as animal performance and diet composition. These potential effects determine different environmental and economic cost-benefit scenarios, which have not yet been quantified. Our objective is to identify relationships between legume proportion and pasture and animal responses. The ultimate goal is to determine the range of legume proportion in C4 warm-season grass pastures which optimizes radiation use efficiency, biological N fixation, and animal performance. Elucidating these relationships will improve the efficiency of grazed legume-grass mixtures in the southern US, making possible the sustainable intensification of regional grazing systems at a lower cost.

Project Objectives:

As part of a research program aimed at increasing integration of legumes into grazed pastures in the southeastern US, the overall objective of this project is to quantify the effect of legume proportion in grass-legume mixtures on pasture and animal responses in order to identify the range in legume proportion that optimizes primary and secondary production. The first specific objective is to evaluate the effect of different proportions of rhizoma peanut and bahiagrass in mixed pastures on sward height, bulk density, and leaf area index (LAI) required to intercept specific target levels of photosynthetically active radiation (PAR) and determine the criteria for optimum grazing management. In addition, we aim to evaluate the radiation use efficiency of swards having different proportions of legumes. Here, we will address the following questions: (1) what is the sward height and LAI that intercepts 95% and 40-50% of PAR, values reported as the ceiling LAI in which primary production is maximized, and the minimum LAI to ensure the persistence and maximize the regrowth of bahiagrass-peanut pastures of varying legume proportion? (2) is there an optimal proportion of legume in bahiagrass pastures where radiation use efficiency is maximized under grazing conditions? (3) how are species distributed vertically in these swards and what is the effect of differences in vertical distribution on bulk density? and (4) is a greater proportion of legume associated with a lesser and earlier optimum LAI, which could affect the target sward height and strategy for grazing management?

The second objective is to evaluate the legume proportion in the diet of grazing heifers, as a function of the legume proportion in the pasture, and assess the role of legume proportion on animal performance during the growing season. Here, the questions addressed are: (1) is there an optimum pasture legume proportion in terms of animal performance? (2) does diet legume proportion approximate pasture legume proportion? and (3) how do differences in vertical pasture structure (rhizoma peanut and bahiagrass distribution) affect animal selectivity and performance? The findings will contribute to the understanding of optimal grazing strategy under different canopy structure and species compositions. The results may identify herbivore strategies, such as selectively grazing and consuming a diet with greater legume proportion than offered, offsetting its lower proportion in pasture. Additionally, we expect to identify the optimum proportion which maximizes animal performance and identify possible negative effects of above-optimum proportion and the associated canopy structural features which limit secondary productivity.

The third objective is to quantify the N derived from the atmosphere (Ndfa) fixed by bahiagrass-rhizoma peanut mixtures of different legume proportions. We aim to address the specific questions (1) what is the total Ndfa fixed by bahiagrass-rhizoma peanut mixtures in pastures varying in legume proportion? and (2) what is the %Ndfa in aboveground biomass in this long-term experiment? This will provide useful numbers for assessing ecosystem services from grass-legume mixtures with different proportions of legume and to estimate the proportion of protein integrated into live weight gain that is explained by Ndfa.


Materials and methods:

To address the problem identified and gaps in the knowledge regarding the effects of legume proportion on bahiagrass in the southeastern US, we propose a grazing trial including bahiagrass monocultures and bahiagrass-rhizoma peanut mixed pastures. The experimental location is the Beef Research Unit of the University of Florida, Gainesville, Florida (29.72° N, 82.35 °W). The study area includes long-term pastures with only ‘Pensacola’ bahiagrass (control, 0% legumes) or with different proportions of ‘Florigraze’ rhizoma peanut mixed with bahiagrass. The rhizoma peanut was planted in 1983 (Ortega et al., 1992), and afterwards was colonized by bahiagrass, creating the pastures with different proportions of each species. Treatments are defined according to the proportion of legume and bahiagrass in total aboveground biomass, where the control area, two experimental units of 0.5 ha each, is dominated by bahiagrass and will not receive N fertilization. The mixed pastures are represented by six experimental units of 0.5 ha each, varying in legume proportion, from low (< 15%), medium (15-30%), to high legume proportion (> 30%).

We will analyze the legume proportion in pastures as a continuous explanatory variable in the analysis. Pastures will be continuously stocked by yearling heifers during late spring (early June) to autumn (mid-October) of two years (2023 and 2024). The grazing period will be 16 weeks (112 days). Throughout the experiment, the grazing intensity will be maintained based on herbage allowance (Sollenberger et al., 2005), where the target herbage allowance is 1.5 kg DM kg-1 LW (Jaramillo et al., 2021). Two tester animals will be assigned to each experimental unit, and additional animals will be added if necessary to maintain the target herbage allowance.

To address the effect of legume proportion on canopy components and radiation use efficiency, the light interception and pasture height will be assessed weekly throughout the grazing season, using three exclusion cages per plot. Caged sites will be chosen to cover a representative range of legume proportion within the experimental unit in which they are placed. The light interception and LAI will be measured in a 0.25-m2 area within the cage, early in the morning using the Plant Canopy Analyzer LAI-2000 (LI-COR). Simultaneously, the sward height will be measured using the sward stick method (Barthram, 1986), and the disk meter will be used to estimate herbage mass using a calibration equation developed with the double sampling method. The herbage inside the exclusion cages (0.25 m2) will be cut at the end of four weeks of regrowth to estimate the herbage mass and the botanical composition. Sampling will occur in 10-cm strata to characterize botanical composition and vertical distribution of herbage throughout the canopy. The samples will be hand separated into bahiagrass, rhizoma peanut, and other species and dried at 60°C for 72 hours to determine plant mass and proportion in each stratum and in the total canopy. Herbage bulk density will be measured and correlated with sward height, light interception, and LAI. The light interception, LAI, sward canopy height, and the radiation use efficiency will be assessed for each cage and the resultant legume proportion.

To address the effect of legume proportion on diet composition and animal performance, two tester animals will be assigned to each experimental unit, and additional animals will be added if necessary to maintain the target herbage allowance. Animal performance will be calculated as the difference in weight of tester animals between consecutive weighings, every 4 weeks. Animals will be fasted for 12 hours before weighing. The proportion of legume in the diet will be quantified addressed using the δ13C technique on fecal samples. Fecal samples will be collected on Days 14, 56, and 96 following the protocol developed by Kohmann et al. (2022). The feces of the two testers  per experimental unit will be collected in the morning immediately after defecation, from the center of dung piles, avoiding soil or herbage contamination. Samples will be combined and dried at 60°C for 72 hours. Dried samples will be ground, and ball milled for isotopic analysis. Botanical composition of each experimental unit will be assessed by clipping the same day 5 samples of 0.25-m2 within each experimental unit, to estimate the species proportion in the aboveground biomass. In each sample, bahiagrass, rhizoma peanut, and other species will be hand separated and subsequently dried at 60°C for 72 hours to estimate the herbage mass. The botanical composition is estimated using the average % of each species in each stratum and the total aboveground biomass in each experimental unit. In addition, the nutritive value (% of crude protein, CP; and in vitro digestible organic matter, IVDOM) of each experimental unit will be estimated during each fecal collection.

To quantify the total N derived from the atmosphere in each pasture and the effect of legume proportion, the total N fixed by bahiagrass monoculture and rhizoma peanut in mixture with bahiagrass will be calculated using the herbage accumulation rate, the total N, and the %Ndfa for each species. The herbage accumulation rate will be estimated using the difference in herbage mass inside exclusion cages at the beginning and end of each grazing period (the clipping samples of 0.25 m2 used to estimate the herbage mass inside the cages, clipped every four weeks). Samples will be hand-separated to estimate the proportion of bahiagrass and rhizoma peanut and dried at 60°C for 72 hours. The samples of bahiagrass in monoculture and rhizoma peanut in mixture will be ground and ball milled to determine total N and the %Ndfa, using the natural abundance technique (δ15N; Dubeux et al., 2017) and the equation developed by Sheared and Kohl (1986). A non-fixing and unfertilized reference species i.e., bermudagrass, Cynodon dactylon, will be collected and dried to analyze the N concentration and estimate the N fixation. The N yield will be estimated using the total aboveground biomass produced and the total N concentration of each clipping sample, while the N fixation will be estimated using the N yield and the %Ndfa (Dubeux et al., 2017).

Research results and discussion:
Participation Summary

Educational & Outreach Activities

3 Consultations
1 Journal articles
3 Webinars / talks / presentations
1 Workshop field days

Participation Summary:

130 Farmers participated
300 Ag professionals participated
Education/outreach description:

Mr. Caram gave three talks at professional meetings as indicated below.

Canopy characteristics and growth rate of bahiagrass monoculture and mixtures with rhizoma peanut - 25th International Grassland Congress - May 2023

Toward an optimum legume proportion in legume-grass pastures: from radiation use efficiency to animal performance. ASA, CSSA, SSSA International Annual Meeting, St. Louis, MO. October  2023

Identifying optimum legume proportion to maximize animal performance: the case of bahiagrass-rhizoma peanut mixtures - Annual Conference of the American Forage and Grassland Council, Mobile, AL, January 2024

Mr. Caram published one refereed article as indicated below.

Caram, N., L.E. Sollenberger, M.O. Wallau, and C.H. Wilson. 2023. Canopy characteristics and growth rate of bahiagrass monoculture and mixture with rhizoma peanut. 461-464. In: R. Smith et al. (eds.) Proc. Int. Grassl. Cong. 25th. 14-19 May 2023. Curran Associates, Inc., Red Hook, NY.

Mr. Caram spoke at one producer field day as indicated below.

Grazing perennial peanut: what proportion do we need in the pasture? - Perennial Peanut Producers' Association - June 2023

Project Outcomes

20 Farmers reporting change in knowledge, attitudes, skills and/or awareness
2 New working collaborations
Project outcomes:

Use of nitrogen fertilizer in pastures is costly to producers and to the environment, the latter due to use of fossil fuels to produce and distribute the fertilizer, emissions of the greenhouse gas nitrous oxide after application to the pasture, and potential leaching of nitrates to ground water. Use of rhizoma peanut in pastures avoids the economic and environmental costs associated with nitrogen fertilizer without sacrificing animal performance. In fact, individual animal gain is increased by approximately 60% by inclusion of rhizoma peanut.

Knowledge Gained:

The early results of this project have provided confirmation that even a relatively small proportion of legume in mixture with grasses can contribute greatly to pasture productivity and nutritive value and to animal performance. Our preliminary data show that approximately 30% of rhizoma peanut in the biomass of a grass-legume mixture can maximize pasture production and animal performance. Why is 30% enough to maximize animal performance? Because the animals actually select a diet that is approximately 60-70% legume when the pasture contains only 30%, thus increasing diet protein and digestibility.

This relatively small amount is achievable by producers at a lesser cost than a large percentage of peanut, and it will encourage greater adoption.

Information Products

    Any opinions, findings, conclusions, or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the view of the U.S. Department of Agriculture or SARE.