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

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Commodities

• Agronomic: grass (misc. perennial), peanuts

Practices

• Animal Production: grazing management

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.

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.