Animal Selectivity in Mixed Grass-Legume Pasture Assessed By Natural Isotopic Abundance and Microhistological Techniques.

The use of mixed grass-legume pastures increases sustainability of pasture-based livestock production systems. Animal diet composition in mixed swards can be used to explain animal performance and effects of management practices. However, the efficiency of techniques for determining animal selectivity is not always high and can be affected by characteristics of the mixture components. The aim of this study was to evaluate two measurement techniques (natural isotopic abundance and microhistological composition in animal feces) for assessing animal selectivity in mixed grass-legume swards and to compare them with sward-based measures of botanical composition. Pastures were mixed ‘Florigraze’ rhizoma peanut (RP; Arachis glabrata) and ‘Pensacola’ bahiagrass (BG; Paspalum notatum) with 17 and 77% RP in dry herbage mass. Pastures were rotationally stocked with a 1-wk grazing period, and each mixture proportion was replicated three times. Pasture botanical composition was determined pre-grazing and expressed on a dry weight basis. Fecal collection occurred in the last 2 d of a grazing period by compositing feces across the animals grazing each pasture. Means vectors were compared using the T² Hotelling test and similarity was determined by Kulczynski’s index (IS). There were differences between botanical composition of the diet and that of the pasture (P<0.05). Pastures with 17% RP had 21.4% RP in the diet, 26% greater than in the pasture. The IS was 98.5% (P>0.05) between the isotope and microhistological techniques. In the 77% RP plots, diet composition was 59 and 47% for the isotope and microhistological techniques, respectively. The similarity indices were less than 82.7% (P<0.05). The similarity of estimates from the two techniques and their ability to approximate pasture-based measures of composition depended upon RP percentage in the pasture, with greater similarity in lower RP percentage swards. Detailed characterization of species distribution in the canopy may help to explain this response.