Tracing sheep binary C3–C4 diet using stable isotope ratio (δ13C)

Abstract

Stable isotopes are an important tool to assess livestock diet in binary mixtures of C3-C4 forages. However, the use of stable isotopes to trace livestock diet using tropical arboreal legumes has been limited. The objective of this study was to evaluate the use of ¹³C stable isotopes to identify the proportions of grass (C4) and legume in sheep diet, when consuming various inclusion levels. We evaluated models correlating faecal δ¹³C to diet δ¹³C and inferred which model can better predict the different proportions of grass or legume consumed by sheep. Thirty male lambs (17 ± 1.3 kg) were used, and the experiment was set up in a randomised complete block design with body weight being the criteria for blocking. Treatments included various inclusion levels of Sabiá (Mimosa caesalpiniifolia Benth) in signalgrass (Urochloa decumbens Stapf) hay (100:0, 75:25, 50:50, 25:75 e 0:100%). Total faecal production and in vivo digestibility were calculated, as well as δ¹³C of diet and faecal samples. Sabia diet (100%) showed the greatest overall intake, and the lowest faecal production was observed when forages were fed alone, both for signalgrass or sabia hays, with 234 and 245 g dry matter animal⁻¹ d⁻¹. The models used within this study estimated with high accuracy the grass-legume proportions within the diets of sheep (R²=0.97). Our results indicate that using additional coefficients (digestibility, discrimination) did not increase the accuracy of the models, since using only isotopes from faeces was sufficient to predict the contribution of C3 or C4 species in the diet.