Evaluating crude protein concentration of leucaena forage and the dietary legume content selected by cattle grazing leucaena and C4 grasses in northern Australia

Abstract

Keynote paper presented at the International Leucaena Conference, 1‒3 November 2018, Brisbane, Queensland, Australia. In Australia’s central and southern Queensland regions, Leucaena leucocephala-grass pastures produce substantially more beef and higher profits than grass-only pastures and annual forage crops. Near infrared reflectance spectroscopy (NIRS) provides a rapid and cost-effective approach to assessing quality of available forage as well as the quality of the diet selected by cattle, but existing calibrations have not been comprehensively validated for leucaena-grass pastures. This study examined the reliability of existing northern Australian calibrations for NIRS to predict the crude protein (CP) concentration of the edible fraction of the leucaena plant, and the proportion of leucaena in the diet of grazing cattle. Samples of edible leucaena and cattle faeces were analyzed by NIRS and the predictions plotted in a linear regression and fitted to a 1:1 line with Dumas analysis of CP for leucaena forage, and mass spectrometry of δ13C for cattle faeces. Results demonstrated that prediction of the CP concentration of leucaena forage and the proportion of leucaena in the diet of grazing cattle using current broad northern Australian NIRS forage calibrations were associated with substantial error. However, it is likely that these errors can be reduced with the inclusion in the calibration data set of more samples representing leucaena forage and faeces of cattle grazing leucaena from varying locations, seasonal conditions and management strategies.