Improving nutritional management of grazing cattle: Improving reliability of faecal NIRS calibration equation

Summary

The primary objective of Project NAP3.121 was to improve faecal NIRS (Near Infrared Reflectance Spectroscopy) calibration equations for predicting the diet quality of cattle grazing pasture systems of northern Australia. With respect to industry application, the reason for the work was to provide the northern beef industry (producers, consultants and agribusiness) with an educational and decision support tool for improved nutritional management of grazing cattle. However, the work was also carried out to provide rangeland scientists with a new and powerful research tool to enhance the capture of knowledge from grazing experiments, to provide new insights and understanding in studies involving grazing cattle, to improve the cost effectiveness of research and to open up new avenues for research.

The project focused on a number of dietary and related attributes, viz. dietary crude protein (CP), the digestibility of the diet, grass and non-grass dietary proportions, faecal nitrogen (N) concentration and liveweight gain in growing cattle. The possibility of predicting faecal phosphorus (P) concentration was also investigated. Moreover, diet and faecal samples generated in the project will enable faecal NIRS calibration equations to be developed for predicting dietary neutral detergent fibre (NDF) and acid detergent fibre (ADF) in the near future. Faecal NIRS calibration equations had already been developed for predicting dietary CP, digestibility, dietary non-grass proportions and liveweight gain but the equations needed further development to improve predictive reliability and accuracy when applied to faecal samples sourced from different locations and pasture communities right across northern Australia.

The calibration equation for each attribute or property is an independent entity derived from separate calibration sets though the same faecal spectra often occur in two or more of the calibration sets. Moreover, apart from dietary non-grass proportions, the reference values used to develop the calibration regression relationships between attribute and faecal spectra are direct measurements. Thus the reference values for dietary CP (diet CP = diet N x 6.25) are determined on samples of the actual diet; digestibility reference values are determined by in vitro analysis of diet samples or by in vivo digestibility trials; faecal N values are determined by chemical analysis of the faecal samples; and LWG values are determined by weighing cattle at regular, short intervals. Dietary non-grass proportions are calculated from faecal δ13C values because this is the conventional method of determining non-grass proportions for

cattle grazing tropical pastures. Thus, for this attribute, the primary faecal NIRS prediction was faecal δ13C. However, the reference values could just as easily have been dietary non-grass (%) calculated from the laboratory determination of faecal δ13C.