Abstract

Grass (monocots) and non-grass (dicots) proportions in ruminant diets are important nutritionally because the non-grasses are usually higher in nutritive value, particularly protein, than the grasses, especially in tropical pastures. For ruminants grazing tropical pastures where the grasses are C₄ species and most non-grasses are C₃ species, the ratio of ¹³C/¹²C in diet and faeces, measured as δ¹³C‰, is proportional to dietary non-grass%. This paper describes the development of a faecal near infrared (NIR) spectroscopy calibration equation for predicting faecal δ¹³C from which dietary grass and non-grass proportions can be calculated. Calibration development used cattle faeces derived from diets containing only C₃ non-grass and C₄ grass components, and a series of expansion and validation steps was employed to develop robustness and predictive reliability. The final calibration equation contained 1637 samples and faecal δ¹³C range (‰) of [12.27]–[27.65]. Calibration statistics were: standard error of calibration (SEC) of 0.78, standard error of cross-validation (SECV) of 0.80, standard deviation (SD) of reference values of 3.11 and R² of 0.94. Validation statistics for the final calibration equation applied to 60 samples were: standard error of prediction (SEP) of 0.87, bias of −0.15, R² of 0.92 and RPD of 3.16. The calibration equation was also tested on faeces from diets containing C₄ non-grass species or temperate C₃ grass species. Faecal δ¹³C predictions indicated that the spectral basis of the calibration was not related to ¹³C/¹²C ratios per se but to consistent differences between grasses and non-grasses in chemical composition and that the differences were modified by photosynthetic pathway. Thus, although the calibration equation could not be used to make valid faecal δ¹³C predictions when the diet contained either C₃ grass or C₄ non-grass, it could be used to make useful estimates of dietary non-grass proportions. It could also be used to make useful estimates of non-grass in mixed C₃ grass/non-grass diets by applying a modified formula to calculate non-grass from predicted faecal δ¹³C. The development of a robust faecal-NIR calibration equation for estimating non-grass proportions in the diets of grazing cattle demonstrated a novel and useful application of NIR spectroscopy in agriculture.

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