Changes in faecal δ¹³C in response to changing proportions of legume (C₃) and grass (C₄) in the diet of sheep and cattle

SUMMARY Feeding studies were conducted in 1986 with penned sheep and cattle fed legume (C 3 ) and grass (C 4 ) hays mixed in different proportions. Large fluctuations in diet δ 13 C within or between days were associated with much smaller fluctuations in the δ 13 C of faeces. When the overall daily legume percentage in the diet was held constant, there was little variation in the δ 13 C of faeces irrespective of when during the day the legume, relative to the grass, was eaten. Cattle set stocked on Stylosanthes/grass pastures in Queensland showed only minor variations in faecal δ 13 C within or between days in the short term. All animals within a group showed similar trends when the diet was changing. The changes in the δ 13 C of faeces from day to day reflected small but real changes in diet selection. It was concluded that the δ 13 C of a single faecal sample reliably reflects the integrated diet over the previous 3–4 days of a free-grazing ruminant so that the frequency and pattern of faecal sampling for estimating diet composition may be determined by considerations other than reliability