The study examined the reliability of predicting faeces output of cattle from chromium oxide (Cr2O3) released from a device (CRD) inserted in the rumen. Sixteen steers were selected from four genotypes and were offered a diet of low quality pasture hay with a protein meal supplement. The steers were placed in metabolism crates which allowed for total collection of faeces over two 5-day periods, days 6–9 (period 1), and days 13–16 (period 2), of a 20-day extrusion cycle of the CRD. Three faeces samples were taken from each steer per rectum; grab samples in the a.m. and p.m. and a daily subsample, that is, a sample of each daily total collected output. Cr2O3 release rates were derived for each sampling regime (a.m., p.m. and daily from the Cr2O3 content of faeces). These derived rates were compared with the release rate specified by the supplier to determine differences and any bias associated with the sampling regimes. Release rates were also derived from the predicted expiry date calculated from plunger travel in the CRD. Organic matter digestibility of the diet was determined from measurements of feed intake and faeces output over the 4 days for each steer and from an in vitro technique on diet samples. Predicted forage intakes were estimated using the mean faeces output collected over 4 days in combination with either the in vivo or in vitro digestibility estimates. Correlations between actual forage intake and predicted forage intake for each method were calculated and the bias in predicted forage intake between methods was compared.

The daily data revealed significant effects of steer (and/or CRD) and sampling period on the estimated release rate of Cr2O3 from the CRD. However, whilst there was only a period effect on the release rate with p.m. sampling there was a linear decline in release rate with the a.m. sampling. The correlation between predicted and actual forage intake was greatly reduced when in vivo digestibility was replaced by in vitro digestibility.

The results of this study suggest that the CRD had release rates less than specified and therefore devices should be calibrated for both the animals and the pastures prior to use. Furthermore, faeces sampling should be restricted to <13 days post-dosing to minimize bias.