ESTIMATING THE PASSAGE OF DIGESTA IN STEERS AND WETHERS USING THE RATIO OF C-13 TO C-12 AND TITANIUM(IV)-OXIDE

The relation between the ratio of the natural C-13 and C-12 isotopes o f carbon in silages of a C-3 species (winter-barley) and a C-4 species (maize) was examined as a potential marker in rate of passage studies in ruminants concurrently with the external marker titanium(IV)-oxide (TiO2). Diets were formulated to contain 140 g crude protein kg(-1) d ry matter. Two ruminally cannulated steers and two wethers were fed on a diet based on whole-plant winter-barley silage (BS) once daily at 0 7.00 h. The animals received one single meal of whole-plant maize sila ge (MS), concurrently with a single dose of TiO2. Animals were then sw itched back to BS for the remainder of the experiment. Faeces were col lected over 5 days post MS feeding. Ruminal pH in steers appeared to b e unaffected by switches from C-3 to C-2 silage feeding, and vice-vers a. In steers, Faecal samples peaked in TiO2 value approximately 22 to 28 h after dosing, and then declined. Carbon isotope values peaked app roximately 31 to 35 h after steers received the C-4 silage, and then d eclined. The time of this peak corresponded closely to that from a stu dy which used intrinsically labelled C-13 forage. Carbon isotope value s from faecal samples of wethers showed no clear peak and decline. How ever, measurement of carbon isotope ratios in neutral detergent fibre of silage and faeces instead of whole material greatly improved curve shaping. Provided that C-3 and C-4 forages of similar chemical composi tion and digestibility are used in order to minimize perturbation of r umen fermentation and subsequent passage processes, naturally occurrin g differences in stable carbon isotope ratios may be used as passage m arkers in ruminants. This restriction will limit the potential use of estimating passage rates from naturally occurring differences in carbo n isotope ratios to only a few specific cases.