ESTIMATION OF MARKER PASSAGE IN THE GASTR OINTESTINAL-TRACT OF STEERSBY DIFFERENT MODELS - INFLUENCE OF AMOUNT OF INTAKE

This study was conducted to investigate the effects of amount of intak e of a mixed diet by steers on the passage of digesta through the reti culorumen and the whole digestive tract. Six ruminally cannulated stee rs received a mixed diet consisting on average of 43 % grass silage, 2 5 % maize silage, 30 % concentrate and 2 % mineral-vitamin mix in DM. The experimental design was a repeated 3 x 3 Latin square with 21 days periods. The diet was offered twice daily (07.00 and 19.00 h) at 1, 1 .5 and 2 times of estimated maintenance energy requirements. At the be ginning of each period, animals received pulse doses of Titanium(IV)-o xide (TiO2) per os and Cr-EDTA intraruminally. Following marker admini stration, faecal marker concentrations were determined over 120 h. Pas sage parameters were estimated by a mono-exponential and a bi-exponent ial model, by models including gamma age-dependency and by a model cal culating total tract mean retention time as mean value of all points o n the marker-excretion curve. Passage rate of TiO2 from the reticuloru men increased with higher intakes, whereas mean retention time in the whole tract decreased. In general, results of different models were co nsistent across intakes. Values for passage of TiO2 were in good agree ment with those reported for small particles, when similar diets were fed to cattle. Rate of passage of Cr-EDTA from the reticulorumen incre ased with higher intakes, and mean retention times of Cr-EDTA in the r eticulorumen and in the whole tract decreased. Differences between mod els with dr without age dependency were greater for Cr-EDTA than for T iO2. Fit to Cr-EDTA excretion curves was not satisfactory for models w ith gamma age dependency. Irrespective of model and marker, passage fr om the reticulorumen accelerated markedly, whereas retention times in the reticulorumen and in the whole tract decreased, as intake increase d from maintenance energy requirements to 1.5 times maintenance.