Evaluation of controlled release devices for administration of chromium sesquioxide using fistulated grazing sheep. 2. Variation in rate of release from the device

Abstract

Merino sheep fitted with rumen cannulae and varying in liveweight, grazed Phalaris aquatica pastures with high and low dry matter mass. One or more controlled release devices were tethered inside the rumen and attached to its cannula. The position of the plunger within the controlled release device was recorded every 2 or 3 days for periods of up to 19 days, and the mean rate of plunger movement was calculated. Faecal output and flow of rumen digesta were measured concurrently in two experiments. Rate of plunger movement was measured in 152 devices and found to be substantially constant over time, but significant differences in mean rate of plunger movement were observed between formulation type, between sheep and between pastures varying in herbage mass. Coefficients of variation between sheep ranged from � 6 to �14%, and differences between pasture means ranged from 1 to 14%. Some of this variation was associated statistically with faecal output; assuming a linear relationship, a change in dry matter output of 100 g/day was associated with a change in plunger rate of 0.05 mm/day or about 6% of the mean rate. Liquid flow through the rumen was the rumen variable which had the highest statistical correlation with the rate of plunger movement.