Effect of herbage type and level of intake on the release of chromic oxide from intraruminal controlled release capsules in sheep

Abstract

Controlled release capsules (CRC) provide for the continuous uniform release of chromic oxide (Cr₂O₃) into the rumen and so offer an improved means of estimating feed intake in grazing animals. Two experiments were conducted with sheep to measure the effect of herbage type and level of herbage intake on the pattern of Cr₂O₃ release from CRC. In Experiment 1, differences in CRC plunger travel within and between rumen-fistulated sheep, fed lucerne chaff indoors over 42 days at an allowance equivalent to 1.1 maintenance (M), were not significant. The relationship between plunger travel and time was almost perfectly linear (P < 0.001), but a slight reduction in the rate of plunger displacement became more pronounced while the animals were at grazing between Day 43 and Day 100. Chromium (Cr) release from Day 3 to Day 42 averaged 62 ± 1 (mean ± SE) mg Cr/day. In Experiment 2, rumen-fistulated and intact wethers, housed indoors, were offered ryegrass, white clover, ryegrass-white clover mixed, or meadow hay at daily allowancesequivalent to 0.8, 1.1, and 1.4 M for 42 days. Plunger travel was linearly related to time (r ≥ 0.988) and yielded an average release of 62 ± 2 mg Cr/day. Although the interaction between feed type and rate of plunger travel was significant (P < 0.001), differences were small and, for field trials with similar feed types, a single linear regression could be used to determine the average rate of Cr release. Cr appeared in the faeces within 24 h of CRC insertion (Day 0) and reached a steady state in the faeces between Day 6 and Day 9. Recovery of Cr from the faeces of the rumen-fistulated sheep was not significantly different from 100%. Cr recoveries for the intact wethers suggested that the rate of plunger travel for these animals was similar to that of the rumen-fistulated animals. Diurnal variation in faecal Cr concentration was low (coefficient of variation = 8.3%). The uniformity of CRC plunger travel, almost complete recovery of Cr, and low diurnal variation in faecal Cr concentration (facilitating flexible faecal sampling routines), together with reduced animal handling required to administer Cr₂O₃, confirm the potential of CRC for estimating voluntary herbage intakes of ruminants grazing at pasture.

Keywords:

• voluntary intake
• herbage
• chromium
• controlled release capsule
• sheep
• ruminants