• Ytterbium marker solution - dissolve 10 g ytterbium (III) chloride hexahydrate (Aldrich Chemical Co., mol. wt 387.54) in 1 l distilled water and adjust the pH to 3.8 using 0.1 M HCl.
Procedure. Weigh 100 g fresh weight of the feed material, previously chopped or unchopped as required for the experiment, and suspend in 1 l of the ytterbium marker solution for 24 h. Subsequently rinse the feed material in distilled water and re-soak in fresh distilled water every hour for 6 h to remove all unbound ytterbium ions. Pour off the water and dry at 50°C until the weight is approximately 100 g, i.e. the same as the original weight of fresh feed material. Accurately weigh approximately 15 g subsample and dry at 60°C to constant weight, weigh and calculate the dry matter. Mill the dried marked feed to 1 mm and retain for ashing before Yb analysis.
Values for the resulting ytterbium concentration in DM for some feeds and corresponding faeces are given by Morrow (1998):
Big bale silage (long chop-length) 17,637 pg g-1
Big bale silage (short chop-length) 29,178 pg g-1
Hay (long chop-length) 12,922 pg g-1
Hay (short chop-length) 18,052 pg g-1
As would be expected, the smaller the feed particle size, the greater the surface area per given weight is available for bonding to the ytterbium ions, hence the greater the concentration of marker. The greatest concentration of ytterbium in faecal DM occurred after approximately 24 h, with the concentration approximately zero after 75 h; the mean retention time was 26.07 h. The maximum ranges for the concentration of ytterbium in the faecal DM after approximately 24 h was approximately as follows:
Big bale silage (long chop-length) 500-800 pg g-1
Big bale silage (short chop-length) 300-500 pg g-1
Hay (long chop-length) 500-750 pg g-1
Hay (short chop-length) 750-850 pg g-1
Calculation of mean retention time. The mean retention time (MRT) may be calculated using equations given in Blaxter et at. (1956) or the simplified version in Pearson and Merritt (1991):
MRT = ZMtrLMj where Mj is the concentration of marker excreted in the faeces at time ti following the administration of the marked feed. This equation was also used for studying flow rates in equines by Nyberg etal. (1995).
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