In vitro digestion studies using 14C-labelled polyethylene glycol (PEG): the effect of sample pretreatment on dry matter and nitrogen digestibility as well as PEG binding of Calliandra calothyrsus

The effect of sample preparation on measures of in vitro digestibility was studied using leaves of Calliandra calothyrsus. They were chopped fresh (FrCh), chopped and oven-dried at 65°C (ChOD), chopped and freeze-dried (ChFD), oven-dried and ground (ODGr) or freeze-dried and ground (FDGr). Samples were analysed for in vitro dry matter and nitrogen digestibility with or without PEG addition to bind tannins. Ground samples gave higher dry matter digestibilities (IVDMD) than chopped samples, freeze-dried samples had higher IVDMD than oven-dried samples, and fresh leaves were intermediate. The IVDMD values for FrCh, ChOD, ChFD, ODGr and FDGr were 45.3, 34.5, 40.5, 48.4 and 50.1%, respectively. Overall, PEG increased IVDMD from 42.1 to 45.4 (P<0.01), and nitrogen digestibility (IVND) from 37.0 to 66.4% (P<0.01). When corrected for PEG bound in the residue, CIVDMD increased by 8.6–15.5% depending on the pretreatment. PEG improved IVND of all samples, although the improvement expressed in percentage units was greatest with the ground samples and least with the dried chopped and fresh samples. The range in IVND was from 27.2% with ChOD−PEG to 86.0% with FDGr+PEG. The relation between PEG binding and IVND for the dried samples was linear (r2=0.96), indicating that pretreatment influenced the ability of PEG to penetrate and bind to tannins. Results emphasise the importance of standardisation of sample pretreatment for PEG binding studies and again show that IVDMD studies with tanniniferous forage and browse species using PEG will give unreliable results because of the PEG-tannin complexes in the residue.