The effect of sample drying conditions on estimates of condensed tannin and fibre content, dry matter digestibility, nitrogen digestibility and PEG binding of Calliandra calothyrsus

Leaf samples of the browse shrub Calliandra calothyrsus were used to study the effect of temperature of drying (25, 45, 65, 85 and 105°C), and method of drying, either in dry air (aerobic) or in dry nitrogen (anaerobic), on estimates of fibre, condensed tannin (CT), digestibility and PEG binding. These estimates were also made on freeze-dried samples. Correlations were made between these variables. In general, there was much less change with increasing temperature for samples dried anaerobically than for samples dried aerobically for all measures. For the condensed tannin measures (using the butanol–HCl with tannins isolated from C. calothyrsus as a standard), there was an interaction of method × temperature of drying (p<0.01 to p<0.0001). At higher temperatures there was more bound tannin (protein- and fibre-bound) with aerobic drying. Free tannin under anaerobic conditions increased slightly with increasing temperature whereas there was a large linear decrease under aerobic drying. A similar response was obtained by the protein precipitation method though the mean level of free tannin using this method was much lower than by the butanol–HCl method (11% vs 21%). Total CT (TCT) increased slightly with temperature under anaerobic drying, but decreased by a similar magnitude (about 10%) with aerobic drying. The freeze-dried samples had values similar to samples dried at the lower temperatures (25 and 45°C) for all measures. The mean TCT for C. calothyrsus in this study of 29% is far higher than those reported in other studies using unrelated tannin standards. etergent fibre (ADF) was higher for the aerobically dried samples, and levels for both drying methods increased slightly with temperature. For neutral detergent fibre (NDF), levels rose at temperatures above 45°C; the rise with aerobic drying was much greater and gave a significant interaction (p<0.01). Both the N% in NDF and the total N in NDF were higher in the aerobically dried samples. For both measures, levels declined as temperature increased from 25 to 45°C and then increased. Levels for the freeze-dried samples were similar to those anaerobically dried at 25 and 45°C. In vitro dry matter and N digestibility (IVDMD and IVND) were measured by a modified Tilley and Terry [J. Br. Grassld. Soc. 18 (1963) 104] technique using ±PEG with 14C-labelled PEG to correct IVDMD for PEG-tannin complexes in the residue (CIVDMD). With anaerobically dried samples there was no change with temperature for IVDMD, CIVDMD, or IVND in the presence of PEG (IVNDP). However, under aerobic conditions, for most measures of digestibility, levels declined above 45°C. PEG binding was unaffected by temperature of drying but, was lower (p<0.001) for aerobic drying compared with anaerobic drying, and far higher for freeze-dried than for oven-dried samples. These data support the hypothesis that the degree of complexing of CT with components of the plant is affected by both temperature and oxidation; the latter having the greater effect particularly at high drying temperatures. Where facilities for freeze drying are not available, drying at 45°C would appear to be the best option.