In vitro digestion studies using 14C-labelled polyethylene glycol (PEG) 4000: comparison of six tanniniferous shrub legumes and the grass Panicum maximum

The in vitro digestibility of dry matter (IVDMD) and of nitrogen (IVND) was measured on six leguminous tropical shrubs and the grass Panicum maximum, in the presence and absence of polyethylene glycol (PEG) 4000. The PEG was spiked with 14C-labelled PEG 4000 so that PEG remaining in the residues after digestion could be calculated. This was subtracted from the residues to enable a corrected IVDMD (CIVDMD) to be calculated. The plant samples were also mixed with labelled PEG in a buffer medium for 24 h, and the remaining PEG in the supernatant measured to estimate the PEG bound to the sample. PEG binding was then related to IVND and to the difference between the IVND of the samples measured with and without PEG. In the absence of PEG, the ranking of the species for IVDMD was: Gliricidia sepium>Panicum maximum>Leucaena leucocephala>L. trichandra>L. pallida>Calliandra calothyrsus>Acacia boliviana. PEG increased IVDMD for A. boliviana, decreased it for L. pallida and had no effect on the other species (p>0.05). The CIVDMD values for the legumes were all higher with + PEG by a mean of 8.6% units, though the species ranking remained the same. IVND varied from 82% for G. sepium to 49% for C. calothyrsus in the absence of PEG. The rankings were similar to those for IVDMD. PEG increased the IVND of all species except the grass and reduced the range from 86.6% with G. sepium to 75.5% with P. maximum. There are no condensed tannins in P. maximum and so PEG had no effect on IVDMD or IVND. PEG binding of the legume species in mg/g DM (x) was negatively related to IVND% (y) in the absence of PEG: y=92.3−0.322x; r2=0.941, and to the IVND difference between the + and − PEG treatments (y): y=−7.5+0.303x; r2=0.923. sults show that the tannins in these shrub species can have a large effect on IVDMD and an even larger effect on IVND, consequently the potential value of A. boliviana and C. calothyrsus for improving animal production should be questioned. The digestibility of the psyllid-tolerant L. pallida and L. trichandra was also shown to be lower than that of the proven high quality but psyllid susceptible L. leucocephala.