Polyethylene glycol, used to alleviate the negative effects of dietary tannins, can also serve as a marker of fecal output in goats

Polyethylene glycol (PEG) is used to neutralize the effects of dietary tannins on nutrient digestibility. We have shown previously that it can be used as a marker of fecal output of goats fed tannin-free diets, and be accurately quantified by using near infrared reflectance (NIR) spectroscopy. However, its analysis can be complicated by the presence of tannins. In experiment 1, using the mixtures of feces, PEG and quebracho tannin (QT), we showed that QT interferes with PEG at NIR segments featuring OH-stretching and bending (2090 nm), but not at segments featuring CH stretch and CH₂ deformation (2280 nm). In experiment 2, we used calibrations of PEG in the feces of goats fed a tannin-free diet to predict PEG content in the feces of goats supplemented with QT. When goats were given 15 g per day of PEG, without QT, the analytical recovery of PEG in feces was 97.8%. However, when they also received QT, the analytical PEG recovery was only 42.7%, when calculated with a calibration equation using the whole NIR range (1100-2498 nm). Analytical recovery increased to 93.5% when a single wavelength (2280 nm) calibration equation was used. The problem of PEG-tannin bonding seems to be almost totally overcome by using calibration equations relying on the NIR segments where no chemical moieties involved in the PEG-tannin bonding are found. PEG, when provided to goats feeding on tannin-containing diets, can also probably serve as a marker of fecal output in goats.