Rapid determination of indigestible NDF and the markers chromium oxide and titanium dioxide of supplement, forages and faecal samples by ICP-OES and near-infrared reflectance spectroscopy

Accurate determination of dry matter digestibility (DMD) and total faecal output (FO) is necessary for estimating ruminant dry matter intake (DMI) under grazing conditions. This study evaluated the efficacy of inductively coupled plasma optical emission spectrometry (ICP-OES) for quantifying external markers [chromium oxide (Cr₂O₃) and titanium dioxide (TiO₂)] and near-infrared reflectance spectroscopy (NIRS) for quantifying external markers and indigestible NDF (iNDF) in samples from grazing ruminant systems under diverse grazing conditions in Colombia. Cr₂O₃ and TiO₂ were processed via wet acid digestion and analysed using ICP-OES, while iNDF was determined through in vitro fermentation (288 h). NIRS equations were developed using modified partial least squares method with cross-validation to prevent model overfitting. ICP-OES methodology yielded detection limits for Ti (2.97–5.95 µg·g⁻¹) and Cr (23.08–46.16 µg·g⁻¹). Ti concentration was analysed in 285 forages samples and Cr concentration in 213 samples, with mean values of 34.52 and 7.06 µg·g⁻¹, respectively. For iNDF, 1285 total samples were processed, including 421 forages, 3 commercial feed supplements, 4 partial mixed rations and 855 faecal samples, with mean values of 18.2%, 4.37%, 2.07% and 23.66% iNDF, respectively. NIRS validation for forages showed satisfactory predictive capacity for iNDF and Ti (r2 = 0.88 and 0.89, respectively) and RPD (2.85 and 2.89, respectively) compared to Cr (r2 = 0.42 and RPD = 1.23). In faecal samples without marker added, Ti showed a satisfactory fit (r2 = 0.86 and RPD = 2.65), while for iNDF demonstrated lower accuracy (r2 = 0.64 and RPD = 1.67). The ICP-OES and NIRS techniques proved efficient methods for Ti, Cr and iNDF marker determination, offering potential for accurate estimation of ruminant DMI under grazing conditions.