Diet, marker and fecal sampling method interactions with internal and external marker pairs when estimating dry matter intake in beef cattle

The use of markers is currently the most used technique for estimating feed intake in production animals when direct measurement is not possible. Twelve growing Nellore bulls were used in a crossover design aiming to evaluate the accuracy of internal markers to estimate dry matter digestibility (DMD), the accuracy of external markers to estimate fecal output (FO), and the combination of internal and external markers to estimate dry matter intake (DMI) at different fecal sampling procedures. Animals received one of the four diets which varied in forage source (corn silage or Tifton-85 hay) and forage to concentrate ratio (F:C; 60:40 or 40:60) in a 2 × 2 factorial arrangement. The internal markers, acetyl bromide lignin (ABL), indigestible aNDFom (iNDF), indigestible ADFom (iADF) and cutin, are naturally present in the feedstuffs and the external markers, Cr2O3 and TiO2, were daily fed to the animals. Three fecal sampling procedures were tested: bulk 72-h (continuous feces collection for 72 h), grab 9 × 3 (sample collection every 9 h over 3-d period), and grab 4 × 4 (4 fecal samples collected during daylight hours over 4-d period). None of the internal or external markers evaluated had complete fecal recovery (FR), being necessary to establish the FR of the markers in order to obtain correct estimates. The FR of ABL, iNDF and iADF were close to 100% when animals received a hay-based diets but close to 50% when feeding a corn silage-based diets, regardless the F:C ratio of the diet. However, ABL produced accurate DMD estimates only with both grab sampling procedures across all diets, while iNDF and iADF produced accurate DMD estimates across all sampling procedures and diets. Cutin failed to produce accurate DMD consistently. Both external markers produced accurate FO under both grab sampling procedures, except for TiO2 when the grab 4 × 4 sampling procedure was performed for animals receiving the diet with high silage proportion. In general, the grab sampling procedures yielded accurate DMD, FO and DMI estimates, which were similar or better than bulk 72 h sampling procedure. The combination of internal and external markers to estimate DMI produced satisfactory and accurate results, particularly when Cr2O3 or TiO2 was paired with iNDF or iADF under both grab sampling procedures. The grab sampling procedures can replace TFC, opening new possibilities for collectively housed animals