Characterization of the variation in the daily excretion of faecal constituents and digestibility predictions in beef cattle fed feedlot diets using near infrared spectroscopy

Abstract Text: The 24-h variation in faecal nutrient excretion and accuracy of digestibility predictions using spot samples collected from feedlot cattle were evaluated using near infrared spectroscopy (NIRS). Six heifers were individually housed and randomly assigned to one of two feeding frequencies; once per day (0900), or twice per day (two equal feedings at 0900 and 1700), first over a backgrounding period, followed by a finishing period. Heifers were fed the backgrounding diet for 21 d, transitioned to the finishing diet over 20 d, which was fed for 21 d. During the last 4 d of both periods, total faecal collections were conducted at 4-h and 4 d-24-h intervals and NIRS calibrations were used to predict faecal organic matter (OM), starch, nitrogen (N), neutral detergent fiber (NDF), acid detergent fiber (ADF) and acid detergent lignin (ADL). Estimated total tract digestibility (eTTD) using NIRS predicted faecal nutrients and ADL and apparent total tract digestibility (aTTD) of DM, OM, starch, NDF, and ADF determined using previously derived NIRS calibrations were calculated at each 4-h interval as well as over 4 d.  Faecal DM (%), NDF, and ADF varied among 4-h interval samples in the backgrounding period, and faecal DM, starch, NDF, ADF, and ADL varied in the finishing period.  Faecal starch was able to predict aTTD during both feeding periods (backgrounding: R2 = 0.96, P < 0.01; finishing: R2 = 0.98, P < 0.01). Most 4-h interval samples could be used to predict eTTD of nutrients and aside from starch in the finishing period, there were no differences for eTTD using faecal samples collected at any of the 4-h intervals versus those collected over 4 d. The NIRS calibrations for predicting aTTD coefficients using the 4-h interval samples or the 4 d-24-h composite were least accurate for NDF and ADF. Spot faecal samples collected at any time point from multiple cattle have potential to predict digestibility. However, timing of sampling after feeding must be standardized to predict starch digestibility during the finishing period, with samples between 0-4 h and 8-16 h generating estimates of both starch concentration and digestibility that were closest to that derived from 4 d-24-h composite samples.

Keywords:

faecal nutrients, faecal starch, 24-h variation, feedlot cattle, near infrared spectroscopy