Calculating total-tract apparent digestibility is not different when adjusting or not adjusting intake for the ort nutrient composition for lactating cow diets

In a method agreement study, as part of a larger randomized controlled trial, the total-tract apparent digestibility (TTAD) of DM, starch, and ash-corrected neutral detergent fiber (aNDFom), was calculated and compared between using the orts-adjusted diet, which accounts for the nutrient content of the TMR and orts and using the TMR-adjusted diet, which only accounts for the nutrient content of the TMR. Multiparous Holstein cows (n = 28) between 32 and 71 DIM were enrolled and moved to a freestall pen with Calan gates and fed a high-corn silage lactating diet for 10 weeks. Total-tract apparent digestibility was measured 3 times during the study with samples collected on 2 consecutive days at each time point. Bland-Altman plots were generated and data were analyzed using SAS software with a paired T-Test to evaluate mean bias between the methods and with Pearson correlations to determine the agreement between methods and between the means and differences of the 2 methods. Lin's concordance correlation coefficient (CCC) was calculated to assess the agreement between the 2 methods. There was significant correlation between the orts- and TMR-adjusted diets for DM, starch, and aNDFom TTAD and excellent agreement, as measured by CCC, between the methods for all nutrient TTAD. There was evidence that the mean difference between methods was different from 0 for DM, starch, and aNDFom TTAD; however, the mean bias was minimal. In addition, we did observe a correlation between the bias and the magnitude for DM and starch TTAD, but not for aNDFom. Overall, there was excellent agreement between methods and the mean bias and correlation between the difference and mean TTAD values observed is minimal and likely not to be analytically significant. In conclusion, analyzing the nutrient composition of the orts may not be necessary when calculating TTAD for lactating cow diets.