PSIV-7 Optimization of Indigestible Neutral and Acid Detergent Fiber Measurement Protocols

Dry matter digestibility (DMD) is a factor that impacts nutrient use efficiency and growth performance of beef cattle. Indigestible neutral (INDF) and acid detergent fiber (IADF) are internal markers commonly used to estimate DMD. The objective of this experiment was to optimize existing protocols for determination of INDF and IADF in feedlot rations and feces. This experiment implemented a 2×2×4 factorial arrangement with a starter or finisher diet, with or without a pepsin and HCl pre-digestion of the diets, and with four ruminal incubation lengths (72, 96, 120, or 288 h). The samples used in this experiment were from diets fed and total fecal collections obtained from 6 steers consuming the starter or finisher diet. Fecal and diet marker concentrations were analyzed using a mixed model ANOVA with ruminally canulated steer (n = 2) as a random effect and diet, pre-digestion, marker, time, and possible interactions as fixed effects. A two-sided t-test was used to test if marker fecal recoveries differed from 100%. When diets were pre-digested, a plateau in dietary marker concentration was achieved, where concentrations of both markers were not different (P ≥ 0.10) when incubated for 120 h compared with 288 h. When diets were not predigested, the dietary content of both markers continued to decrease with increasing ruminal incubation length so that marker content of 120-h incubations were significantly greater (P < 0.01) than 288-h incubations, except for IADF content of the finisher diet where the 120 and 288 h were not different (P = 0.77). The dietary content of INDF and IADF for samples that were not pre-digested and incubated for 288 h were not different (P ≥ 0.64) from the samples that were pre-digested and incubated for 120-h. For the INDF marker across both diets, fecal concentrations were lowest (P ≤ 0.01) when ruminally incubated for 288 h compared with all other ruminal incubation durations. The fecal concentrations of IADF were less (P < 0.01) when incubated for 288 h compared with those incubated for 120 h for the starter diet, but only tended (P = 0.07) to be less at those same time points for the finisher diet. When pre-digestion was conducted, only 120-h ruminal incubations had fecal recoveries that were not different (P ≥ 0.26) from 100% for all markers and diet types. If a pre-digestion was not conducted, then only the 288 h length provided fecal recoveries that were not different (P ≥ 0.12) from 100% for all diets and markers. Based on these results, using a pepsin and HCl pre-digestion followed by a 120-h ruminal incubation shortens time requirements and provides acceptable measurements of INDF and IADF concentrations in feeds and feces. If pre-digestion of diet samples is not possible then a 288-h ruminal incubation length is required.