97 Can Fecal Volatile Fatty Acids be Used as an Indicator of Ruminal Volatile Fatty Acids?

Abstract

Abstract For in vitro digestibility experiments addressing both horses and cattle, feces have long been thought to be an equivalent surrogate for ruminal or cecal inoculum that would eliminate the need for fistulated animals. However, this assumption requires similar microbial and chemical properties among inoculum sources. Validation of the relationship of fecal to ruminal metabolic properties would allow for more readily accessible information on fermentation across nutrition experiments. Thus, the purpose of this study was to compare ruminal and fecal VFA production from heifers fed four bermudagrass (Cynodon dactylon [L.] Pers.) cultivars. Ruminally-fistulated heifers (n = 4) were assigned randomly to one of four bermudagrass cultivars (Coastal [COS], Russell [RUS], Tifton 44 [T44], or Tifton 85 [T85]) for four 30-d in vivo periods (21-d adaptation and 9-d collection). On d 28, rumen fluid (RF) samples were collected at 13 timepoints (0, 0.25, 0.5, 0.75, 1, 2, 3, 4, 8, 12, 16, 20, 24 h) and rectal fecal samples were collected at 7 timepoints (0, 0.5, 1, 2, 4, 8, 12 h). Samples were acidified with meta-phosphoric acid, and 2-ethylbutyric acid was included as an internal standard. Samples were assayed for acetic (A), propionic (P), isobutyric (IB), butyric (B), isovaleric (IV), valeric (V), 4-methylvalteric (4MV), and caproic (C) acids by gas-liquid chromatography. Data were analyzed using SAS v. 9.4. Heifers receiving COS produced less (P < 0.05) RF concentrations of A (25.01 mM), P (5.21 mM), and total VFA (34.2 mM) than T85 (43.4, 8.9, and 58.0 mM, respectively), T44 (42.0, 9.0, and 57.3 mM, respectively), and RUS (39.8 8.9, and 55.3 mM, respectively), which were all similar. Ruminal B was greatest (P < 0.05) from RUS (5.4 mM) and least from COS (3.1 mmol/L), with T44 and T85 intermediate (4.9 and 4.4 mM, respectively). There was no effect of cultivar (P ≥ 0.10) on RF IB, IV, V, or 4MV concentrations. Fecal IB was correlated (P < 0.05) with RF IV (r = 0.59) and tended to be correlated (P ≤ 0.09) with RF A (r = 0.39), P (r = 0.39), C (r = 0.37), and total VFA (r = 0.38). Fecal P was correlated (P < 0.05) with RF IV (r = 0.51). No other fecal VFA were correlated with RF VFA. Results are interpreted to mean that COS results in decreased ruminal fermentation relative to the other cultivars tested. However, there is little indication that fecal VFA can serve as an indicator for ruminal fermentation.