Modeling digesta retention in horses fed high or low neutral detergent fiber concentration forages

Authors: HANSEN, TAYLER - University Of Kentucky; FOWLER, ASHLEY - University Of Kentucky; Harlow, Brittany; HAYES, SUSAN - University Of Kentucky; CRUM, ANDREA - University Of Kentucky; LAWRENCE, LAURIE - University Of Kentucky

Submitted to: Livestock Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/3/2021
Publication Date: 6/11/2021
Citation: Hansen, T.L., Fowler, A.L., Davis, B.E., Hayes, S.H., Crum, A., Lawrence, L.M. 2021. Modeling digesta retention in horses fed high or low neutral detergent fiber concentration forages. Livestock Science. 250. Article 104592. https://doi.org/10.1016/j.livsci.2021.104592.
DOI: https://doi.org/10.1016/j.livsci.2021.104592

Interpretive Summary: Residence time of digesta in the gastrointestinal tract (GIT) influences the extent of feed degradation. In general, the longer feed is present at the site of digestion, the greater the degradation and consequently the nutrients available to meet energy requirements. Previous research has suggested that longer mean retention time may increase nutrient availability and compensate for low nutrient content of high fiber diets, but there have been conflicting results reported in horses. By modeling retention time in specific segments of the horse GIT (multi-compartment modeling) we may gain a better understanding of the relationship between digesta transit and digestibility. The objective of this study was to evaluate passage kinetics in horses fed diets containing different amounts of fiber. Geldings were fed either a high-fiber (timothy cubes) or low-fiber diet (alfalfa cubes) with added concentrate. Geldings were orally dosed with Co-EDTA and Yb-labeled NDF residue markers to determine the liquid and particulate GIT retention time, respectively. Total feces were collected for 120 h after dosing and analyzed for Co and Yb concentrations. In result, there was no effect of diet on total tract mean retention time of digesta. However, by utilizing a multi-compartment model, it was demonstrated that both liquid and particulate transit time was longer in the high-fiber diet. The low-fiber diet was more digestible, but there was no relationship between diet digestibility and mean retention time. In conclusion, multi-compartment models can be used to identify differences in some digesta rate parameters between high- and low-fiber diets that are not detected with total tract mean retention time measures in horses. The impact of this research is that multi-compartment models can be used to detect differences in passage kinetics between equine diets.

Technical Abstract: The extent of feed degradation is influenced by the length of time feed resides at the site of digestion. Traditionally, mean retention time (MRT) is measured for the entire equine gastrointestinal tract (GIT), but retention time in specific organs of the GIT may be more relevant to understanding digestion. Multi-compartment models theoretically partition the GIT allowing for the potential representation of MRT in individual segments. The objective of this study was to apply a multi-compartment model to digesta passage in horses fed forage-based diets. We hypothesized an increased fiber concentration would lengthen total tract MRT, transit time (TT), and compartment retention time. Six geldings (4 ± 1.5 y, 545.2 ± 64.6 kg, mean ± SD) were randomly assigned a diet of timothy cubes (HF, n = 3) or alfalfa cubes (LF, n = 3) fed at 1.53% BW and concentrate at 0.22% BW (DM basis). After a 15-d dietary adaptation, geldings were orally dosed with Co-EDTA and Yb-labeled NDF residue to determine liquid and particulate retention time, respectively. A total fecal collection was conducted for 120 h after marker dosing. Samples were analyzed for Co and Yb concentrations by inductively coupled plasma spectrometry. Gamma distributed age-dependent two compartment models were fit to marker excretion curves for each gelding (MATLAB, vR2014a). Diet effects were analyzed by ANOVA (SAS, v9.3). There was no effect of diet on total tract MRT (liquid 21.78 ± 0.79 h, particulate 25.80 ± 0.79 h, mean ± SEM). However, liquid TT was longer (P = 0.041) in the HF diet (7.25 ± 0.91 h) than the LF diet (3.86 ± 0.91 h), and liquid age-dependent compartment retention time was shorter (P = 0.019) in the HF diet (8.40 ± 0.84 h) compared with LF diet (11.72 ± 0.84 h). Particulate TT was longer (P = 0.029) in the HF diet (8.24 ± 0.96 h) than the LF diet (4.54 ± 0.96 h). Although there was a difference in DM digestibility (HF: 50.1 ± 2.31%; LF: 60.0 ± 2.31%; P = 0.002), covariate analysis did not show a significant relationship between MRT or compartment parameters to diet digestibility. Additional research should pursue methods to ascribe physiological meaning to mathematically determined compartments; nonetheless, the current data indicates the use of a multi-compartment model can identify differences in some digesta rate parameters between high and low fiber diets that were not apparent using total tract MRT alone.