Comparison of in vitro and in situ methods in evaluation of forage digestibility in ruminants1

Abstract

The objective of this study was to compare the application of different in vitro and in situ methods in empirical and mechanistic predictions of in vivo OM digestibility (OMD) and their associations to near-infrared reflectance spectroscopy spectra for a variety of forages. Apparent in vivo OMD of silages made from alfalfa (n = 2), corn (n = 9), corn stover (n = 2), grass (n = 11), whole crops of wheat and barley (n = 8) and red clover (n = 7), and fresh alfalfa (n = 1), grass hays (n = 5), and wheat straws (n = 5) had previously been determined in sheep. Concentrations of indigestible NDF (iNDF) in all forage samples were determined by a 288-h ruminal in situ incubation. Gas production of isolated forage NDF was measured by in vitro incubations for 72 h. In vitro pepsin-cellulase OM solubility (OMS) of the forages was determined by a 2-step gravimetric digestion method. Samples were also subjected to a 2-step determination of in vitro OMD based on buffered rumen fluid and pepsin. Further, rumen fluid digestible OM was determined from a single 96-h incubation at 38°C. Digestibility of OM from the in situ and the in vitro incubations was calculated according to published empirical equations, which were either forage specific or general (1 equation for all forages) within method. Indigestible NDF was also used in a mechanistic model to predict OMD. Predictions of OMD were evaluated by residual analysis using the GLM procedure in SAS. In vitro OMS in a general prediction equation of OMD did not display a significant forage-type effect on the residuals (observed - predicted OMD; P = 0.10). Predictions of OMD within forage types were consistent between iNDF and the 2-step in vitro method based on rumen fluid. Root mean square error of OMD was least (0.032) when the prediction was based on a general forage equation of OMS. However, regenerating a simple regression for iNDF by omitting alfalfa and wheat straw reduced the root mean square error of OMD to 0.025. Indigestible NDF in a general forage equation predicted OMD without any bias (P ≥ 0.16), and root mean square error of prediction was smallest among all methods when alfalfa and wheat straw samples were excluded. Our study suggests that compared with the in vitro laboratory methods, iNDF used in forage-specific equations will improve overall predictions of forage in vivo OMD. The in vitro and in situ methods performed equally well in calibrations of iNDF or OMD by near-infrared reflectance spectroscopy.

Predicting feeding value of forage maize hybrids harvested at different maturities and sites

“…Despite this, EFOS is the recommended laboratory method for whole crop maize in the Nordic feed evaluation system NorFor (Åkerlind et al, 2011). Poor prediction of OMD IN SITU might be due to limitations of enzymes compared to rumen fluid microorganisms used in vitro, in agreement with other recent comparisons in the literature (Koukolová et al, 2004;Jančík et al, 2011;Krizsan et al, 2012). Further, differences between determined in vitro and in vivo digestibility may vary among forage types and species.…”

“…This is in accordance with previous reports that the VOS method is a good predictor of in vivo OMD . The under-prediction of OMD adapting the VOS technique was greater in this study than what Krizsan et al (2012) found when comparing the predictions with values of in vivo OMD in maize silage samples. The VOS technique has the advantage over other in vitro methods of using smaller amount of rumen fluid and longer incubation period thereby being less sensitive to fluctuations in condition of the inoculum .…”

“…The objective of this study was to compare different analytical methods used for predicting OMD and iNDF concentrations in forage maize, taking into account effects of hybrid, site and maturity on the difference between observed and predicted values. Unlike other recent assessments of analytical techniques for evaluating forage quality (e.g., Gosselink et al, 2004;Krizsan et al, 2012) the plant material in this study only included one forage type. The NIRS predictive ability of feed value traits of forage maize covering a wide variation in phenological growth stages at high latitudes is not well-known.…”

“…Instead of OMD determined in vivo, we used predictions of OMD based on forage iNDF and NDF concentrations as reference method in the present study. This decision relied on findings by Huhtanen et al (2006) and Krizsan et al (2012) that OMD predicted from forage iNDF concentration generated least prediction error compared to in vivo digestibility data. The method was also more uniform across forage types than any other laboratory method evaluated.…”

“…Predictions of forage OMD performed almost equally well based on the 96-h in vitro incubation method as when predicted from indigestible NDF (iNDF) determined from a 288-h ruminal in situ incubation . It has been shown, however, that in situ was a more uniform method and would improve overall predictions of forage in vivo OMD at commercial laboratories in Nordic countries (Huhtanen et al, 2006;Krizsan et al, 2012). The performance of these analytical methods as predictors of OMD has not been evaluated for forage maize grown at higher latitudes before.…”