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A Comparison of Indirect Methods of Predicting in vivo Digestibility of Grazed Forage¹

Colorado Agricultural Experiment Station Fort Collins, 80521

Abstract

Six in vivo digestion trials were conducted with forage collected via the esophageal fistulae of grazing steers. In vivo digestibility coefficients were compared to those derived from various indirect techniques of estimating digestibility. The modified two-stage Tilley and Terry method proved to be the most reliable predictor of in vivo digestibility (Y), the relationship being described by = 10.2 + .79X ± 2.4 (r = .93**) where X = in vitro DM digestibility. A one-stage in vitro system was comparable in predictive ability to the two-stage techinque. Cell wall in vitro digestibility resulted in slightly less accurate predictions of in vivo digestibility, but like the summative equation technique was within acceptable limits of accuracy (r = .85**). In vitro studies employing inoculum obtained from grazing animals did not provide as reliable prediction of in vivo digestibility as did inoculum obtained from an animal fed a standard diet of grass hay. The nylon bag technique did not prove to be a valid estimator of in vivo digestibility. Digestibility coefficients obtained by this technique were up to 23% higher than corresponding in vivo values. The lignin ratio technique and forage lignin determined by KMnO⁴ procedures also gave unsatisfactory estimation of in vivo digestibility, although forage lignin determined by the H₂SO⁴ procedures was satisfactory (r = .94**). No advantage was obtained in predictive ability by expressing lignin as a percentage of ADF, compared with dry matter Fecal nitrogen proved to be a valid estimator of in vivo digestibility (r = .96**), although as with H₂SO₄ lignin, the number of observations were limited. In vivo energy digestibility was reliably predicted from a modified two-stage Tilley and Terry in vitro system by the equation Y = .97 – 1.2X ± 2.3 (r = .96**) where Y = in vivo digestible energy (%) and X = in vitro organic matter digestibility (%).

These results indicate that several in vitro methods will provide reliable estimates of the digestibility of range forage. This conclusion, however, does not apply to the nylon bag or the lignin ratio technique.

¹ Supported by the Colorado State University Experiment Station and published as Scientific Series Paper No. 1847.

² Present address: Tara Hills High Country Research Station, Kurow, New Zealand.

³ Department of Animal Science, University of Kentucky, Lexington.

⁴ Eastern Colorado Range Station, Akron.

⁵ Department of Animal Sciences, Colorado State University, Fort Collins.