Differences between the in vitro digestibility of extrusa collected from oesophageal fistulated steers and the forage consumed
David B. Coates A C and Robert J. Mayer BA Davies Laboratory, CSIRO Sustainable Ecosystems, PO, Aitkenvale, Townsville, Qld 4814, Australia.
B Department of Primary Industries and Fisheries, PO Box 1085, Townsville, Qld 4810, Australia.
C Corresponding author. Email: david.coates@csiro.au
Animal Production Science 49(7) 563-573 https://doi.org/10.1071/EA08285
Submitted: 21 November 2008 Accepted: 25 February 2009 Published: 11 June 2009
Abstract
In a study that included C4 tropical grasses, C3 temperate grasses and C3 pasture legumes, in vitro dry matter digestibility of extrusa, measured as in vitro dry matter loss (IVDML) during incubation, compared with that of the forage consumed, was greater for grass extrusa but not for legume extrusa. The increase in digestibility was not caused by mastication or by the freezing of extrusa samples during storage but by the action of saliva. Comparable increases in IVDML were achieved merely by mixing bovine saliva with ground forage samples. Differences were greater than could be explained by increases due to completely digestible salivary DM. There was no significant difference between animals in relation to the saliva effect on IVDML and, except for some minor differences, similar saliva effects on IVDML were measured using either the pepsin–cellulase or rumen fluid–pepsin in vitro techniques. For both C4 and C3 grasses the magnitude of the differences were inversely related to IVDML of the feed and there was little or no difference between extrusa and feed at high digestibilities (>70%) whereas differences of more than 10 percentage units were measured on low quality grass forages. The data did not suggest that the extrusa or saliva effect on digestibility was different for C3 grasses than for C4 grasses but data on C3 grasses were limited to few species and to high digestibility samples. For legume forages there was no saliva effect when the pepsin–cellulase method was used but there was a small but significant positive effect using the rumen fluid–pepsin method. It was concluded that when samples of extrusa are analysed using in vitro techniques, predicted in vivo digestibility of the feed consumed will often be overestimated, especially for low quality grass diets. The implications of overestimating in vivo digestibility and suggestions for overcoming such errors are discussed.
Acknowledgements
We thank MLA for funding support for projects that provided the data for this paper. We also thank Jennifer Stanford and Kylee Verry for dedicated technical support in assisting with pen experiments and laboratory analyses.
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Appendix 3. Calculation of feed in vitro dry matter loss (IVDML) from extrusa IVDML for grass legume mixtures
Let IVDML (%) of the grass/legume extrusa sample = Sext
Let IVDML (%) of the grass in the extrusa = Gext
Let IVDML (%) of the legume in the extrusa = Lext
Let the feed IVDML (%) of the grass/legume sample = Sfeed
Let the feed IVDML (%) of the grass in the mixed sample = Gfeed
Let the feed IVDMD (%) of the legume in the mixed sample = Lfeed
Now Lext = Lfeed because there is no extrusa effect of digestibility of legumes
Gext = (0.8 × Gfeed) + 16.8 from regression in paper
Let the difference between Gfeed and Lfeed = D
Therefore: Lext = Lfeed = Gfeed + D
Let grass proportion (%/100) in the mixture = X
And therefore legume proportion in the mixture = 1 – X
Then: Sext = X(0.8 × Gfeed + 16.8) + (1 – X) × (Gfeed + D)
As Gfeed in the above equation is the only unknown, then Gfeed can be calculated
and Gext, Lfeed can be calculated from Gfeed
And Sfeed = X(Gfeed) + (1 – X) × (Lfeed).
Example
If Sext = 58%, and the proportion of grass (X) in the extrusa sample = 0.6, and the difference in IVDML between plucked grass and legume samples = 8%, then:
58 = 0.6(0.8 × Gfeed + 16.8) + 0.4(Gfeed + 8)
Therefore: 58 = 0.48(Gfeed) + 10.8 + 0.4(Gfeed) + 3.2
Therefore: Gfeed = 50.818 and Lfeed = 58.818
And Sfeed = 0.6(Gfeed) + 0.4(Lfeed) = 54.018.
