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RESEARCH ARTICLE
Contents Vol 48(7)

Ruminal protein degradability of a range of tropical pastures

M. K. Bowen A C D , D. P. Poppi A and S. R. McLennan B
+ Author Affiliations
- Author Affiliations

A Schools of Animal Studies and Veterinary Science, University of Queensland, St Lucia, Qld 4072, Australia.

B Department of Primary Industries and Fisheries, Locked Mail Bag 4, Moorooka, Qld 4105, Australia.

C Present address: Department of Primary Industries and Fisheries, PO Box 6014, CQMC Rockhampton, Qld 4702, Australia.

D Corresponding author. Email: maree.bowen@dpi.qld.gov.au

Australian Journal of Experimental Agriculture 48(7) 806-810 https://doi.org/10.1071/EA07414
Submitted: 18 December 2007  Accepted: 6 March 2008   Published: 20 June 2008

Abstract

The rumen degradability parameters of the diet selected by two to four oesophageal-fistulated Brahman steers grazing a range of tropical pastures were determined by incubation of extrusa in nylon bags suspended in the rumen of rumen-fistulated (RF) Brahman steers. The effective protein degradability (Edg) was determined by measuring the rate of disappearance of neutral detergent insoluble nitrogen (NDIN) less acid detergent insoluble nitrogen (ADIN) in the incubated extrusa. Six to eight RF steers also grazed each of the pastures along with the oesophageal-fistulated steers, to allow determination of key rumen parameters and rumen particulate matter fractional outflow rates (FOR). The seven pastures studied included: native tropical grass (C4) pasture (major species Heteropogon contortus and Bothriochloa bladhii), studied in the early wet (NPEW), the wet/dry transition (NPT) and the dry (NPD) seasons; introduced tropical grass (C4) pasture (Bothriochloa insculpta), studied in the mid wet season (BB); the introduced tropical legumes (C3), Lablab purpureus (LL) and Clitoria ternatea (BP); and the temperate grass (C3) pasture, ryegrass (Lolium multiflorum, RG). Using the measured particle FOR values in calculations, the Edg estimates were very high for both C4 and C3 species: 0.82–0.91 and 0.95–0.98 g/g crude protein (CP), respectively. Substitution of an assumed FOR (kp = 0.02/h) for the measured values for each pasture type did not markedly affect estimates of Edg. However, C4 tropical grasses had much lower effective rumen degradable protein (ERDP) fractions (23–66 g/kg DM) than the C3 pasture species RG and LL (356 and 243 g/kg DM, respectively). This was associated with a lower potential degradability and degradation rate of organic matter (OM) in sacco, lower in vitro organic matter digestibility (IVOMD) and CP concentrations in the extrusa, and lower ammonia-N and branched-chain fatty acid concentrations in rumen fluid for the tropical grasses. As tropical grass pastures senesced, there was a decline in Edg, the ERDP and rumen undegradable protein (UDP) fractions, the potential degradability and degradation rate of OM and the IVOMD. These results provide useful data for estimating protein supply to cattle grazing tropical pastures.


Acknowledgements

This study was mainly funded by the Department of Primary Industries and Fisheries, Queensland (DPI&F), who provided postgraduate support for M. K. Bowen. M. K. Bowen was also in receipt of an Australian Government Postgraduate Research Award. We are grateful to the Health and Nutritional Biochemistry Laboratory of DPI&F for conducting laboratory analyses and to Vivienne Doogan for assistance with statistical analyses. We also thank the staff of Brian Pastures Research Station, Gayndah for providing assistance with field work.


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