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Ruminal Digestion and Fermentation of High-Producing Dairy Cows with Three Different Feeding Systems Combining Pasture and Total Mixed Rations

Department of Dairy and Animal Science, The Pennsylvania State University, University Park 16802

Corresponding author:
L. D. Muller; e-mail:
lmuller{at}psu.edu.

Six multiparous Holstein cows fitted with rumen cannulas were used to study the effect of three feeding systems combining pasture and total mixed rations (TMR) on ruminal digestion in a 21-wk repeated measures experiment. The three treatments were: 1) pasture plus concentrate (PC), 2) pasture plus partial TMR (pTMR), and 3) TMR (nonpasture). Ruminal NH₃-N concentration was lower on both the pTMR and TMR treatments (10.2 ± 0.5 mg/dL) than on the PC treatment (19.9 ± 0.5 mg/dL). Ruminal pH was not affected by treatments and averaged 5.87. Neither total volatile fatty acid concentration (137.5 mmol/L) nor individual volatile fatty acid proportions (63.1, 20.6, and 12.0 mol/100 mol for acetate, propionate, and butyrate, respectively) differed among treatments. The pTMR treatment reduced the total potentially degradable fraction of dry matter (85.5 vs. 82.3%) and the potentially digestible fraction of neutral detergent fiber (82.1 vs. 74.9%) of pasture compared to the PC treatment. Ruminal NH₃-N losses were reduced when combining pasture and TMR; however this combination decreased the ruminal digestion of pasture, indicating the presence of associative effects in the rumen.

Key Words: pasture • partial total mixed ration • rumen fermentation • in situ digestion

Abbreviation key: ED = effective degradability, PC = pasture plus concentrate, pTMR = pasture plus partial TMR

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