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Pasture Intake and Substitution Rate Effects on Nutrient Digestion and Nitrogen Metabolism during Continuous Culture Fermentation

F. Bargo^1,*, G. A. Varga^*, L. D. Muller^* and E. S. Kolver

^* Department of Dairy and Animal Science, The Pennsylvania State University, University Park 16802
Dexcel Ltd., Private Bag 3221, Hamilton, New Zealand

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

A continuous culture system was used to investigate ruminal digestion in response to increased pasture intake and three different substitution rates (SR) in a 4 x 4 Latin square design. The treatments were 1) low pasture (55 g dry matter (DM)/d, 2) medium pasture (MP, 65 g DM/d), 3) high pasture (75 g DM/d), and 4) pasture (45 g DM/d) plus concentrate (PC, 30 g DM/d). Treatments were designed to produce a low (0.33), medium (0.67), and high (1.00) SR (g of pasture/g of concentrate) by contrasting the low, medium, and high pasture intake treatments with the pasture plus concentrate treatment, respectively. Pasture was fed at 0630, 1000, 1730, and 2100 h, and concentrate at 0600 and 1700 h. Digestibility of DM and neutral detergent fiber were not affected by the amount of pasture. As the amount of pasture increased, pH decreased linearly, and total volatile fatty acid and NH₃-N concentrations, and nonammonia N and bacterial N flows increased linearly. Concentrate supplementation did not affect DM digestibility at high SR but increased DM digestibility at low SR. Concentrate supplementation reduced pH and NH₃-N concentrations at the three SR. Concentrate supplementation reduced the ratio of rumen degradable N to rumen degradable organic matter; however, the mechanism depended on the SR. High SR, concentrate supplementation reduced rumen degradable N, which reduced NH₃-N concentration without affecting bacterial N flow. At low SR, concentrate supplementation increased rumen degradable organic matter, which reduced NH₃-N concentration and increased bacterial N flow. Based on these results, at low SR, concentrate supplementation may enhance animal performance because of higher total DM intake and synthesis of microbial protein.

Key Words: pasture • substitution rate • nutrient digestion • N metabolism

Abbreviation key: BN = bacterial N, HP = high pasture, LP = low pasture, MP = medium pasture, NI = N intake, OMI = OM intake, PC = pasture plus concentrate, RDN = rumen degradable N, RDOM = rumen degradable OM, SR = substitution rate, TN = total N

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