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Fermentation of eastern gamagrass (Tripsacum dactyloides [L.] L.) by mixed cultures of ruminal microorganisms with or without supplemental corn¹

J.-S. Eun^*, V. Fellner^*,2, J. C. Burns^*, and M. L. Gumpertz

^* Department of Animal Science and and Department of Statistics, and Department of Crop Science and USDA-ARS, North Carolina State University, Raleigh 27695-7621

² Correspondence—phone: 919-515-4018; fax: 919-515-7780; e-mail: Vivek_Fellner{at}ncsu.edu.

Abstract

Five dual-flow fermentors (700 mL) were used to determine the effects of eastern gamagrass (Tripsacum dactyloides [L.] L.) diets on microbial metabolism by mixed rumen cultures. Fermentors were incubated with filtered ruminal contents and allowed to adapt for 4 d to diets followed by 3 d of sample collection. Five dietary treatments were tested: 1) gamagrass hay (GH) + no corn (GHNC), 2) gama grass silage (GS) + no corn (GSNC), 3) GS + low corn (GSLC), 4) GS + medium corn (GSMC); and 5) GS + high corn (GSHC). The experiment was conducted as a randomized complete block design with five treatments and three replications. Total VFA concentrations were not affected by diets. Corn addition linearly decreased (P < 0.001) molar proportion of acetate. In contrast, molar proportion of propionate was reduced in GSLC (cubic effect, P < 0.001) but remained similar across other diets. Corn supplementation linearly increased molar proportion of butyrate (P < 0.001). The acetate + butyrate-to-propionate ratio was highest in cultures offered GSLC (cubic effect, P < 0.001) but similar across other diets. Feeding GSNC resulted in a higher ruminal pH compared with GHNC (P < 0.03). Increasing the level of corn supplementation in GS linearly decreased culture pH (P < 0.001). All diets resulted in similar methane production, with the exception of GSMC, which lowered methane output (quadratic effect, P < 0.004). Total substrate fermented to VFA and gas tended to be greater with GHNC than with GSNC (P < 0.06) and linearly increased with the addition of corn (P < 0.004). Neutral detergent fiber digestibility was similar between GH and GS and was not affected by supplemental corn. Microbial N flow increased in cultures offered GSHC (quadratic effect, P < 0.02). Corn supplementation at the medium and high level linearly decreased C _18:0 (P < 0.02) and increased trans-C_18:1 (P < 0.004). Including corn at the high level with GS did not have a detrimental effect on fermentation in dual-flow fermentors.

Key Words: Corn Supplementation • Gamagrass • Microbial Nitrogen

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