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Sucrose concentration alters fermentation kinetics, products, and carbon fates during in vitro fermentation with mixed ruminal microbes¹

US Dairy Forage Research Center, USDA-ARS, Madison, WI 53706

² Corresponding author: mbhall{at}wisc.edu

Effects of sucrose (Suc) concentration on fermentation kinetics and products were evaluated using 3 concentrations of Suc, with 1 concentration of isolated NDF from Bermudagrass fermented together in batch culture in vitro with rumen inoculum. Fixed amounts of medium and inoculum were the protein sources, so protein:Suc decreased with increasing Suc. Kinetics were calculated from gas production over 48 h in a randomized complete block design (n = 28), and product yield was evaluated with sampling every 4 h for 24 h in a split-split plot in time design (n = 84). Fermentation vial was the experimental unit. Increasing Suc increased the lag time of rapidly (P < 0.01) and slowly fermented (P < 0.01) fractions and tended to decrease the rate of gas production from the rapid fraction (P = 0.07). Gas production from the slow fraction decreased linearly with increasing Suc (P = 0.02), suggesting a decrease in NDF fermentation. Sucrose was the predominant substrate at 8 h of fermentation. Maxima for microbial CP (MCP) production were detected at 8 h of fermentation. At detected MCP maxima, MCP production increased linearly (P = 0.02) and total organic acids (sum of lactate, acetate, propionate, and butyrate; mmol) tended to increase linearly (P = 0.07) with increasing Suc. Maximum lactate production at 0 and 4 h increased (P = 0.01), and yield of lactate from Suc tended to increase, linearly (P = 0.09) with increasing Suc. At detected MCP maxima, yield of C in products (total organic acids, MCP, CO₂, CH₄, glycogen) from utilized Suc C declined linearly for total products (P = 0.01) and organic acids (P = 0.01) and tended to decline for MCP (P = 0.12) as Suc increased. This may be a function of increased catabolic inefficiency of microbes with increasing Suc, as evidenced by increasing yields of lactate, or the use of C for products not measured. Product C yields were 1.28, 0.98, and 0.81 from lowest to greatest Suc inclusion, respectively. Values >1 indicate incorporation of C from the medium, likely from AA and peptides. The results support the premises that direct effects of Suc concentration and perhaps protein:Suc alter yields of fermentation products. That substrate concentration altered fermentation products and kinetics, possibly due to interactions with the run conditions, advises the clear definition of substrates and fermentation conditions to determine how the results integrate into our knowledge of ruminant nutrition.

Key Words: cattle • fermentation kinetics • fermentation product • nonfiber carbohydrate • rumen fermentation • sucrose

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