Submitted to: Journal of Dairy Science
Publication Type: Abstract Only
Publication Acceptance Date: March 15, 2006
Publication Date: July 12, 2006
Citation: Hall, M., Weimer, P.J. 2006. Implications of a carbon balance study: organic acid and protein supplies change with fermentable carbohydrate:protein ratio [abstract]. Journal of Dairy Science. 89:Suppl 1:404. Technical Abstract: Three concentrations of sucrose (Suc, trt) with one of isolated neutral detergent fiber (isolNDF) from bermudagrass were fermented together in vitro with rumen inoculum to evaluate the effects of Suc concentration on partition of carbon (C) in Suc into fermentation products. Nitrogen sources were inoculum, isolNDF and casein hydrolysate and NH4HCO3 in the medium. Yield of C in fermentation products [organic acids (OA:acetate, propionate, butyrate, lactate), microbial crude protein (MCP), CO2, CH4, glycogen] from C from fermented Suc was evaluated at peak MCP production from Suc (detected at 4 to 8 h of fermentation; Suc almost entirely fermented at this point). Yield of total products, MCP and OA decreased or tended to decrease linearly with increasing Suc. This may be a function of decreased catabolic efficiency of the microbes with increasing Suc as evidenced by increasing yields of lactate, but increased energy spilling is also a possibility. Values > 1.0 indicate incorporation of C from the medium, likely from the inoculum and casein hydrolysate. When corrected for estimates of microbial cell C not in MCP, product C from Suc for the greatest Suc trt was 0.96. Even with C from MCP excluded, yield of C in other products per unit of Suc utilized still differed among trt (1.03, 0.77, and 0.61 from lowest to greatest Suc inclusion; linear effect of Suc, P = 0.017; SED = 0.054). In this study, the ratio of available protein to fermentable Suc decreased with increasing Suc. Published studies report increased yield of MCP in vitro and increased ruminal acid concentrations in vivo as the ratio of degradable protein:fermented carbohydrate was increased. Results of this C balance support the premise that the ratio of available N to Suc and direct effects of Suc concentration altered partitioning of C into products. This has implications for prediction of ruminal pH and supply of nutrients from ruminal fermentation.