Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: January 30, 2007
Publication Date: June 1, 2007
Citation: Hall, M., Weimer, P.J. 2007. Sucrose concentration alters fermentation kinetics, products and carbon fates in in vitro fermentations with mixed ruminal microbes. Journal of Animal Science. 85:1467-1478. Interpretive Summary: Sugars are a common component of forages and many by-product feedstuffs. The sugar content of feeds can vary greatly, and so sugar content of dairy cattle diets will also vary. There is some information to suggest that sugars can affect how well fiber is fermented in cattle. If sugars change fiber digestion, they have potential to change the amount of nutrients that cattle receive from their diets. Little is known about the specifics of how different amounts of sugar affect fiber fermentation. Under controlled conditions in the laboratory, we examined how sucrose (sugar) affects fiber fermentation and the products of the fermentation produced by rumen microbes. Increasing sucrose increased the lag in time before slowly (fiber) and rapidly (sucrose) fermented fractions began fermenting. Sucrose did not change the fermentation rate of the slowly fermented fraction. Production of fermentation products such as microbial crude protein, organic acids, glycogen and gases from sucrose increased as sucrose levels increased. But the amount of products produced per unit of sucrose decreased as more sucrose was added. Other factors such as the amount of true protein available to the microbes relative to the amount of sucrose may also have affected product yield. The changes in fiber fermentation and fermentation products suggest that amount of sugar in a diet may change how well cattle digest their feed and the amount of nutrients they receive from the diet. This is important to know as we use information about sugars and other components in diets to establish recommendations so that dairy cattle diets can be formulated to make most efficient use of feeds.
Technical Abstract: Three concentrations of sucrose (Suc) and one of isolated neutral detergent fiber from bermudagrass were fermented together in vitro with rumen inoculum to evaluate the effects of Suc concentration on fermentation products and on fermentation kinetics as assessed by gas production. In gas production measurements collected over 48 h, increasing Suc increased the lag time of both rapidly- and slowly fermented fractions, tended to decrease rate for the rapid fraction. Gas production from the slow fraction decreased linearly with increasing Suc, suggesting a decrease in fiber fermentation. In 24 h fermentations, Suc was the predominant substrate for microbial product formation at <8 h of fermentation. Maxima for acetate+propionate+butyrate (VFA), microbial crude protein (MCP), glycogen, lactate and CO2 achieved at or before 8 h increased or tended to increase linearly with increasing Suc. Maxima for lactate and glycogen were detected at 4 h. At 24 h, production of VFA, MCP, CO2, and CH4 increased linearly with increasing Suc. Molar proportions of VFA at 8 h did not change with Suc concentration, whereas 24 h values decreased linearly for acetate and increased linearly for propionate with increased Suc. At 8 and 24 h, the molar proportion of butyrate tended to increase linearly with increasing Suc. Directional response of yield per unit of sucrose utilized differed among fermentation products. Linear responses in yield tended to increase for lactate at its maxima, decrease for MCP at 8 h, and tend to decrease for maximal glycogen, and VFA, CO2, and CH4 at 24 h. Decline in MCP yield may be related to a decrease in peptides available relative to Suc amount. Calculations of C in products / C in Suc utilized gave values of 1.27, 1.36 and 0.97 at 8 h (tended to be linear), and 1.61, 1.23 and 1.08 (linear) at 24 h, from lowest to greatest Suc inclusion. A value >1.0 indicates uptake of C from non-Suc sources: likely peptides and AA from the medium at 8 h, and these sources plus fermented fiber at 24 h. The 8 h results are supportive of the premise that the ratio of true protein to Suc altered MCP yields. In conclusion, changes in Suc concentration altered fermentation kinetics as well as production and yields of fermentation products. Further evaluation of carbohydrate fractions and inclusion rates is needed to assess their effects on ruminal fermentation, feedstuff digestion and nutrient supply to the animal.