Divergent utilization patterns of grass fructan, inulin, and other nonfiber carbohydrates by ruminal microbes

Authors: Hall, Mary Beth; Weimer, Paul

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/29/2015
Publication Date: 1/1/2016
Publication URL: http://handle.nal.usda.gov/10113/62092
Citation: Hall, M., Weimer, P.J. 2016. Divergent utilization patterns of grass fructan, inulin, and other nonfiber carbohydrates by ruminal microbes. Journal of Dairy Science. 99:245-257.

Interpretive Summary: Fructans are an important carbohydrate in cool-season grasses that are consumed by ruminants such as dairy cattle. Little research has been done to describe their fermentation by rumen microbes; from the perspective of nutritionists who balance dairy cattle diets, this is essential to understanding the nutritional value of grass fructans to ruminants. In this study, we compared the fermentation of phlein (a naturally occurring grass fructan) to glucose (a sugar that is commonly used as a standard), and to inulin (a commercially available fructan from chicory). We found that phlein in grass fermented more slowly and produced more rumen microbes when compared to glucose, and it grew microbes more quickly and made more acetic acid than the chicory fructan. This information indicates that, to more accurately describe the nutrient supply of phlein to the cow, predictions should not be based on fermentation data for other sugars or fructans from other plants.

Technical Abstract: Fructans are an important nonfiber carbohydrate in cool-season grasses. Their fermentation by ruminal microbes is not well described, though such information is needed to understand their nutritional value to ruminants. Our objective was to compare kinetics and product formation of orchardgrass fructan (phlein; PHL) to other nonfiber carbohydrates when fermented in vitro with mixed or pure culture ruminal microbes. Studies were carried out as randomized complete block designs. All rates given are first-order rate constants. With mixed ruminal microbes, rate of substrate disappearance statistically tended to be greater for glucose (GLC) than for PHL and chicory fructan (inulin; INU) which tended to differ from each other (0.74, 0.62, and 0.33 h-1, respectively). Disappearance of GLC had almost no lag time (0.04 h), whereas the fructans had lags of 1.4 h. The maximum microbial N accumulation, a proxy for cell growth, tended to be 20% greater for PHL and INU than for GLC. The N accumulation rate for GLC (1.31 h-1) was greater than for PHL (0.75 h-1) and INU (0.26 h-1) which also differed. More microbial glycogen (+57%) was accumulated from GLC than from PHL, though accumulation rates did not differ (1.95 and 1.44 h-1, respectively); little glycogen accumulated from INU. Rates of organic acid formation were 0.80, 0.28, and 0.80 h-1 for GLC, INU, and PHL, respectively, with PHL tending to be greater than INU. Lactic acid production was more than 7-fold greater for GLC than for the fructans. The ratio of microbial cell carbon to organic acid carbon tended to be greater for PHL (0.90) and INU (0.86) than for GLC (0.69), indicating a greater yield of cell mass per amount of substrate fermented with fructans. Reduced microbial yield for GLC may relate to the greater glycogen production which requires ATP, and lactate production which yields less ATP. Together, these processes could have reduced ATP available for cell growth. Acetate molar proportion was less for GLC than for fructans, and less for PHL than for INU. In studies with pure cultures, all microbes evaluated showed differences in specific growth rate constants (mu) for GLC, fructose, sucrose, maltose, and PHL. Selenomonas ruminantium and Streptococcus bovis showed the highest mu for PHL (0.55 and 0.67 h-1, respectively), which were 50 to 60% of the mu achieved for GLC. The ten other species tested had mu between 0.01 and 0.11 h-1 with PHL. This study concluded that ruminal microbes utilize PHL differently than they do GLC or INU.