Utilization of lactose by mixed ruminal microbes is affected by nitrogen type and level, and differs from utilization of glucose

Abstract Text: The objective of this study was to evaluate the effects of providing lactose (Lac) at differing nitrogen (N) levels and types, and glucose (Glc) at a low N level on products formed and substrate utilized by mixed ruminal microbes. The 3 N treatments were applied via modification of Goering & Van Soest medium : LoN (60% of enzymic digest of casein removed; 300 mg N/L), and HiU or HiT with urea or an enzymic digest of casein, respectively, added to LoN to give 451 mg N/L. Glc and Lac were added at 3 g/L (79.5 mg / fermentation vial). Two replicated in vitro fermentations with mixed ruminal microbes were performed. Inoculum donors were 2 ruminally cannulated cows individually provided with 200 g each of Lac and Glc per day via their diets for 14 d before the fermentations. Vials, the experimental units, were destructively sampled hourly from 0 through 6 h of fermentation. Data were analyzed as a randomized complete block design. Organic acid carbon is the sum of carbon in acetate, propionate, butyrate, valerate, and lactate. Detected maxima and endpoints were analyzed with the factors treatment (trt), and fermentation run (random variable) in the statistical model. Orthogonal contrasts of Glc vs. all Lac, Lac LoN vs HiN, and Lac HiT vs HiU were evaluated. Significance was declared at P<0.05, and tendency at 0.05<P<0.10. Glc was utilized more rapidly than Lac, giving less residual Glc than Lac at 6 h (P=0.02); HiT tended to have a lower value than HiU at 6 h (P=0.07). Maximum detected glycogen was greater for Glc than for all Lac (P<0.01), which did not differ from each other (P>0.66). Maximum detected organic acid carbon values did not differ among treatments, except for a tendency for a greater amount with HiT than with HiU (P=0.06). Maximum detected microbial N accumulation (a proxy for cell growth) was greater for Glc than Lac (P=0.04), and tended to be greater for Lac with HiN than with LoN (P=0.07). Rumen microbes utilize Glc and Lac differently; N level and type alters utilization of Lac.

Keywords: Lactose, Rumen, Fermentation