Nitrogen source and concentration affect utilization of glucose by mixed ruminal microbes in vitro

Authors: Hall, Mary Beth

Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/21/2016
Publication Date: 4/1/2017
Publication URL: https://handle.nal.usda.gov/10113/5654207
Citation: Hall, M. 2017. Nitrogen source and concentration affect utilization of glucose by mixed ruminal microbes in vitro. Journal of Dairy Science. 100:2739-2750.

Interpretive Summary: Rumen microbes digest much of the feed a cow consumes. In turn, they provide nutrients the cow uses to grow and make milk. Protein in diets has potential to affect how rumen microbes use carbohydrates. Little research has been done to describe this effect, but we need to understand it because it may alter nutrient supply to the cow. In fermentations of glucose with three levels of true protein or urea, microbes utilized glucose and the glycogen made from glucose most rapidly with true protein. Microbial cell growth increased with increasing true protein, which was 66% higher than the results from urea. Overall, supplementation with true protein, but not urea, improved microbial cell production and glucose utilization. Both of these effects can increase the supply of protein and energy to the cow.

Technical Abstract: Availability of ruminally degradable protein (RDP) changes the utilization of carbohydrates by ruminal microbes. However, the effects are not well described, though such information is needed to understand the potential impact on nutrient supplies for ruminants. The objective of this study was to compare the effects of different levels of RDP (0.15, 0.31, 0.46 g N/L) from tryptone (Trp) or urea (Ur) on product formation from glucose (Glc) in fermentations with mixed ruminal microbes in vitro. Studies were carried out as randomized complete block designs in 2 replicated fermentation runs with hourly destructive sampling from 0 through 5 h. All rates (k) given are first-order rate constants. Glc disappearance k tended to be greater for Trp (0.64 h-1) than for Ur (0.51 h-1), but did not differ by nitrogen (N) level. Lag time for microbial N accumulation decreased linearly with increasing N (1.86, 0.74, and 0.70 h), but differences in rates were not detected. Maximum detected microbial N increased linearly with increasing Trp (1.47, 2.23, and 2.47 mg), but increasing additions of Ur did not change the response appreciably from that of the basal media (1.41 mg). Increasing levels of N tended to linearly decrease maximum detected glycogen accumulation (16.4, 16.1, and 15.3 mg). A greater amount of glycogen remained at 5 h with Ur (11.0 mg) as compared to Trp (7.2 mg). Organic acid k was greater with Trp (0.58 h-1) than with Ur (0.22 h-1), but lag time was less with Ur (0.14 vs. 0.48 h). At 5 h of fermentation, there tended to be less organic acid carbon (C) with Ur (14.8 mg) than with Trp (20 mg). At maximum detected microbial N production, the yield of non-glycogen cell C from non-glycogen use of Glc substrate was greater for Trp (39.5 mg/mg) than for Ur (27.0 mg/mg). Similarly, the yield of non-glycogen cell C per amount of organic acid C produced tended to be greater with Trp (59.1 mg/mg) than with Ur (44.2 mg/mg). Overall, addition of Trp resulted in more rapid and extensive utilization of the glucose substrate, as well as more extensive use of glycogen produced from Glc. The improved efficiencies in cell yield from utilized Glc or from organic acids produced suggested that the provision of Trp makes cell growth a more energetically efficient process than with provision of Ur.