The effects of rumen nitrogen balance on in vitro rumen fermentation and microbial protein synthesis vary with dietary carbohydrate and nitrogen sources

Kand, D.; Raharjo, I. B.; Castro-Montoya, J.; Dickhoefer, U.

Animal Feed Science and Technology 241: 184-197


ISSN/ISBN: 0377-8401
DOI: 10.1016/j.anifeedsci.2018.05.005
Accession: 066365123

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The objectives were to understand the effects of different levels of rumen nitrogen balance (RNB) on carbohydrate (CHO) fermentation, protein degradation, and microbial crude protein synthesis and to determine whether these effects differ depending on the dietary CHO or nitrogen (N) sources. Three RNB levels RNBO (ie., RNB similar to 0 g/kg dry matter), RNB-5 (i.e., RNB similar to -5 g/kg dry matter), and RNB-9 (ie., RNB similar to -9 g/kg dry matter) were tested in two in vitro experiments. Three CHO sources (i.e., sucrose, corn starch, and cellulose) were incubated with urea as main source of N in experiment 1, whereas experiment 2 tested three N sources (ie., wheat gluten, soy protein, and casein) with corn starch as main source of CHO. During three runs, 1 g each (as-fed basis) of grass hay and a mixture with different proportions of the N and CHO sources were incubated in duplicate in buffered rumen fluid for 24 h. Cumulative gas production was higher for RNB-9 than for RNBO for all diets (P < 0.01; except sucrose), likely due to higher proportions of CHO in the diets. Yet, total SCFA concentrations in inoculum only increased with declining RNB for soy protein and casein (P <= 0.02), and there were no differences in proportions of acetate (P >= 0.31), butyrate (P >= 0.31), and propionate (P >= 0.33). Ammonia-nitrogen concentrations in inoculum decreased with declining RNB for all diets (P < 0.01), with greater differences in wheat gluten and soy protein than in casein diets. Proportions of branched-chain fatty acids increased in experiment 1 (P <= 0.03; except cellulose) and decreased from RNBO to RNB-9 in experiment 2 (except wheat gluten; P < 0.01). Additionally, microbial crude protein synthesis was lower for RNB-9 than RNBO in experiment 2 (P < 0.01; except casein), with no or only minor RNB effects observed in experiment 1. In conclusion, decreasing RNB has only minor effects on in vitro CHO fermentation. However, feed protein degradation and microbial crude protein synthesis decrease with declining RNB, with less pronounced effects in slowly degradable CHO and rapidly degradable N sources.