Changes in the ruminal fermentative activity by including urea as a protein replacement in fattening lamb diets

Trabajo presentado al XIIIth International Symposium on Ruminant Physiology. Leipzig, Germany, 3-6 september 2019.

Rumen contents from twelve lambs fed three isonitrogenous diets (n=4) were used as inocula for in vitro incubations to study fermentation of different substrates. Diets consisted of a standard fattening diet (CON), a diet with 0.6 % urea (URE1) and a diet with 0.95% urea (URE2) partially replacing soybean meal of CON. Substrates of varying composition (CON diet, starch, cellulose or pectin) were incubated in vitro (300 mg of substrate) in batch cultures inoculated with rumen fluid from lambs fed one of the experimental diets. Gas production, volatile fatty acid (VFA) production and ammonia concentration were measured after 24 h of incubation. Data were subjected to one-way ANOVA using a mixed model with diet as the fixed effect and animal as random effect. No differences (P>0.05) were observed in ammonia or VFA concentrations, VFA molar proportions or pH in the ruminal fluid used as inocula. When the CON diet was incubated no differences (P>0.05) were observed among the three inocula in the fermentation parameters. Gas production ranged from 2.1 to 2.2 mol/300 mg substrate, ammonia concentrations from 352 to 391 mg/L and total VFA production from 2.63 to 2.80 mol. Accumulation of fermentation end-products in the cultures was not affected by the source of inoculum when either pectin or starch were incubated. Conversely, when cellulose was incubated, the amount of gas produced decreased linearly (P<0.01) from 2.1 mol/300 mg substrate with the CON inoculum to 1.9 (URE1) and 1.2 mol/300 mg substrate (URE2) in cultures inoculated with rumen liquid from lambs fed diets in which soybean was partially replaced with urea. Concurrently, VFA production was also linearly decreased (P<0.01) as urea was increased in the diet of rumen donor lambs (2.51 mol/300 mg substrate for CON, 2.26 mol/300 mg substrate for URE1 and 1.39 mol/300 mg substrate for URE2). Only the molar proportion of butyrate was affected by the source of inoculum when cellulose was incubated, with no differences in the molar proportions of other VFA or in the acetate to propionate ratio. The results suggest a similar fermentative activity in the rumen contents of lambs fed the three diets, except for an apparent decreased degradation of cellulose when soybean meal was substituted for urea as a replacement for a part of the protein in fattening lamb diets.

Peer reviewed