Effect of nitrogen supplementation on urea kinetics and microbial use of recycled urea in steers consuming corn-based diets1

Abstract

We studied the effects of supplementing N as distillers dried grains with solubles (DDGS) or urea to steers consuming corn-based diets. Six ruminally and duodenally cannulated steers (244 kg) were used in 2 concurrent 3 x 3 Latin squares and fed 1 of 3 corn-based diets: control (10.2% CP), urea (13.3% CP), or DDGS (14.9% CP). Periods were 14 d, with 9 d for adaptation and 5 d for collection of urine and feces. Urinary (15)N(15)N-urea enrichments, resulting from venous infusions of (15)N(15)N-urea, were used to measure urea kinetics. Dry matter intake (6.0 kg/d) was not affected by treatment, but N intake differed (99, 151, and 123 g/d for the control, DDGS, and urea treatments, respectively). Urea-N synthesis tended to be greater (P = 0.09) for DDGS (118 g/d) than for the control treatment (52 g/d), with the urea treatment (86 g/d) being intermediate. Urea-N excreted in the urine was greater (P < 0.03) for the DDGS (35 g/d) and urea treatments (29 g/d) than for the control treatment (13 g/d). Gastrointestinal entry of urea-N was not statistically different among treatments (P = 0.25), but was numerically greatest for DDGS (83 g/d), intermediate for urea (57 g/d), and least for the control (39 g/d). The amount of urea-N returned to the ornithine cycle tended to be greater (P = 0.09) for the DDGS treatment (47 g/d) than for the urea (27 g/d) or control treatment (16 g/d). The fraction of recycled urea-N that was apparently used for anabolism tended (P = 0.14) to be greater for the control treatment (0.56) than for the DDGS treatment (0.31), with the urea treatment (0.45) being intermediate, but no differences were observed among treatments in the amount of urea-N used for anabolism (P = 0.66). Urea kinetics in cattle fed grain-based diets were largely related to the amount of N consumed. The percentage of urea production that was captured by ruminal bacteria was greater (P < 0.03) for the control treatment (42%) than for the DDGS (25%) or urea treatment (22%), but the percentage of duodenal microbial N flow that was derived from recycled urea-N tended (P = 0.10) to be greater for the DDGS treatment (35%) than for the urea (22%) or control treatment (17%). Thus, ruminal microbes were more dependent on N recycling when the protein supplement was largely resistant to ruminal degradation.

Effects of varying ruminally undegradable protein supplementation on forage digestion, nitrogen metabolism, and urea kinetics in Nellore cattle fed low-quality tropical forage1

“…If all GER entered the rumen, it would be expected that the relationship between GER and GER plus dietary RDP would be similar to the percentage of MN that was synthesized from recycled urea (Brake et al, 2010). In our study, GER as a percentage of GER plus measured RDP (data not presented tabularly) was greater (P < 0.001) for the control (65%) than for the supplemented treatments (41%).…”

“…Considering that supplementation had only modest effects on nutritional characteristics (i.e., forage intake and digestion), it can be inferred that the most prominent effects were on the whole body N metabolism, N retention, and efficiency of N utilization, as observed by other researchers working with cattle in tropical conditions (Costa et al, 2011;Rufino, 2011;Batista, 2012). Detmann et al (2014b) used a meta-analysis to evaluate the efficiency of N utilization (% of N intake) in cattle fed tropical forage and receiving N supplementation.…”

“…For the supplemented treatments, MNU:MN (10%) was approximately one-fourth of the GER:(GER + RDP). These differences between control and supplemented treatments for the relationships between GER:(GER + RDP) and MNU:MN may be due to urea N that was recycled to the rumen but did not mix with the RAN pool before absorption (accounted for as ROC by the methodology and expected to be greater for the supplemented treatments than for control) as well as to urea recycled to the postruminal GIT (Brake et al, 2010).…”

The effect of CP concentration in the diet on urea kinetics and microbial usage of recycled urea in cattle: a meta-analysis

“…Theoretically, increases in the proportion of GER being recycled to the rumen, rather than to postruminal segments of the GIT, would increase the opportunity of ruminal microbes to assimilate recycled urea N. The fraction of recycled urea N assimilated by ruminal microbes (i.e. MNU:GER) appeared to be dependent on the ruminal NH 3 ; when ruminal NH 3 increased the efficiency of recycled N assimilation by microbes decreased (Brake et al, 2010). In addition, urea N can be recycled to the postruminal GIT.…”

“…According to Brake et al (2010), efficiency of microbial capture of recycled urea N is associated with the availability of fermentable energy to the ruminal microbes, the fraction of GER recycled to the rumen, and the availability of competing N sources within the rumen. Theoretically, increases in the proportion of GER being recycled to the rumen, rather than to postruminal segments of the GIT, would increase the opportunity of ruminal microbes to assimilate recycled urea N. The fraction of recycled urea N assimilated by ruminal microbes (i.e.…”