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

Abstract

In ruminants, urea recycling is considered an evolutionary advantage. The amount of urea recycled mainly depends of the nitrogen (N) intake and the amount of organic matter (OM) digested in the rumen. Because recycled N contributes to meeting microbial N requirements, accurate estimates of urea recycling can improve the understanding of efficiency of N utilization and N losses to the environment. The objective of this study was to evaluate urea kinetics and microbial usage of recycled urea N in ruminants using a meta-analytical approach. Treatment mean values were compiled from 25 studies with ruminants (beef cattle, dairy cows and sheep) which were published from 2001 to 2016, totalling 107 treatment means. The data set was analyzed according to meta-analysis techniques using linear or non-linear mixed models, taking into account the random variations among experiments. Urea N synthesized in the liver (UER) and urea N recycled to the gut (GER) linearly increased (P < 0.001) as N intake (g/BW 0.75 ) increased, with increases corresponding to 71.5% and 35.2% of N intake, respectively. The UER was positively associated (P < 0.05) with dietary CP concentration and the ratio of CP to digestible OM (CP:DOM). Maximum curvature analyses identified 17% dietary CP as the point where there was a prominent increase in hepatic synthesis of urea N, likely due to an excess of dietary N leading to greater ammonia absorption. The GER:UER decreased with increasing dietary CP concentration (P < 0.05). At dietary CP ⩾ 19%, GER:UER reached near minimal values. The fraction of UER eliminated as urinary urea N and the contribution of urea N to total urinary N were positively associated with dietary CP ( P < 0.05), both reaching values near the plateau when dietary CP was 17%. The fractions of GER excreted in the feces and utilized for anabolism decreased, whereas the fraction of GER returned to the ornithine cycle increased with dietary CP concentration (P < 0.05). Recycled urea N assimilated by ruminal microbes (as a fraction of GER) decreased as dietary CP and CP:DOM increased (P < 0.05). The efficiency of microbial assimilation of recycled urea N was near plateau values at 194 g CP/kg DOM. The models obtained in this study contribute to the knowledge on N utilization, and they could be used in feeding models to predict urea recycling and thus to improve formulation of diets to reduce N losses that contribute to air and water pollution.Keywords: cattle, mixed models, protein, recycling, ruminal bacteria ImplicationsIn cattle systems, many farmers utilize feeding models to estimate protein requirements. However, most models recognize the importance of urea nitrogen (N) recycling in the rumen, but not all account for it. In such situation, a tendency to oversupply dietary CP may occur, leading to higher feed costs and N excretion. As N excreted can contribute to air and water pollution, more efficient feeding strategies are required to maximize N utilization. In this study, we used a meta-analytical approach to obtain models that cou...

Effect of dietary nitrogen level and source on mRNA expression of urea transporters in the rumen epithelium of fattening bulls

“…In our study, the CP difference of diets (11.0 and 12.5% DM) was much lower and, probably, this had a limited impact on urea influx. In a recent meta-analysis Batista et al (2017) studied the relationship between dietary CP levels and the rumen urea N entry rate in a data set of trials involving several cattle categories. According to such a model, the different CP intakes in the trial of Røjen et al (2011) caused a variation of approx.…”

Invited review: Nitrogen in ruminant nutrition: A review of measurement techniques

“…In line with Reynolds and Huntington (1988), Spek et al (2013b) observed that the fraction of total urea entry rate that returned to the GIT was negatively related to dietary protein content. Reports on this type of measurements allowed Reynolds and Kristensen (2008), and more recently Batista et al (2017), to demonstrate that GER accounts for almost the entire urea entry rate in systemic blood when dietary DM contains 8% or less CP.…”

Effects of energy-protein supplementation frequency on performance of primiparous grazing beef cows during pre and postpartum

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