Urea Nitrogen Metabolite Can Contribute to Implementing the Ideal Protein Concept in Monogastric Animals

Abstract

The ideal protein concept refers to dietary protein with an amino acid profile that exactly meets an animal’s requirement. Low-quality protein levels in the diet have negative implications for productive and reproductive traits, and a protein oversupply is energetically costly and leads to an excessive N excretion, with potentially negative environmental impact. Urea Nitrogen (UN), which corresponds to the amount of nitrogen in the form of urea circulating in the bloodstream, is a metabolite that has been widely used to detect amino acid imbalances and deficiencies and protein requirements. This review aims to critically analyse how UN can contribute to accurately implementing the ideal protein concept in monogastric animals, particularly in pig, poultry, and rabbit nutrition (14,000 animals from 76 published trials). About 59, 37, and 4% of trials have been conducted in pigs, poultry, and rabbits, respectively. UN level was negatively correlated to main performance traits (Pearson Correlation Coefficient [PCC] of −0.98 and −0.76, for average daily gain and feed conversion ratio, respectively), and lower UN level was related to higher milk yield and concentration. High level of UN was positively correlated to N excretion (PCC = 0.99) and negatively correlated to protein retention (PCC = −0.99). Therefore, UN in blood seems to be a proper indicator of amino acid imbalance in monogastric animals. Great variability in the use of UN was observed in the literature, including uses as determination medium (blood, plasma, or serum), units, and feeding system used (ad libitum or restricted), among others. A standardization of the methods in each of the species, with the aim to harmonize comparison among works, is suggested. After review, UN measurement in plasma and, whenever possible, the utilization of the same nutritional methodology (ad libitum conditions or restriction with blood sampling after refeeding at standardised time) are recommended. More studies are necessary to know the potential of UN and other bioindicators for amino acid deficiencies evaluation to get closer to the ideal protein concept.

Effect of Increasing the Methionine Level and Reducing the Threonine Level in the Diet of Fast-Growing Rabbits

“…The reason why the differences in growth traits were mainly observed at the end of the fattening period may be due to different causes. It may be because the choice of diet that minimised the PUN level was made at 48 d of age [28], this feed being more representative of the amino acid requirements at that age. However, it could also be due to a greater impact of an adequate dietary amino acid formulation on growth at that age.…”

Biomarkers for ideal protein: rabbit diet metabolomics varying key amino acids

“…Urea is the result from ammonia produced by the deamination of amino acids. Urea nitrogen level is the main result of urine excretion and has been widely used to detect amino acid imbalances in animal diets 44 . Urea levels have previously been analysed in growing rabbits, and the levels found in this work are similar to those obtained previously 2 , 45 , 46 .…”

Feeding, Nutrition and Rearing Systems of the Rabbit

“…Thus, the development of tools for an early detection of poor nutrients use efficiency is fundamental to improve the environmental sustainability of rabbit farms. Urea nitrogen seems to be an appropriate indicator of amino acid imbalance in monogastric animals, including rabbits [ 9 ]. Furthermore, bioelectrical impedance has been proposed as a non-invasive method to estimate in vivo carcass composition and to determine the nutrient retention and overall energy and nitrogen retention efficiencies of fattening rabbits [ 10 ]; however, more studies on these topics are required.…”