Using blood urea nitrogen to predict nitrogen excretion and efficiency of nitrogen utilization in cattle, sheep, goats, horses, pigs, and rats1

Abstract

The objectives of this study were to evaluate the potential for using blood urea N concentration to predict urinary N excretion rate, and to develop a mathematical model to estimate important variables of N utilization for several different species of farm animals and for rats. Treatment means (n = 251) from 41 research publications were used to develop mathematical relationships. There was a strong linear relationship between blood urea N concentration (mg/100 mL) and rate of N excretion (g x d(-1) x kg BW(-1)) for all animal species investigated. The N clearance rate of the kidney (L of blood cleared of urea x d(-1) x kg BW(-1)) was greater for pigs and rats than for herbivores (cattle, sheep, goats, horses). A model was developed to estimate parameters of N utilization. Driving variables for the model included blood urea N concentration (mg/100 mL), BW (kg), milk production rate (kg/d), and ADG (kg/d), and response variables included urinary N excretion rate (g/d), fecal N excretion rate (g/d), rate of N intake (g/d), and N utilization efficiency (N in milk and gain per unit of N intake). Prediction errors varied widely depending on the variable and species of animal, with most of the variation attributed to study differences. Blood urea N concentration (mg/100 mL) can be used to predict relative differences in urinary N excretion rate (g/d) for animals of a similar type and stage of production within a study, but is less reliable across animal types or studies. Blood urea N concentration (mg/100 mL) can be further integrated with estimates of N digestibility (g/g) and N retention (g/d) to predict fecal N (g/d), N intake (g/d), and N utilization efficiency (grams of N in milk and meat per gram of N intake). Target values of blood urea N concentration (mg/100 mL) can be backcalculated from required dietary N (g/d) and expected protein digestibility. Blood urea N can be used in various animal species to quantify N utilization and excretion rates.

Low degradable protein supply to increase nitrogen efficiency in lactating dairy cows and reduce environmental impacts at barn level

“…Nitrogen deficiency in the LowN diet therefore led to nitrogen saving in the kidney (Rojen et al, 2011). As discussed by Kohn et al (2005), keeping urea in the blood may enable herbivores to recycle greater amounts of N in the gut compared with omnivores, which explains how herbivores can survive on low amounts of low-quality protein.…”

Response of growing ruminants to diet in warm climates: a meta-analysis

“…This RPB is the major criterion to predict UN excretion. The increase in UN level with increased NI can be attributed to the high blood and plasma urea nitrogen levels, which cause high nitrogen clearance rates of the kidneys and increase urea excretion via urine (Kohn et al, 2005). A debate is remaining about the minimum loss of N in urine, which can be adopted as endogenous loss in protein unit system.…”

Aspects of nitrogen metabolism in sheep-fed mixed diets containing tree and shrub foliages

“…It is noteworthy to mention that a wide range of true digestibility of N is found in the literature ranging from 760 (12) to 950 g/kg (49) . However, the true N digestibility decreased in FL-H compared to FL-0, which might be attributed to (1) the binding of protein with tannins present in foliages and thus excreted in faeces as NDF-N (4,5) ; (2) availability of free tannins, which may bind to the gut epithelium (50) and then reduce amino acid absorption (51) ; and (3) the presence of high content of NDF-N in foliages, which generally have low digestibility (52) .…”

“…Fodder trees are more particularly important in the arid and semi-arid regions because trees can survive harsh climatic conditions compared to grasses, and hence may be more reliable feed resources in these areas. These multipurpose tree foliages contain moderate levels of N, minerals and vitamins (1,2) , and have been suggested as a solution to feeding of ruminants in the tropical areas, especially as supplementary feeds to low-quality forages (3 -6) .Many models have been published, particularly in recent years, in an effort to reduce N excretion in the environment, to develop prediction equations for N excretion in faeces and urine and to increase the efficiency of utilisation of dietary N in dairy and beef cattle based on the dietary chemical composition (7 -11) , and sheep and goats based on blood urea N levels (12) . Although tree leaves contain moderate level of N, they also contain tannins and other secondary compounds that may influence intake and utilisation of nutrients.…”

“…Many models have been published, particularly in recent years, in an effort to reduce N excretion in the environment, to develop prediction equations for N excretion in faeces and urine and to increase the efficiency of utilisation of dietary N in dairy and beef cattle based on the dietary chemical composition (7 -11) , and sheep and goats based on blood urea N levels (12) . Although tree leaves contain moderate level of N, they also contain tannins and other secondary compounds that may influence intake and utilisation of nutrients.…”