Milk urea nitrogen variation explained by differences in urea transport into the gastrointestinal tract in lactating dairy cows

Milk urea nitrogen (MUN) and blood are correlated with balance excretion; however, there is also a genetic component to MUN concentrations that could be associated differences in transport. It was hypothesized portion of the variation among cows caused by gastrointestinal kidney clearance rates. Eight lactating varying while fed common diet were infused [15N15N]urea determine N entry rate (UER), rate, returned ornithine cycle, used for anabolism, excretion feces urine. Urea rates kidneys tract calculated from isotopic enrichment urine gut respectively, plasma (PUN). Over course experiment, animals weighed an average 506 ± 62 kg produced 26.3 4.39 milk/d, ranging 11.6 17.3 mg/dL (average 14.9 2.1 mg/dL). Plasma positively UER, urine, anabolism. negatively ratio UER. This relationship supports hypothesis transport activity causes PUN concentrations, high less efficient at recycling thus may more susceptible ruminal deficiencies when low RDP diets. Such biological metabolism necessitates adequate safety margin setting regulations maximal levels as indicator herd efficiency.