title: Effects of high ambient temperature on urea-nitrogen recycling in lactating dairy cows.
abstract: Effects of exposure to hot environment on urea metabolism were studied in lactating Holstein cows. Four cows were fed ad libitum a total mixed ration and housed in a temperature-controlled chamber at constant moderate (18°C) or high (28°C) ambient temperatures in a cross-over design. Urea nitrogen (N) kinetics was measured by determining urea isotopomer in urine after single injection of [(15) N(2) ]urea into the jugular vein. Both dry matter intake and milk yield were decreased under high ambient temperature. Intakes of total N and digestible N were decreased under high ambient temperature but urinary urea-N excretion was increased. The ratio of urea-N production to digestible N was increased, whereas the proportion of gut urea-N entry to urea-N production tended to be decreased under high ambient temperature. Neither return to the ornithine cycle, anabolic use nor fecal excretion of urea-N recycled to the gut was affected by ambient temperature. Under high ambient temperature, renal clearance of plasma urea was not affected but the gut clearance was decreased. Increase of urea-N production and reduction of gut urea-N entry, in relative terms, were associated with increased urinary urea-N excretion of lactating dairy cows in higher thermal environments.
authors.affiliation: Graduate School of Biosphere Science, Hiroshima University, Higashi-Hiroshima National Agricultural Research Center for Kyushu Okinawa Region, Koshi, Kumamoto, Japan. tobitsu@hiroshima-u.ac.jp
authors.forename: Taketo
authors.initials: T
authors.lastname: Obitsu
authors.forename: Mitsuru
authors.initials: M
authors.lastname: Kamiya
authors.forename: Yuko
authors.initials: Y
authors.lastname: Kamiya
authors.forename: Masahito
authors.initials: M
authors.lastname: Tanaka
authors.forename: Toshihisa
authors.initials: T
authors.lastname: Sugino
authors.forename: Kohzo
authors.initials: K
authors.lastname: Taniguchi
mesh_terms: D000818:Animals; D002417:Cattle / Q000378:metabolism*; D005260:Female; D006358:Hot Temperature; D007774:Lactation / Q000502:physiology*; D009584:Nitrogen / Q000378:metabolism*; D013312:Stress, Physiological; D014508:Urea / Q000378:metabolism*
chemical_list: D014508:Urea; D009584:Nitrogen