Variation of milk urea in dairy cattle : a study on factors that affect the relationship between urea concentration in milk and urea excretion in urine / Jan Wouter Spek . Wageningen :  Wageningen University , 2013 . 156 s . Doktorandská dizertačná práca (PhD.). - Wageningen University . ISBN 978-94-6173-664-2 . [1, z toho požičaných 0]