RESUMEN
This study describes concentrations of Pregnancy Associated Glycoproteins (PAG), progesterone (P4), estrone (E1) and estrone-sulfate (E1S) in American Bison sera. In 2 ranches, mature American Bison were sampled once a year for 2 yr. Subsequent American Bison cows calving days were reported. PAG concentration was determined by Radio-Immuno Assay, whereas P4, E1 and E1S were assayed using Liquid Chromatography and Mass Spectrometry. Concentrations were compared between American Bison bulls (B, n = 7), Nonpregnant cows (NP, n = 32), first (1TP, n = 3), second (2TP, n = 26) and third (3TP, n = 15) trimester of pregnancy. Seven American Bison bulls and 92 cows were sampled, 51 calved during these 2 yr. Calving occurred mostly in spring (74.5%), but also in summer (13.7%) and fall (11.8%). PAG and P4 were higher in 2TP and 3TP than B and NP (P< 0.0001). P4 was non-basal in B and NP. E1 and E1S were correlated (P< 0.0001; r = 0.76) and increased in 2TP and 3TP when compared with B and NP (P< 0.01). Moreover, E1S was higher in 3TP than in 2TP (P< 0.0001) and correlated to pregnancy day (P< 0.0001; r = 0.60). Breeding American Bison in Belgium induces a calving seasonality loss. P4 slowly increases in 1TP and remains steady and high in 2 and 3TP. P4 non-basal and variable concentrations in B or NP disable its use as gestation marker. American Bison produce PAG in the 2 and 3TP, but Estrone-sulfate assay seems to be the best pregnancy marker during the 2 last trimesters as it could help to estimate the gestation period.