Pregnancy-Specific Protein B in Serum of Postpartum Beef COWS^^^

To determine whether the uterus was the source of serum pregnancy-specific protein B (PSPB) after calving, five beef cows were hysterec- tomized at 21 d postpartum and five served as intact controls. A single blood sample was taken from all cows immediately after calving and then twice weekly until 21 d postpartum. Beginning on d 22, blood samples were taken from all cows at 3-h intervals for 4 d and then twice weekly until 53 d postpartum. When the half-life calculated for the interval from d 1 to 21 was used as a covariate, the adjusted d-22 to -53 half- lives were 8.4 d for control cows vs 7.3 d for hysterectomized cows (P = .044). Data show that PSPB has a long half-life in the circulation and that the uterus is a minimal, if any, source of postpartum circulating PSPB. In another experiment, PSPB was measured weekly after calving in serum of 58 Polled Hereford and Simmental cows. Cows were exposed to fertile bulls and allowed to mate at every estrus. Observations were made for estrus, and progesterone concentrations in serum were measured to estimate the time of ovulations. Levels of PSPB were highest at approximately the time of calving, then decreased rapidly. Concentrations of PSPB were 39 d), PSPB disappearance curves did not differ for the three groups; however, cows with highest PSPB in the circulation during the first 2 wk postpartum had a longer interval to first ovulation. Similar curves plotted for cows grouped according to the interval from calving to conception (5 45, 46 through 69, and 2 70 d), also were not different. Levels of PSPB were not affected by body condition score, weight of cows, sex of calf, or calf birth weight. The slow disappearance rate of PSPB in serum of beef cows after calving may impose limitations on the use of an assay for PSPB as a pregnancy test unless a cut-off at < 1 ng/mL of serum can be established.