131 Late-Breaking: Use of fetal age and concentrations of pregnancy-specific protein B as a tool to determine pregnancy status and to assign commercial beef females to calving management groups.

Abstract Pregnancy detection is a way to identify nonpregnant cows and decide how to best market or manage the females. In addition, producers can use data from pregnancy checking to sort females into groups with similar anticipated calving dates. Transrectal ultrasonography and determining circulating concentrations pregnancy-specific protein B (PSPB) are two methods to determine pregnancy status. The objectives of this study were to compare the results of ultrasonography and a PSPB for determining final pregnancy status and anticipated calving management groups in beef females. A total of 558 crossbred Angus females were evaluated at two North Dakota research stations. Females either received fixed time insemination on the first day of the breeding season followed by cleanup bulls (n = 272) or were immediately exposed to bulls for the duration of the 49 or 65 d breeding season. Ultrasound evaluations were conducted on day 49 and again ≥40 days after bull removal to confirm pregnancy and estimate fetal age based on crown-rump length or biparietal distance (range: 40 to 111 days). Blood samples collected at the final exam were analyzed for PSPB concentration and optical density (OD). After ultrasonographies, females were placed into anticipated calving groups based on fetal age to represent those that became pregnant in the first, second, or third estrous cycle of the breeding season. Females were then ranked within location by concentrations of PSPB at final pregnancy check and placed into anticipated calving groups using the same distribution as used for ultrasound groups. Data were analyzed using generalized least squares (GLM procedure, SAS) in a completely randomized design with calving group, location, and their interaction as model terms. A secondary model assessed differences between AI- and natural service-sired pregnancies among the 272 AI-exposed females. Final pregnancy rate determined via ultrasound was 88.4% (493/558), while PSPB testing indicated 476 pregnant (85.3%), 70 nonpregnant (12.5%), and 12 “Recheck” (2.2%) females. Of the “Recheck” group, 10 were confirmed pregnant by ultrasound and excluded from diagnostic accuracy calculations. For the remaining 546 females, the sensitivity of the PSPB test was 99.8% (with one false negative) and the specificity was 98.6%. Females pregnant to AI had older fetuses and greater (P < 0.0001) PSPB concentrations at pregnancy check (99.1 ± 0.69 days and 2.91 ± 0.08 ng/mL) compared to those bred by natural service (71.1 ± 0.88 days and 1.87 ± 0.095 ng/mL). Ultrasound-derived calving groups aligned more closely with actual calving intervals (78.2%) than PSPB-based groups (62.3%; P < 0.001). These findings support PSPB as a sensitive and specific pregnancy test in beef herds, offering a potentially cost-effective alternative to ultrasonography. However, utilization of PSPB concentration for grouping females into anticipated calving cohorts is less accurate than using ultrasound-derived data.