Environmental regulation of pregnancy-specific protein B concentrations during late pregnancy in dairy cattle

Environmental factors, such as photoperiod and heat stress, can be manipulated during the dry period to influence health, productivity, and reproductive performance of dairy cows in their subsequent lactation. The impacts of photoperiod and heat stress on subsequent lactation are related to alterations in prolactin (PRL) signaling and may affect the expression of pregnancy-specific protein B (PSPB). Additionally, exposure of cows to heat stress during the dry period decreases gestation length; however, the mechanism involved in this process is unknown. The objective of these experiments was to evaluate the influence of environmental factors (i.e., heat stress and photoperiod) during late gestation (i.e., dry period) on PSPB concentrations in plasma of dairy cows. In Exp. 1, cows were dried off in the summer months approximately 46 d before expected calving and assigned randomly to heat stress (HT; n=30) or cooling (CL; n=30) treatment. Cooling cows were housed with sprinklers, fans, and shade, whereas HT cows were provided only shade. In Exp. 2, cows were dried off at approximately 60 d before expected calving in summer/fall months and randomly assigned to 3 treatments: long day photoperiod (LDPP: 16L:8D; n=15), short day photoperiod (SDPP: 8L:16D; n=14) and SDPP+PRL implant (12 mg/d of PRL at 28 d or 16 mg/d of PRL at 39 d; n=11). In both experiments, plasma samples were collected at dry off and at -32, -18, -7, -3 and 0 d relative to calving. In Exp. 1, greater concentrations of PSPB were detected in plasma of CL versus HT cows (388.3±24.7 vs. 287.4±23.8 ng/mL; P<0.01). Concentrations of PSPB did not differ between -46 to -18 d before calving (66.0 ng/mL). However, PSPB concentrations were greater (P<0.01) for CL cows at d -7 (534.7>357.2 ng/mL), -3 (807.2>572.2 ng/mL) and 0 (800.8>563.5 ng/mL) relative to calving. Additionally, HT cows in Exp. 1 had increased PRL plasma concentrations compared with CL cows (21.01±1.6 vs. 13.78±1.6 ng/mL). In Exp. 2, no differences were detected in plasma concentrations of PSPB (ng/mL) among LDPP, SDPP, or SDPP+PRL groups on d -60 (41.5), -32 (51.7), -18 (58.5), -7 (532.9), -3 (838.2), and 0 (729.4) relative to parturition. Photoperiodic PRL concentrations were 10.81, 7.84, and 4.22 ng/mL for LDPP, SDPP+ PRL, and SDPP, respectively. Results indicate that HT alters PSPB concentrations in late pregnancy, suggesting that placental activity is altered in cows exposed to excessive elevated temperatures around the time of calving. However, the mechanism involved likely is not associated with changes in PRL secretion.