Road Transport of Late Pregnant Mares Advances the Onset of Foaling

Abstract Cortisol is involved in the initiation of parturition and we hypothesized that increased maternal cortisol release advances the onset of foaling. Transport is a stressor for horses and induces an increase in cortisol release. To determine stress effects on the time of foaling, late-pregnant mares were transported by road for 3 hours (n = 12) or remained in their stable as controls (n = 4). Starting on day 325 of gestation, saliva and blood samples were taken for cortisol and progestin analysis, respectively. Fetomaternal electrocardiograms were recorded repeatedly. Mares were checked for impending parturition and changes in precolostrum pH. When pH decreased to 6.5, mares were either transported or left untreated. After birth, saliva was collected repeatedly from mares and their foals and heart rate (HR) was recorded. Foals were checked for maturity and health. Gestation length was 337 ± 2 days in stressed and 336 ± 2 days in control mares. Cortisol concentration increased from 3.3 ± 0.9 to 8.4 ± 0.8 ng/mL in transported mares (P