Electrolytes and pH of Mammary Gland Secretions Assessments to Detect Impending Parturition and Associations With Placental and Neonate Features in Donkeys

The objectives of this study were to determine (i) the usefulness of serial assessment of mammary gland electrolytes concentrations and correspondent pH to detect impending parturition in jennies; and (ii2) the associations between mammary gland secretions, and gestation length, foal sex, maternal, placental, and foal birth weights. Multiparous jennies (n = 37) were monitored daily from 350 to 355 days of gestation until parturition. The pH of mammary gland secretions was assessed daily with a hand-held device. Aliquots of mammary secretions were frozen daily and then retrospectively assessed for electrolyte concentrations (Ca²⁺, Mg²⁺, K⁺, and Na⁺) with an automated analyzer from five days until the last sampling pre-partum. Mammary gland electrolytes and pH were analyzed with mix-models. The gestational length, newborn, maternal, and fetal membranes weights were analyzed with t-test grouped by foal's sex. Associations across all variables were assessed with Pearson's coefficient of correlation. Sensitivity, specificity, and negative predictive value (NPV) and positive predictive values (PPV) were evaluated for pH (≤ 6.4), Ca²⁺ (>10 mmol/L), and a combination of both. Each jenny had pH profiles assessed visually and classified as fast pH drop (1), slow pH drop (2), and alkaline pH (3) as previously described for horses. The overall gestation length was 374 ± 8.7 days, ranging from 357 to 390 days. There were no differences for gestation lengths for jennies delivering colts (374 ± 2.1 range 357–385 days), versus the ones delivering fillies (373 ± 2.3 range 358–390 days) (P > .05). Of all the foals, there were 61.8% colts and 38.2% fillies. The ratio of foal birthweight with the dam's bodyweight was 9.7%, and the ratio with fetal membranes was 11%. The majority of parturitions happened during the night (91.9%). There was a significant reduction in Na⁺ and an increase in Ca²⁺, Mg²⁺, and K⁺ concentrations leading to foaling. The pH showed a 90% sensitivity for foaling within 24 hours, whereas the specificity was 70%, and the PPV and NPV values were 40% and 97%. Of interest, Ca²⁺ (>10 mmol/l) displayed a sensitivity and specificity of 71% and 85%, whereas the PPV and NPV were 72% and 84%. In the present study, jennies exhibited profiles 1 to 3 as previously described for mares. Herein, 65% of the jennies displayed profile 2 and foaled with a mean acidic pH of 6.4 ± 0.02. Conversely, 32% of the jennies showed a fast reduction in the pH of mammary secretion (profile 1) from day -1 (7.3 ± 0.2) to the day of foaling with an average pH of 6.6 ± 0.08. One jenny foaled with high and alkaline pH (i.e., 7.5). There were weak and negative correlations between pH and Ca²⁺, Mg²⁺, and K⁺ (P < .05). In addition, Ca²⁺ displayed a weak but significant correlation with Mg²⁺, Na⁺, and K⁺. In conclusion, daily pH measurements of the mammary gland secretions can predict foaling in jennies, whereas Ca²⁺ was not as useful. Contrary to horses that most mares present a fast pH profile, most jennies showed a slow pH profile. The sex of foal did not affect the gestational length and fetal/maternal and fetal membrane proportions in donkeys.