Factors contributing to the absence of post-weaning estrus in hyperprolific sows and their treatment using prostaglandin F2α

Currently, a growing concern in the swine production sector is the increasing number of sows that fail to return to estrus within 7 days after weaning. The objectives of this study were: i) to investigate the occurrence of ovulation during lactation in hyperprolific sows in a tropical region, ii) to assess the accuracy and practicality of using a serum progesterone strip test to determine serum progesterone concentration in sows that did not return to estrus within 6 days after weaning, and iii) to evaluate the efficacy of PGF2α administration in inducing estrus in sows that do not return to estrus within 6 days post-weaning, particularly those with elevated circulating progesterone levels. A total of 344 crossbred Canadian Landrace × Yorkshire post-weaning sows with an average parity of 2.9 ± 1.3 (range: 1 to 5) were included in the study. On the 6th day after weaning, the sows were divided into two groups. The control group consisted of sows that exhibited normal standing estrus behavior within 6 days post-weaning (n = 278), while the treatment group comprised sows that did not display estrus behavior within the same 6-day period (n = 66). Data collection in the subsequent phase focused on the treatment group (n = 66). To assess serum progesterone levels, blood samples were collected from these sows and analyzed using a serum progesterone strip test. A positive result was indicated if the serum progesterone concentration exceeded 5 ng/ml. Additionally, serum was extracted from the blood samples and stored at -20°C before being sent for progesterone level analysis using enzyme-linked immunosorbent assay. For the sows in the treatment group that tested positive on the serum progesterone strip test, an intramuscular injection of 10 mg of exogenous PGF2α was administered. Following this, vulva scores were visually assessed on the sixth day post-treatment. Additionally, the sows' responses were recorded, with a score of 0 indicating no response and a score of 1 indicating signs of estrus within 6 days of treatment. Serum progesterone levels were reassessed in the sows 6 days after treatment. Out of the 344 sows, 66 (19.2%) did not return to estrus within 6 days following weaning. Among the weaned sows that did not display standing estrus within 6 days post-weaning, 80.3% (53 out of 66 sows) tested positive using the serum progesterone strip test. Using transrectal real-time B-mode ultrasonography, cystic ovaries were detected in 9 of the 66 sows, representing 13.6%. The incidence of sows failing to return to estrus within 6 days post-weaning was greater in multiparous sows compared to primiparous sows (21.6% vs. 8.1%, P = 0.008). The serum progesterone test indicated that a significantly higher proportion of multiparous sows had elevated serum progesterone levels (51/58, 79.7%) compared to primiparous sows (0/6, 0%, P < 0.001). The proportion of sows with elevated serum progesterone levels, as measured by a progesterone strip test, was significantly higher in sows with a backfat thickness of ≥17 mm compared to those with backfat thickness <17 mm (32/35 sows, 91.4% vs. 21/31 sows, 67.7%, P = 0.016). The occurrence of sows that failed to return to estrus within 6 days after weaning was significantly higher in sows with a lactation length of ≥25 days (32.9%) compared to those with lactation lengths of 18–21 days (18.2%, P = 0.032) and 22–24 days (14.2%, P < 0.001). In conclusion, factors associated with a high incidence of delayed estrus included cystic ovaries (13.6%), higher parity, greater backfat thickness at weaning (≥ 17 mm), and a lactation period exceeding 25 days. Of the 66 sows that failed to return to estrus within 6 days post-weaning, 53 (80.3%) had ovulated during lactation, as indicated by the serum progesterone strip test. Compared to the standard ELISA, the serum progesterone strip test demonstrated a sensitivity of 94.6% and a specificity of 100%. However, only 21.6% of the sows treated with PGF2α exhibited standing estrus within 6 days post-treatment.