Effects of lactation length and an exogenous progesterone and estradiol-17β regimen during embryo attachment on endogenous steroid concentrations and embryo survival in sows

Abstract

The hypotheses that short lactation lengths increase embryo mortality by altering endogenous post-weaning steroid concentrations, and that an exogenous steroid regimen during embryo attachment might increase embryo survival were tested using 36 s parity sows assigned randomly to a 2×2 factorial. Sows were subjected to either a short lactation (SL, 13.0 days, n=25) or a long lactation (LL, 31.5 days, n=11), artificially inseminated at first estrus and treated daily with 2 ml i.m. of either 25 mg progesterone (P 4) and 1.25 μg estradiol-17β (E 2) (steroid treatment, ST, n=17) or the vehicle alone (control treatment, CT, n=17) on Days 14–20 post-insemination. Blood samples were collected by jugular venipuncture from weaning to 24 days post-insemination on alternate days. Sows subjected to the SL compared to the LL tended to have a longer weaning-to-estrus interval (WEI) (5.3 versus 4.6 days; P<0.10), but did not have a significantly reduced conception rate (CR) (71 versus 90%; P>0.10). The SL and LL sows had a similar ovulation rate (19.9 versus 21.3 corpora lutea, CL; P>0.05), but SL sows had fewer viable embryos than LL sows (11.5 versus 15.3; P<0.05) when reproductive tracts were recovered 28–32 days post-insemination. In addition, even after correction for the difference in number of embryos between groups, viable embryos from the SL versus the LL group weighed less (1.63 versus 1.79 g; P<0.05), had a decreased amnion volume (1.02 versus 1.22 ml; P<0.05) and apparently produced less estrogens since estrone sulfate concentration was decreased at 24 days post-insemination in SL versus LL sows (4.3 versus 6.3 ng/ml; P<0.05). Embryo survival (percentage of CL represented by a viable embryo) however, was not different between SL and LL sows (60 versus 74%; P>0.05) and no differences in post-weaning P 4 or E 2 concentrations were apparent. Sows that received the ST only tended to have increased P 4 concentrations at 16 days post-insemination compared to CT sows and neither the number of viable embryos, nor embryo survival, was increased in ST versus CT sows (14.7 versus 12.2; P>0.05 and 66 versus 68%; P>0.05, respectively). These data suggest that short lactations do not increase embryo mortality by inducing aberrant endogenous post-weaning P 4 or E 2 concentrations. It is unclear whether or not small, repeated doses of exogenous P 4 and E 2 during attachment can increase embryo survival.