Carry-over effects of periparturient endocrine changes on postpartum reproductive function of Holstein heifers bred to genetically different service sires.

Effects of fetal sire on postpartum reproductive changes of the dam were studied in 21 Holstein heifers whose pregnancy had been initiated by either Angus (n = 7), Holstein (n = 7) or Brahman (n = 7) bulls. After parturition, all heifers were managed uniformly. Heifers in each service-sire-breed group were bled via jugular venipuncture thrice weekly from d 160 to 265 of pregnancy, daily thereafter until 15 d postpartum, and three times per week until d 60 postpartum. Ability of heifers to release prolactin (PRL) and luteinizing hormone (LH) was evaluated on d 10 postpartum after a simultaneous injection of thyrotropin releasing hormone (TRH; 100 micrograms) and gonadotropin releasing hormone (GnRH; 100 micrograms). Between d 5 and 60 postpartum, the reproductive tract of each heifer was examined rectally thrice weekly after collection of blood samples. Basal concentrations of LH from d 1 to 10 postpartum, as well as ability of the pituitary gland to release LH and PRL after the GnRH-TRH challenge, did not differ between service-sire-breed groups (P greater than .1). Means and profiles of progesterone concentrations during the first 60 d postpartum did not differ between service-sire-breed groups (P greater than .1). However, increases in progesterone concentrations following the GnRH-TRH challenge were synchronized more precisely in Angus (P less than .02) than in Holstein- and Brahman-service-sire groups. Daily rates of reduction in cervical and uterine horn diameters were higher (P less than .01) in Holstein- and Brahman- than in Angus-service-sire groups and were associated with higher profiles of postpartum 15-keto-13,14-dihydro-prostaglandin F2 alpha (PGFM) concentrations. Within-cow PGFM concentrations were correlated positively with cervical (r = .36) and uterine horn (r = .32) diameters. Postpartum ovarian responsiveness and uterine involution in Holstein heifers may be affected by genotypes of the conceptus they bore during pregnancy.