Materials and Methods
Animals
Forty-six Yorkshire (Y × Y), 27 Large White (LW × LW), and 69 crossbred (26 LW × Y, 43 Y × LW) females were selected for use in this study.
All animals were raised at the Western Branch of OARDC (South Charleston, Ohio).
PRLR Genotype Analysis
For each animal, genomic DNA was extracted from lymphocytes and the PRLR gene amplified using a PCR-RFLP procedure that has been outlined previously (Vincent et al., 1998). This procedure has been patented by Iowa State University for use in improving reproductive efficiency in swine. Two PRLR alleles (1 and 2) were identified and each animal was classified as 11, 12, or 22 with respect to PRLR genotype.
Reproductive Tract Collection and Analysis
All 142 females were bred to Hampshire boars and slaughtered at approximately 75 days of gestation in a commercial slaughter facility. Animals were slaughtered in five separate groups, with approximately 30 animals in each group. Following slaughter, gravid uterine tracts were collected and analyzed. Data collected on these tracts included ovulation rate, horn length, number of fetuses in each horn, fetal weight, average fetal weight, uterine weight, number of mummies, fetal sex, fetal placement, fetal survival ([number of fetuses / ovulation rate] * 100), and fetal space (uterine length / [number of fetuses + number of mummies]).
Statistical Analysis
Allele and genotype frequencies were calculated for the total population of 147 animals and within each of the breed subgroups.
All reproductive tract data were analyzed using the General Linear Model Procedures of SAS (SAS Inst. Inc., Cary NC). Data were analyzed using a model that included the fixed effects of PRLR genotype, breed, parity, and all significant two-way interactions. Uterine horn was also included as a fixed effect in some analyses to determine the presence of between horn effects.