Influence of sire, sex of fetus and type of pregnancy on conceptus development.

Estrus was synchronized in 93 crossbred cows 3 to 7 yr old which were then superovulated with 10 mg FSH-P and bred artificially to either a Jersey or Charolais bull. Females pregnant to the Jersey sire were slaughtered at 95, 180 or 250 d of gestation; low pregnancy rate from the Charolais sire resulted in enough dams for slaughter and valid comparisons at 95 d only. Conceptus tissue and organ weights and dam carcass weights and measurements were collected at slaughter. At 95 d of gestation, sire differences were not significant; type of pregnancy (single vs multiple) had significant effects on placentome and membrane weight; fetus sex had highly significant effects on fetus weight and crown-rump length. Fetal brain and heart weights showed little effect of sire, fetus sex or type of pregnancy, indicating these early differentiating tissues are highly competitive for available nutrients for growth. Fetal data at 180 and 250 d of gestation showed highly significant effects of gestation stage on placentome, membrane and fetus weights, crown-rump and metatarsal lengths, and liver, kidney, heart and total brain weights. Type of pregnancy had greater effects on the conceptus traits than did fetus sex at these stages. Nonorthogonal analyses of data from the Jersey-sired pregnancies indicated growth of fetuses in multiple pregnancies was suppressed, and the reduction in fetus weight became more pronounced as gestation progressed. Correlations among conceptus traits and dam carcass data showed different relationships within single and multiple pregnancies. We suggest that the genetic contribution for fetal growth has a positive effect on fetal size throughout gestation while the importance of metabolite availability may change during gestation depending on the genetic growth potential of the fetus and on whether the pregnancy is single or multiple. We speculate that these fetus-dam relationships involve systems controlling fetus growth that arise from the fetus genetic growth potential interacting with positive and negative dam effects that are linked or related to the carcass skeletal-fat-muscle endpoints studied.