The relationship between placental and piglet birth weights and growth traits

Our laboratory recently reported that the ratio of a piglet's weight to tha t of its placenta (RATIO) varies approximately three-fold within a litter, due predominantly to variation in placental size which is known to be a lim iting factor in litter size. It was further determined that selection of bo ars and gilts with a higher than average RATIO for breeding resulted in an increased litter size and reduced placental weights. The objective of this experiment was to examine the effects of RATIO on economically important pr oduction traits (piglet survival to weaning, 21-day weights, days to 105 kg , backfat depth and loin muscle area at 105 kg) in a purebred breeding stoc k herd of Finnish Yorkshire and Landrace pigs. Sows were monitored througho ut farrowing and each piglet was matched to its placenta. Each piglet and i ts placenta were individually weighed and a RATIO determined Placental weig ht but not piglet birth weight exhibited a strong (P < 0.001) negative corr elation with RATIO (r = -0.73). Up to three-fold differences in RATIO were observed within a single litter. These differences were much greater than t he variation noted between litters and litter mean RATIO was not significan tly associated with litter size. The distribution of individual piglet RATI OS across the entire group of pigs ranged from 2.8 to 11.4 suggesting a mar ked difference in placental efficiencies. No significant association was se en between RATIO and 21-day weight, days to 105 kg, backfat depth or loin m uscle area. Thus, an above average ratio of a piglet's weight to that of it s placenta is indicative of a smaller move efficient placenta and appears t o have Mo subsequent detrimental effects on growth efficiency.