Selection of Piglets with a Reduced Placental Size Does Not Hinder Production Traits
Using a purebred population of Yorkshire (Y) females, our laboratory recently reported that the ratio of a piglet's weight to that of its placenta (RATIO) varies ≈threefold within a litter. This variation in RATIO is due predominantly to variations in placental weight that are known to be a limiting factor in litter size. It was further determined that selection of boars 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 production traits (21- day weight, days to 105 kg and backfat and loin area at 105 kg) in a purebred breeding stock herd of Y and Landrace pigs. Sows were monitored throughout farrowing, and as each piglet appeared its umbilical cord was clamped close to the dam's vulva and again near the neonates body then cut between the clamps. A number designating birth order was then tied around the exposed umbilical cord with surgical silk and the tagged umbilical cord was allowed to retract back into the birth canal. Each piglet was then notched to match its numbered placenta. Following expulsion, placentae were separated, piglets and placentae weighed, and RATIO determined for each. Piglets were individually weighed at 21 days and scanned by ultrasound at 105 kg for backfat thickness and loin area. Placental weight exhibited a significant (P<.0001) negative correlation with RATIO (r=- .73), whereas birth weight did not (r=.09, P>.25). Further, neither 21-day weight nor days to 105 kg were associated with RATIO. Additionally, carcass quality was not associated with RATIO. These data indicate that piglet selection based on a high RATIO (increased placental efficiency) is not detrimental to economically important production traits. Data will continue to be compiled on the tagged gilts as they reach breeding age to determine if their selection for increased placental efficiency results in increased litter size.