Novel insight into the control of litter size in pigs, using placental efficiency as a selection tool

Chinese Meishan pigs produce three to five more pigs: per litter than less- prolific U.S. or European pig breeds as a result of a markedly decreased pl acental size and an increased pig weight: placental weight ratio(placental efficiency). We hypothesized that as a result of their intense selection fo r prolificacy, the Chinese had indirectly selected for a smaller, more effi cient placenta in the Meishan breed. The goals of this study were to determ ine whether 1) significant variation in placental size and efficiency exist ed within our population of purebred Yorkshire pigs and 2) selection of pig s (boars and gilts) based on clear differences in placental size and effici ency would affect litter size. There was significant (approximately threefo ld) variation in placental efficiency in our herd of Yorkshire pigs, and ma rked (approximately twofold) variation existed within individual litters. W e then selected pigs boars and gilts) that had either a higher (A Group) or lower (B Group) than average placental efficiency. Although the birth weig hts of selected A Group pigs were similar to those of the B Group pigs, the y had markedly smaller placentae. Males from each group (A or B) were bred to the females of the same group, and farrowing data were collected from pa rities 1 and 2. In both, parities, A Group females farrowed more live pigs per litter than did B Group females (12.5 +/- .7 vs 9.6 +/- .5, P < .05), A lthough A Group pigs were on average approximately 20% lighter than B group pigs (1.2 +/- .1 vs 1.5 +/- .1 kg, P < .05), their placentae were approxim ately 40% lighter(250 +/- 10 vs 347 +/- 15g,P < .01),resulting in a marked increase in placental efficiency. The results of this study suggest that se lection on placental size and efficiency may provide a valuable tool for op timizing litter size in commercially important pig breeds.