Lifetime parental productivity in twenty-seven crosses of XXXX . I. Birth traits.

@entity19 @entity30486
Twenty-seven crosses derived from three random-mating, unselected strains of @entity19 (SWO, J, PGH) with 16 matings/pureline or cross were cohabited continuously and monogamously for 365 d to evaluate long-term reproductive performance of the parents. Data from 3,249 litters born were analyzed to study the effects of parity (number of litters) and type of cross (pureline, two-way, backcross, three-way) on the following traits: 1) continuity of reproduction ( @entity30486 ), 2) litter size at birth (LSB), 3) age at first litter (AFL), 4) fraction alive at birth-the ratio of number born alive to LSB (FAB), 5) birth to birth interval (BBI) and 6) reproductive rate-the ratio of number born alive to birth to birth interval (RRT). Reproductive rate was also studied in a cumulative manner rather than on a per parity basis by regressing cumulative number born alive on cumulative birth to birth interval. Parity effects were important (P less than .01) for all traits measured. Differences among cross-types were highly significant for @entity30486 , LSB and RRT. Three-way crosses were more productive than backcrosses, and both were superior to two-way and pureline matings. Uniformity was greater among three-way crosses for all traits except @entity30486 . Cross-types differed importantly in the pattern of change in performance over parities for @entity30486 , LSB, FAB, RRT and cumulative number of live young (P less than .01). Number of parities were most important for variation in longevity of reproduction ( @entity30486 ) and somewhat for litter size (LSB). Mean and cumulative reproductive rates were influenced more by longevity and litter size than by interval between births.
