Adjusting preweaning pig weights to a standard age.

Quadratic regression equations were fitted to preweaning weight records for 3,587 Duroc and Yorkshire pigs in 533 litters farrowed in the spring and fall from 1974 through 1978 at the Beltsville Agricultural Research Center. The objective was to derive factors for adjusting pig and litter weights to uniform ages for use in evaluating sow productivity. Only records for litters with at least two boar and two gilt pigs alive at weaning at 42 d of age were included. Records for up to four healthy pigs of each sex in each litter were used. Pig weights were recorded shortly after birth at approximately 21 and 42 d of age. Actual weights were transformed to natural logarithms prior to analysis. Quadratic regression equations were fitted for each of the 1,066 litter-sex subgroups. The computed least-squares mean and linear and quadratic regression coefficients from all subgroups were analyzed with a model that included effects of years, seasons, breeds, sex and all interactions. On the basis of significant season and year x season x breed interaction effects, the quadratic coefficient was required for a good fit of the equation to the data. The equation Y = .2127 + .07545 (days of age)--.000558 (days of age)2, where Y = estimated pig weight in natural logarithms, was then used to calculate multiplicative factors for adjusting preweaning pig and litter weights to ages of 14, 21, 28, 35 and 42 d.