Objective: To model the effects of birth weight on preweaning survival and weights at weaning and at 42 days post weaning.

Materials and methods: Individual birth-weight and weaning-weight records of 2467 pigs weaned at 14 to 21 days old were partitioned into nine categories incrementally increasing or decreasing by 0.5 SD (0.16 and 0.68 kg, respectively) from the mean (1.57 and 5.80 kg, respectively). To study the effect of birth weight on weight at subsequent phases, fixed effects of birth-weight category and parity of dam were included in the model for weights at weaning and 42 days post weaning. To evaluate the effect of weaning weight on weight at 42 days post weaning, fixed effects of weaning-weight category and parity of dam were included.

Results: From birth to weaning, maximum piglet survival (93.8% to 97.1%) was attained in birth-weight categories 2, 3, and 4, and poorest survivability (71.2% to 79.6%) was observed in birth-weight categories 1, 5, and 7. Weight at 42 days post weaning increased with heavier birth-weight category (P < .001) and with heavier weaning-weight category (P < .001). Parity of dam was a source of variation (P < .01) for weights at weaning and 42 days post weaning.

Implications: Differences in body weight at birth are perpetuated, so that pigs lighter at birth are still lighter at 42 days post weaning. Pigs of primiparous sows have a growth disadvantage. Producers should consider alternatives for managing underweight pigs on an individual herd basis.