EFFECT OF DIVERGENT SELECTION FOR POSTWEANING AVERAGE DAILY GAIN ON FRONT-END SOUNDNESS OF MARKET-WEIGHT PIGS

A total of 975 barrows and gilts from lines divergently selected for p ostweaning ADG were evaluated for front-end structural soundness (scor ed on a scale from 1, extreme leg weakness, to 8, superior leg structu re) at approximately 100 kg, as well as growth performance and backfat thickness. Selection was for either fast (line Fl or slow (line S) AD G from 9 wk of age to 100 kg and was replicated in spring-and fall-far rowing groups. The cumulative divergent selection differential corresp onding to the animals evaluated was .47 kg/d (approximately 5 standard deviations). There was a line x sex x farrowing group interaction (P < .05) for ADG. Pigs from F grew faster (P < .01) than pigs from S, bu t the difference between lines was greater in gilts than in barrows. T his interaction between line and sex was more pronounced in the spring - than in the fall-farrowing group. Average daily feed intake was 23% greater (P < .01) for F pigs than for S pigs. Even though pigs from F consumed more feed than those from S, their relatively faster ADG resu lted in a greater(P < .01) feed efficiency (gain/feed) in F than in S. There was a line x farrowing group interaction (P < .05) for average backfat thickness adjusted to 105 kg. Barrows and gilts from F had 3.9 and 8.3% greater backfat thickness than those from S in the fall- and spring-farrowing groups, respectively. Soundness scores were not sign ificantly affected by selection line (P > .35), but gilts had greater (P < .05) front-end soundness than barrows, and pigs had greater (P < .05) soundness in the fall-farrowing group than in the spring-farrowin g group. Phenotypic correlations between soundness score and ADG durin g the growing and finishing phases were -.011 and .214, respectively. The phenotypic correlation between soundness score and average backfat thickness was .095. Genetic selection for ADG significantly changed p ostweaning performance but did not affect front-end structural soundne ss in this population.