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The Nebraska Index Line excels in reproduction and is being used in industry breeding programs. However, because it has been selected only for litter size since 1981, growth and carcass merit of pure-line pigs are below industry standards. The objectives of this experiment were 1) to compare growth and carcass traits of Index cross pigs with either 50% or 25% Line I genes in a crossbreeding system typical of how the line is used in the industry and 2) to determine the effects of feeding 18 g PAYLEAN® per ton during the last 28 days of the feeding period on Index cross pigs. Line I was crossed with Danbred® USA Landrace (L) boars and Duroc-Hampshire terminal boars to produce F1 pigs with 50% Line I genes and terminal cross pigs with 25% Line I genes. Pigs with 25% Line I genes grew faster (2.03 vs. 1.97 lb/d) from 65 days of age to approximately 240 lb than pigs with 50% Line I genes (P < 0.05). They also ate more feed per day although the difference was not significant (5.82 vs 5.76 lb per d). Thus, the difference between groups in feed conversion was small and not significant. Terminal cross pigs with 25% Line I genes had only 0.02 in less backfat at the end of the experiment than F1 pigs with 50% Line I genes, but they had significantly larger longissimus muscle area (6.42 vs 6.10 in2) and greater percentage carcass lean (52.4 vs 51 %). Pigs of both genetic groups and both barrows and gilts responded similarly to a diet with 18 g PAYLEAN® per ton. Feeding PAYLEAN® at 18 g/ton for 28 days significantly increased growth rate (2.19 vs. 1.80 lb/d), reduced feed intake (6.49 vs. 6.81 lb/d) improved efficiency of growth (0.33 vs 0.26 gain/ feed ratio, corresponding with 3.03 and 3.85 feed/gain ratios), increased carcass weight (185.4 vs 177.2 lb), increased dressing percentage (75.2 vs 74.3%), and increased carcass lean (53.6 vs 49.9%). Performance and carcass merit of pigs with 25% Line I genes were greater than for F1 pigs with 50% Line I genes, and feeding PAYLEAN® at the rate of 18 g per ton produced similar increases in performance and carcass merit of both groups.