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Two trials involving 216 growing boars were conducted to study the effects of different levels of crude protein on daily gains, feed efficiency, feed intake and estimated carcass characteristics. The boars were fed either a 16, 18 or 20% crude protein ration from 23.8 to 55.8 kg (period 1). Upon completion of period 1, the protein level was reduced 2% for each treatment (14, 16 and 18%, respectively) from 55.8 to 99.6 kg (period 2). Average daily gain increased linearly (P<.01) and feed required per unit gain decreased quadratically (P<.05) as the percent protein in the diet increased from 16 to 20% in period 1 and 14 to 18% in period 2. Average daily feed intake was similar for boars on each treatment. Scanned backfat thickness decreased linearly (P<.01) and scanned longissimus muscle area increased linearly (P<.05) as protein in the diet increased.