The effects of level of crude protein on performance of growing boars.

Five trials involving 432 growing boars were utilized to study the effects of six levels of crude protein on gain, feed efficiency, feed intake, backfat thickness and longissimus muscle area. The boars were fed ad libitum either a 14, 16, 18, 20, 22 or 24% crude protein diet from about 24.6 to 54.4 kg (period 1). From 54.4 to 98.8 kg (period 2), the crude protein in each diet was reduced by 2%. During period 1, gain increased and feed to gain ratio decreased quadratically (P<.05)with increasing dietary crude protein in corn-soybean meal based diets and were maximum at protein levels of 20 to 22%. Feed intake was not affected by dietary protein level. During period 2, protein level did not significantly affect rate of gain, feed efficiency or feed intake. Overall, gain increased quadratically (P<.05)with increasing protein levels and was maximum for boars fed the 20/18% protein sequence during periods 1 .and 2, respectively. Feed to gain ratio decreased linearly (P<.05) with increasing dietary protein intake. Feed intake was not affected by dietary protein level. Scan backfat thickness and longissimus muscle area obtained at the end of the trial indicated that backfat decreased linearly (P<.05) with increasing protein intake, while longissimus muscle area increased quad-ratically (P<.05) as protein in the diet increased. These results indicate that small improvements in average daily gain, feed efficiency, longissimus muscle area and backfat thickness can be made by feeding protein levels to growing boars approximately 2% higher than the currently recommended 18% protein diet during the growing period (24.6 to 58.4 kg), followed by an 18% diet during the finishing period (54.8 to 98.8 kg).