The influence of protein concentrations in basal diet on metabolizable energy of full-fat soybeans and soy protein isolate determined by the difference procedure in pigs

Abstract

In the difference procedure, a potential influence of crude protein (CP) concentrations of basal diets on accurate metabolizable energy (ME) determination of a test ingredient is of an issue. The objective of the present study was to determine the influence of CP concentrations in the basal diet and test ingredients on the ME:digestible energy (DE) of full-fat soybeans (FFSB; 377 g/kg CP) and soy protein isolate (SPI; 876 g/kg CP) using the difference procedure. Twelve barrows (73.7 ± 5.5 kg body weight) individually housed in metabolism crates were used. A low-protein basal diet (LPBD; 74 g/kg CP) was composed of 975 g/kg corn as the sole energy source and a high-protein basal diet (HPBD; 143 g/kg CP) was mainly composed of 780 g/kg corn and 196 g/kg soybean meal as the sole energy sources. Four additional diets were prepared by including FFSB or SPI at 200 g/kg at the expense of the energy sources in the basal diets. The 6 experimental diets were fed to the 12 pigs employing a replicated 6 × 4 incomplete Latin square design with 4 periods. While the urinary energy output from the pigs fed the SPI diet in the HPBD group was greater (P < 0.05) than that in the LPBD group, the urinary energy output was not affected by CP concentration in the basal diet in pigs fed the basal diets or the FFSB diets. The ME:DE of the SPI diet in the LPBD group was greater (P < 0.05) than that in the HPBD group, whereas ME:DE was not affected by the basal CP concentrations in pigs fed the basal diets or the FFSB diets. The one-slope broken-line analysis indicated that the CP concentration in the diet needs to be less than 170 g/kg (P < 0.05) to avoid potential interaction between a basal diet and a test ingredient on ME:DE. While ME:DE of the SPI in the LPBD group was greater (P < 0.05) than that in the HPBD group, ME:DE of the FFSB was not different between the 2 basal diet groups. Overall, ME in a high-protein ingredient can be underestimated if a relatively high-protein basal diet is used in the difference procedure mainly due to increased urinary energy output from pigs. Experimental diets in the difference procedure need to contain less than 170 g/kg of protein for finishing pigs to accurately determine biologically available energy concentrations of soybean protein sources.