Development and validation of equations to predict the metabolizable energy value of corn for pigs

Three experiments were conducted with the aim of developing and validating an equation to predict the ME of corn for pigs from its chemical composition, physical characteristics and particle size. Exp. 1: Eight lots of corn were ground in a hammer mill, using 5 sieves with different screen opening sizes, generating 40 batches of ground corn. The chemical composition (DM, CP, ether extract, crude fiber, ADF, NDF, and ash) and physical characteristics (bulk density-BD and 1,000-kernel weight-TKW) were determined in the 8 lots and geometric mean diameter (GMD) and N-corrected ME (AMEn) were determined in the 40 batches of corn. The AMEn values were determined in 16 metabolism assays with pigs. Mathematical models were adjusted by regression analysis, based on the Akaike Information Criterion. Based on statistical parameters (R-2 = 0.76 and prediction error = 1.05%), number of predictor variables, and easiness of measurements, an equation with 2 segments was chosen: y = 2845.41 + 0.9385 x BD -20.8784 x CP, if GMD <= 522.98 and y = 3105.75 -0.4978 x GMD + 0.9385 x BD -20.8784 x CP, if GMD > 522.98. Exp. 2 and 3: Sixty four gilts (Exp. 1; 29.5 +/- 3.8 kg) and 64 barrows (Exp. 2; 29.3 +/- 3.6 kg), 1 lot of corn, and 3 particle sizes (GMD = 483, 632, and 904 mu m) were used in a 3 x 2 factorial arrangement with 2 methods of diet formulation, differing in ME value of corn: "FIX" (value from nutrient composition table) vs. "ESTIMATED" (estimated for each particle size using the equation developed in Exp. 1). In Exp. 2, ADFI was greater (P < 0.05) and feed efficiency was lower (P < 0.05) in the diet with GMD of 904 mu m compared to the diets with GMD of 632 or 483 mu m, but only for diet formulation by the FIX method. In the treatments with GMD of 483 mu m, gilts fed with the diet formulated by the ESTIMATED method had greater (P < 0.10) ADFI, backfat, fat area, and fat: meat ratio than gilts fed the diet formulated by the FIX method. In Exp. 3, particle size and formulation method did not affect (P > 0.10) growth performance and carcass traits of barrows. The equation developed was effective to adjust the ME value of corn, considering particle size variation. However, improvement to the proposed equation is necessary to achieve greater precision for predicting corn ME. Validation of the equation with more lots of corn of different chemical compositions and densities appears necessary to assess the efficacy of the equation regarding the variation of the other predicting variables.