4217 INTRODUCTION Requirements for indispensable AA for pigs are often expressed as a ratio to Lys in the diet (Wang and Fuller, 1989; Chung and Baker, 1992; NRC, 2012), but there is some confusion about the ideal Thr:Lys ratio. A standardized ileal digestible (SID) Thr:Lys ratio of 0.60 for 25- to 50-kg growing pigs was re- cently estimated as being adequate (NRC, 2012), and this value is in agreement with a previous estimate Effects of dietary fber on the ideal standardized ileal digestible threonine:lysine ratio for twenty-fve to ffty kilogram growing gilts 1 J. K. Mathai,* J. K. Htoo,† J. E. Thomson,‡ K. J. Touchette,§ and H. H. Stein* 2 *Department of Animal Sciences and Division of Nutritional Sciences, University of Illinois, Urbana 61801; †Evonik Nutrition & Care GmbH, Rodenbacher Chaussee 4, Hanau-Wolfgang, Germany, ‡Evonik Degussa Corporation, Kennesaw, GA; and §Ajinomoto Heartland Inc., Chicago, IL, 60631 ABSTRACT: Four experiments were conducted to determine effects of fber on the ideal Thr:Lys ratio for 25- to 50-kg gilts. In Exp. 1, the objective was to determine the requirement for standardized ileal digestible Lys for gilts from 25 to 50 kg BW. Seventy gilts (24.54 ± 3.28 kg BW) were used in a growth assay with 2 pigs per pen, 5 diets, and 7 replicate pens per diet. The 5 diets were based on corn and soybean meal and contained between 0.80 and 1.32% SID Lys. Results indicated that 1.09% SID Lys was needed to optimize ADG and G:F. In Exp. 2, the objective was to determine the standardized ileal digestibility of AA in corn, soybean meal, feld peas, fsh meal, and soy- bean hulls. Six ileal-cannulated gilts (26.5 ± 0.74 kg BW) were allotted to a 6 × 6 Latin square design with 6 diets and 6 periods. Values for standardized ileal digestibility of AA were calculated for all ingredients. In Exp. 3, the objective was to determine the effect of fber on the ideal SID Thr:Lys ratio for gilts from 25 to 50 kg BW. A total of 192 gilts (26.29 ± 4.64 kg BW) were used in a growth assay with 2 pigs per pen and 8 replicate pens per treatment. Six low-fber diets and 6 high-fber diets were formulated using the same batches of ingredients as in Exp. 2. Within each level of fber, diets with SID Thr:Lys ratios ranging from 45:100 to 90:100 were formulated using the SID values calculated in Exp. 2. In both types of diets, ADG and G:F linearly and quadratically (P < 0.05) increased as the Thr:Lys ratio increased. Regression analysis estimated the ideal SID Thr:Lys ratio at 0.66 and 0.63 for ADG and G:F, respectively, for pigs fed low-fber diets and at 0.71 and 0.63, respectively, for pigs fed high-fber diets. In Exp. 4, the objective was to determine the N balance in pigs fed low-fber or high-fber diets that were formulated to have SID Thr:Lys ratios of 45:100 or 60:100. The 4 diets were formulated using the same batches of ingredients as in Exp. 2, and the SID values determined in Exp. 2 were used in diet formulations. Thirty-six gilts (29.0 ± 0.74 kg BW) were individually housed in metabolism crates with 9 replicate pigs per diet. Retention of N (% of intake) was greater (P < 0.05) for pigs fed the low-fber diets compared with pigs fed the high-fber diets regardless of the Thr:Lys ratio. Results of these experiments indicate that increased fber levels in diets fed to growing gilts increase the requirement for Thr and that diets with higher fber levels should be formulated to a greater SID Thr:Lys ratio. Key words: amino acids, fber, ideal protein, lysine, pigs, threonine © 2016 American Society of Animal Science. All rights reserved. J. Anim. Sci. 2016.94:4217–4230 doi:10.2527/jas2016-0680 1 Financial support for this research was provided by Ajinomoto Heartland, Chicago, IL, and by Evonik Nutrition & Care GmbH (Hanau-Wolfgang, Germany). 2 Corresponding author: hstein@illinois.edu Received May 28, 2016. Accepted July 28, 2016. Published October 24, 2016