Evaluation of High Standardized Ileal Digestible Tryptophan:Lysine Evaluation of High Standardized Ileal Digestible Tryptophan:Lysine Ratios with Ractopamine HCl on Growth and Carcass Ratios with Ractopamine HCl on Growth and Carcass Performance of Pigs from 240 to 295 lb Performance of Pigs from 240 to 295 lb

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Introduction
Tryptophan (Trp) is generally considered the second or third limiting amino acid in corn-soybean meal-based diets fed to growing and finishing swine. 4Although considerable research has been conducted to determine the optimum Trp requirement for swine, there are important discrepancies between studies. 5The NRC 6 SID Trp:Lys ratio requirement estimate for pigs above 165 lb is 17.7% of Lys.Zhang 7 et al. suggested an ideal SID Trp:Lys ratio ranged from 19.7 to 23.6% for finishing pigs depending on the response variable.Gonçalves 8 et al. reported that increasing SID Trp: Lys ratio to 24.5% in finishing pigs fed ractopamine HCl (RAC) during summer months improved ADG by 0.15 and 0.08 lb/d in comparison with ratios of 18 and 21%, respectively.These results were later supported by the findings of Soto 9 et al., where Trp:Lys ratios of 24 and 28% improved ADG by 0.11 and 0.09 lb/d, respectively, compared with pigs fed a Trp:Lys ratio of 20% ratio when diets contained RAC.With a limited number of ratios to determine the maximum performance response, the objective of this experiment is to evaluate the effects of feeding higher levels of SID Trp:Lys than used in our earlier study in diets containing RAC on growth and carcass performance of pigs from 240 to 295 lb.

Procedures
The Kansas State University Institutional Animal Care and Use Committee approved the protocol used in this experiment.The study was conducted at a commercial research-finishing site in southwest Minnesota.The barn was naturally ventilated and double-curtain-sided.Each pen was equipped with a 5-hole stainless steel feeder and bowl waterer for ad libitum access to feed and water.Feed additions to each individual pen were made and recorded by a robotic feeding system (FeedPro; Feedlogic Corp.,  Wilmar, MN).
In April 2017, a total of 935 pigs (PIC 1050 × 337, initially 237.2 lb BW) were used in a 22-d trial.There were 23 or 24 mixed-gender pigs per pen at a floor space of 7.4 ft 2 per pig, and 8 replications per treatment.Pigs were allotted based on initial BW and assigned to 1 of 5 treatments in a completely randomized block design.The dietary treatments included 5 SID Trp:Lys ratios (20, 22, 24, 26, and 28% of Lys, Table 1).All diets were formulated with 0.90% SID Lys and contained 10 ppm ractopamine.Prior to the trial, from 200 to 240 lb, these pigs were fed a corn-soybean meal-dried distillers grains with solubles-based diet that contained 13.0% CP, 0.70% SID Lys, 20% SID Trp:Lys ratio, and 1,150 kcal NE/lb.Pigs were weighed on d 0, 9, and 22, to determine ADG, ADFI, and F/G.Prior to marketing, pigs were individually tattooed with a pen ID number to allow for carcass measurements to be recorded on a pen basis.On d 22, final pen weights were taken, and pigs were sent to a commercial packing plant (JBS Swift and Company, Worthington, MN) for processing and carcass data collection.Carcass measurements taken at the plant included HCW, loin depth, backfat, and percentage lean.
Diet samples were taken from 6 feeders per dietary treatment 3 d after the beginning of the trial and 3 d prior to the end of the trial and stored at -20°C until they were 7 Zhang, G. J., Q. L. Song, C. Y. Xie, L. C. Chu, P. A. Thacker, J. K. Htoo, and S. Y. Qiao.2012.Estimation of the ideal standardized ileal digestible tryptophan to lysine ratio for growing pigs fed low crude protein diets supplemented with crystalline amino acids.Livest.Sci.149:260-266. 8Gonçalves, M. A. D. 2015.Effects of standardized ileal digestible tryptophan:lysine ratio on growth performance of finishing gilts under commercial conditions.PhD diss., Kansas State University, Manhattan, KS. 9 J. A. Soto, M. D. Tokach, K. J. homogenized, subsampled, and submitted for total AA analysis (except Trp; method 994.12;10 ) and Trp (method 994.13; 10 ) by Ajinomoto Heartland, Inc. (Chicago, IL).Samples of the diets were also submitted to Cumberland Valley Analytical Service (Hagerstown, MD) for analysis of DM, CP, Ca, P, ether extract, and ash.Data were analyzed using the GLIMMIX procedure of SAS version 9.4 (SAS Institute, Inc., Cary, NC) in a randomized complete block design with pen serving as the experimental unit and initial BW serving as the blocking factor.Dietary treatments were the fixed effect and block served as the random effect in the analysis.Preplanned linear and quadratic orthogonal contrast were built using coefficients for equally spaced treatment and used to determine the main effects of increasing SID Trp.Hot carcass weight served as a covariate for the analysis of backfat, loin depth, and lean percentage.Results from the experiment were considered significant at P < 0.05 and a marginally significant between P > 0.05 and P ≤ 0.10.

Results
The analyzed total amino acids, DM, CP, Ca, P, ether extract, and ash contents of experimental diets (Table 2) were reasonably consistent with formulated estimates.
For overall growth performance (d 0 to 22), increasing SID Trp:Lys ratio increased (linear, P < 0.05) ADFI and SID Trp g/kg gain.However, there was no evidence for treatment differences for ADG or F/G.Similarly, for carcass characteristics, there was no evidence for treatment differences for HCW, carcass yield, backfat loin depth, lean, carcass ADG, or carcass feed efficiency.
In summary, increasing SID Trp:Lys did not provide any benefits in overall growth or carcass performance.The ADFI results of this study are consistent with the findings of Gonçalves et al., 8 where ADFI was 0.07 lb/d higher in finishing gilts fed 24.5% SID Trp:Lys ratio compared with pigs fed 21% SID Trp:Lys ratio in diets containing RAC.Consequently, ADG was 0.08 lb/d higher in their experiment.Furthermore, Soto et al. 9 observed improvements in growth, where ADG was 50 g/d higher in finishing pigs fed 24.0%SID Trp:Lys ratio compared with pigs fed 20% SID Trp:Lys ratio in diets containing RAC.Conversely, ADG did not improve with increasing Trp:Lys in the current study.Tryptophan is involved in the regulation of behavioral and physiological processes such as susceptibility to stress and feed intake. 11,12In addition, extended supplementation of Trp may be required to observe performance improvements. 13Further research is necessary to determine why inconsistencies are observed when feeding high SID Trp:Lys ratios to finishing pigs as well as to identify the greatest opportunities to yield economic benefits.  Caloric efficiency is expressed as kcal/lb of gain.
3 Adjusted using HCW as a covariate. 4Carcass average daily gain = overall ADG × carcass yield. 5 Carcass G:F = overall average feed intake:carcass average daily gain.

Table 2 .
Chemical analysis of experimental diets (as-fed basis) 1

Table 3 .
The effects of feeding high standardized ileal digestible (SID) tryptophan to lysine ratio on growth performance, carcass characteristics, and economics of finishing pigs 1 A total of 935 pigs (PIC 1050 × 337) were used with 23 or 24 pigs per pen and 8 replications per treatment. 1