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The objective of this study was to compare the gain-response curve to dietary levels of 2-hydroxy-4(methylthio) butanoic acid (HMTBA) and DL-Met (DLM) across 4 floor pen trials in which different diets were used. Six replicates of 38 or 41 birds per pen (trials 1 to 2 and 3 to 4, respectively) were used in a 2 x 3 factorial arrangement. A control with 12 replicates was also included. The 2 Met sources were fed at 3 equimolar levels equally spaced, with the highest level added at requirements from 1 to 48, 49, 43, or 49 d for trials 1, 2, 3, and 4, respectively. Commercial-type TSAA-deficient control diets contained sorghum, wheat, corn, or corn plus meat and bone meal for trials 1, 2, 3, and 4, respectively. Performance improved at all times for most parameters after supplementing with HMTBA or DLM (P < 0.05). No differences were found in the birds fed HMTBA or DLM at any age and trial (P > 0.05), except for trial 1, in which 17-d-old birds performed better when fed HMTBA than DLM (P < 0.05). In each trial, linear, quadratic, and exponential regressions were conducted upon the gain response of birds fed HMTBA and DLM separately. Equations with better goodness of fit were used to compare the estimated gain responses to feeding HMTBA vs. DLM. In 3 trials, the shape of the gain-response curve differed when feeding HMTBA vs. DLM. In trials 3 and 4, feeding HMTBA at commercial levels resulted in greater gain responses than DLM (P < 0.05), whereas, in trials 2 and 4, at very deficient levels, DLM-fed birds outperformed those fed HMTBA (P < 0.05). When the 4 trials were combined, the dose-response curve with the best goodness of fit was linear for HMTBA and quadratic for DLM. It can be concluded that the 2 Met sources have a different dose-response form, HMTBA could outperform DLM at commercial levels, and DLM could outperform HMTBA at deficient levels.

BackgroundThe goal of this study was to compare the antagonism of elevated dietary Cu (250 mg/kg) from CuSO4 on three different Zn sources (ZnSO4 · H2O; [Zn bis(−2-hydroxy-4-(methylthio)butanoic acid)], Zn(HMTBa)2, a chelated Zn methionine hydroxy analogue; and Zn-Methionine), as measured using multiple indices of animal performance in ROSS 308 broilers.MethodsThree experiments were conducted in broiler chicks fed a semi-purified diet. All birds were fed a Zn-deficient diet (8.5 mg/kg diet) for 1 wk, and then provided with the experimental diets for 2 wks.ResultsExperiment 1 was a 2 × 2 factorial design with two levels of Cu (8 vs. 250 mg/kg diet from CuSO4) and two Zn sources at 30 mg/kg [ZnSO4 · H2O vs. Zn(HMTBa)2]. Elevated Cu impaired growth performance only in birds fed ZnSO4. Compared to ZnSO4 · H2O, Zn(HMTBa)2 improved feed intake (12 %; P < 0.001) and weight gain (12 %, P < 0.001) and the benefits were more pronounced in the presence of 250 mg/kg diet Cu. Experiment 2 was a dose titration of ZnSO4 · H2O and Zn(HMTBa)2 at 30, 45, 60, and 75 mg/kg diet in the presence of 250 mg/kg CuSO4. Feed:gain was decreased and tibia Zn was increased with increasing Zn levels from 30 to 75 mg/kg. Birds fed Zn(HMTBa)2 consumed more food and gained more weight compared to birds fed ZnSO4, especially at lower supplementation levels (30 and 45 mg/kg; interaction P < 0,05). Experiment 3 compared two organic Zn sources (Zn(HMTBa)2 vs. Zn-Methionine) at 30 mg/kg with or without 250 mg/kg CuSO4. No interactions were observed between Zn sources and Cu levels on performance or tissue mineral concentrations. High dietary Cu decreased weight gain (P < 0.01). Tibia Cu and liver Cu were significantly increased with 250 mg/kg dietary Cu supplementation (P < 0.01). No difference was observed between the two Zn sources.ConclusionsDietary 250 mg/kg Cu significantly impaired feed intake and weight gain in birds fed ZnSO4 · H2O, but had less impact in birds fed Zn(HMTBa)2. No difference was observed between the two organic zinc sources. These results are consistent with the hypothesis that chelated organic Zn is better utilized than inorganic zinc in the presence of elevated Cu.

In broilers, 2-hydroxy-4(methylthio) butanoic acid (HMTBA) can elicit a different dose response relative to dl-Met (DLM) such that birds could have lower gain responses at deficient TSAA concentrations but greater gain responses at maximum response concentrations. Two experiments tested if the 2 Met sources have a different dose response in 1-d-old turkeys using a 2 x 4 factorial plus a control design with 8 replicates of 12 toms per treatment. 2-Hydroxy-4(methylthio) butanoic acid and DLM were supplemented at equimolar concentrations of 0.05, 0.10, 0.15, and 0.20% or 0.04, 0.08, 0.16, and 0.32% for experiments 1 and 2, respectively, in commercial-type TSAA-deficient (0.99 to 1.02%) diets for 21 d. No differences in any performance parameter tested were found between HMTBA and DLM in either trial by ANOVA. Linear (LIN), quadratic (QUAD), and exponential regressions were fitted to the gain response of birds fed HMTBA or DLM. Equations with better goodness of fit as determined by Schwarz's Bayesian information criteria index were used for further calculations of predicted differences between HMTBA and DLM. In both trials, the shape of the dose response differed according to the Met source used, and best-fit equations were obtained when using Met intake over control rather than dietary Met concentration as the dependent variable. In experiment 1, the best-fit equations were an inverse QUAD for HMTBA and a LIN for DLM, and in experiment 2 with higher Met concentrations, the best-fit equations were a QUAD for DLM and a LIN for HMTBA. Feeding HMTBA at deficient TSAA resulted in lower (P <0.05) gains in experiment 1 but greater gains at maximum response concentrations (P <0.05) in both experiments. Plasma-free Met increased at 3 times the rate for DLM than HMTBA (P <0.01) with increasing Met concentration, which may play a role in the evolution of different dose responses at the extremes of the Met dose response. These results demonstrate that Met sources elicit a differential dose response in turkeys such that feeding HMTBA at deficient TSAA concentrations can be lower than DLM and can reach a higher maximum performance than with DLM.

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