We compared the effectiveness of 2-hydroxy-4-(methylthio) butanoic acid (HMB) and DL-methionine (DLM) as sources of L-methionine activity in methionine-deficient primary cultures of pig liver cells and methionine-deficient early-weaned pigs. Viable hepatocytes were obtained from minced pig liver and maintained in a high density, differentiated, nonproliferation cell culture system. Culture medium was supplemented with HMB, DLM, or L-methionine, and cells were pulse-dosed with L-[14C(U)]leucine for 24 h to determine the level of protein synthesis. Leucine incorporation per milligram of protein indicated a six-to eightfold increase in protein synthesis (P < .01) with methionine levels between 5 and 10 microM, regardless of source of methionine activity. Two 24-pen replicate methionine dose titrations were conducted with 95 early-weaned commercial crossbred pigs. The pelleted corn, dried whey, and porcine plasma basal diet contained 1.5% lysine, .23% methionine, and .48% cystine and was supplemented with 0, .05, or .10% methionine activity as DLM or HMB for 21 d. There was a 134, 104, and 61% increase (P < .01) in cumulative ADG for each successive week on study with a 30 and 19% improvement in feed/gain (P < .01) after 7 and 14 d. Performance responses due to source of methionine activity did not differ and slope ratio potency determinations (gain vs intake of methionine source) of HMB vs DLM indicated a 119, 111, and 95% relative activity for cumulative weekly performance. These results support the hypothesis that HMB and DLM provide equimolar levels of methionine activity in swine.

The objective of these experiments was to study the effect of intermittent high temperature (IHT) on performance of broilers fed different sources of methionine activity. Two 20-d experiments were conducted in which individually caged 28-d-old cockerels were exposed to a 5-d period of constant high temperature (HT, 30 to 32 C) followed by an equal period of thermoneutral (TN) temperatures (22 C) with the 10-d temperature cycle repeated twice. Birds held at 22 C were TN controls. Grower diets contained 3,275 kcal/kg ME and 20% CP and were supplemented with either 2-hydroxy-4-(methylthio)butanoic acid (HMB, Alimet Feed Supplement) or DL-Met up to a maximum of .88 to .90% total sulfur amino acids (TSAA). In Experiment 1, gain:feed but not average daily gain was greater (P < .05) for HMB than DL-Met birds subjected to IHT (.451 vs .413, respectively), but no treatment differences were observed for TN birds. Results of Experiment 2 demonstrated a linear response to HMB and DL-Met dose in TN (P < .01); however, only HMB-supplemented birds responded similarly in IHT (P < .01). These results are consistent with lower availability of DL-Met as a result of IHT. In vitro experiments indicated that capacity for uptake of DL-Met into intestinal epithelial cells was reduced in heat-stressed birds. Uptake of D-Met was more severely affected than was L-Met. Consequently, a third performance experiment compared the ability of D-Met and L-Met to support growth under conditions of HT. The results indicated that the effect of HT on broiler performance was mediated through reduced utilization of D-Met.

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.