We showed that N-acetyl-L-cysteine (NAC), as a source of cystein, improved performance of chronic cyclic heat stressd finisher broilers. Here, different sulfur amino acid sources were compared. The control diet had a ratio dig M+C to dig LYS of 0.73 (d25-41). Three experimental diets were prepared: control supplemented with NAC (2,000 mg/kg), L-cysteine (1,479 mg/kg), or Ca-DL-HMTBa (2,168 mg/kg). Treatment were replicater in 9 pens with 20 Ross 308 male each. A chronic cyclic heat stress model (34oC, 50-60% rh for 7 h daily) was initiated at d28. One bird per pen was sampled on d29 (acute heat stress) and d41 (chronic heat stress) to determine malondialdehyde and activity of enzyms involved in glutathione metabolism in various tissue. Adg for Ca-DL-HMTBa supplemented birds was higher than control (90.8 vs 83.3 g/d; P< 0.05), whereas for other treatment is was intermediate (P> 0.05). F:G was not affevted, but all supplemented diets showed app. 0.1 lower F:G. Enzyme activities were not affected by treatment. Liver malondialdehyde was lowerd in CA-DL-HMTBa supplemented birds compared to other groups on d219 (P<0.05). whereas it was higher in all supplemented birds as compared to control on d41 (P<0.05). In conclusion, additional Ca-DL-HMTBa showed beneficial effects on growth in chronic cyclic heat stressed finisher broilers.

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