| Popis: |
SUMMARY Heat stress is a life-threatening factor in commercial broilers raised in the tropics (e.g., Thailand) without temperature-controlled housing facilities. However, these temperature-controlled facilities are hardly affordable by small-scale farmers. To avoid such limitations, a Thai indigenous crossbred line (C line), selected as a candidate meat-type chicken to survive in the tropical environment, was examined for its capacity to tolerate the tropical climate and for its immune functions. Comparisons, in terms of the stress and innate and humoral immune responses using the heterophil-to-lymphocyte ratio, abdominal exudative cell phagocytic activities, and serum anti-SRBC titer, were made between C-line chicks and the corresponding cohorts from their low-meat-yielding Thai indigenous (T line) parents and from high-meat-yielding commercial broiler (B line) chickens. The responses were evaluated in the 3 different seasons of Thailand (monsoon, summer, and winter). Significantly higher stress levels (P < 0.001), based on the heterophil-to-lymphocyte ratios, were detected in the B line chicks compared with those in the T and C lines at all ages regardless of the season, suggesting that the B line chicks were vulnerable to tropical heat stress whereas the T and C lines were well adapted, with no significant differences detected between the latter two. The innate and humoral immunities of B-line chicks were significantly lower (P < 0.001) than those of T- and C-line chicks. The differences were immense in the summer, when the immunity of the C- and T-line chicks outperformed that of the B-line chicks, with mean opsonized-SRBC phagocytic activities of 7.90, 8.31, and 4.74 and mean IgG titers of 8.00, 8.40, and 5.10, respectively. This could be a consequence of the heat stress causing immunosuppression in B-line chicks, and could represent a noteworthy adaptation to the tropical conditions of the C- and T-line chicks. The apparent climate-tolerant capacity conserved in the C-line chickens, with approximately 50% T genetics, could serve as a guideline for further genetic improvement toward a high-meat-yielding chicken that retains a suitable heat tolerance. |
