Effect of relative humidity at either acute or chronic moderate temperature on growth performance and droppings' corticosterone metabolites of broilers

The present study aimed to investigate the effect of relative humidity (RH) at either acute or chronic moderate ambient temperature (Ta) on growth performance and droppings' corticosterone metabolites of broilers. Two experiments were conducted: effect of RH (35, 60 or 85%) on average daily feed intake (ADFI) and droppings' corticosterone metabolites at acute (1 d: 20–26 or 31–20°C, 26 or 31°C for 6 h d^–1 at 10:00–16:00) moderate Ta (experiment 1) and effect of RH (35, 60 or 85%) on growth performance and droppings' corticosterone metabolites at chronic (step-wisely increasing temperature by 3°C every 3 d from 20 to 32°C within 15 d: 20–23–26–29–32°C) moderate Ta (experiment 2). Droppings were collected at the 2, 4, 6, 8, and 22 h after Ta-RH controlled in experiment 1 and at the 2, 4, 6, and 22 h after Ta controlled to 32°C in experiment 2. The results showed that: 1) In experiment 1, 85% RH increased (P<0.05) the droppings' corticosterone metabolites at the 2, 6, 8, and 22 h and 35% RH increased (P<0.05) it at the 2 and 22 h compared to the 60% RH. Moreover, 85% RH further increased (P<0.05) it compared to the 35% RH, however, no difference (P>0.05) was found in ADFI among the three RH groups at acute moderate 26°C; 35 and 85% RH increased (P<0.05) droppings' corticosterone metabolites at the 2, 6, 8 and 22 h and decreased (P<0.05) ADFI compared to the 60% RH, moreover, 85% RH further increased (P<0.05) droppings' corticosterone metabolites and further decreased (P<0.05) ADFI compared to the 35% RH at acute moderate 31°C; and the average of droppings' corticosterone metabolites in the whole period had a negative correlation (P<0.02) with the ADFI. 2) In experiment 2, 85% RH increased (P<0.01) droppings' corticosterone metabolites only at the 2 h and decreased (P<0.02) ADFI and average daily gain (ADG) compared to the 60% RH, no difference (P>0.05) in droppings' corticosterone metabolites was found between the 35 and 60% RH, however, 35% RH decreased (P<0.01) ADG compared to the 60% RH, and the average of droppings' corticosterone metabolites in the whole period also had a negative correlation (P<0.02) with ADFI and ADG. In conclusion, droppings' corticosterone metabolites could be used as a RH stress index and low and high RH, especially high RH, reduced growth performance possibly through inducing RH stress at moderate temperature.