Effects of stocking density and group size on thermoregulatory responses of laying hens under heat challenging conditions

Sectors of the US cage layer industry have begun adopting practices of reduced stocking density (i.e.. increased cage floor space) and varying group sizes. This study was conducted with 24 groups of 48 W-36 laying hens (39 to 46 weeks old) to assess the effects of cage floor space or stocking density (SD) (348, 387, 465, or 581 cm²/bird; 54, 60, 72, or 90 in ²/bird) and group size (GS) (8 or 16 birds/cage) on the ability of the hens to cope with heat challenge. Data were collected at thermoneutral (24°C or 76°F) and warm conditions (32°C or 90°F and 35°C or 95°F). No differences in core body temperature (CBT) of the hens were observed among the treatment regimens at 24°C. In general, mean CBT increased with heat exposure duration (P<0.0001) but leveled offafter the 32°C phase. At 32°C, CBT was higher for GS of 16 vs. 8 (42.3 vs. 42.1°C, P=0.05); higher for SD of 348 and 387 cn²/bird than for 465 or 581 cm²/bird (42.4 and 42.2°C vs. 41.9 and 42.1°C, respectively, P=0.009). Bird body mass decreased as heat exposure duration increased (P<0.0001), but no differences were observed among the treatments. No mortalities were observed during the thermoneutral period, and the mortality rate increased with heat exposure duration. The results indicate that, while CBT was lower for lower stocking density, the increased space was not sufficient to offer a clear benefit for coping with heat challenge of 32°C or 35°C.