The effects of long-term caging and molt of Single Comb White Leghorn henson herterophil to lymphocyte ratios, corticosterone and thyroid hormones

Two commercial strains of 18-wk-old Single Comb White Leghorn (SCWL) hens, HyLine W-36 and DeKalb XL, were housed six hens per cage in layer cages at two densities (361 and 482 cm(2) per bird) with two replications each per s train and density combination. The high density treatment contained 24 hens per replication, and the low density treatment contained 18 hens per repli cation. Egg production was measured during the first egg production cycle, a molt (fast) period, and the first 4 wk of the second lay cycle (20 to 68 wk of age). Blood samples were obtained from six hens from each replicate in each strai n and density combination (total of 48) at 20, 26, 34, 43, 51, 62, 64, and 68 wk of age. In addition, blood samples were obtained in a random order fr om hens in each cage, and the sequence of sampling was recorded (1 to 6). B lood smears were made, from which heterophil to lymphocyte ratios (H:L) wer e determined. Radioimmunoassays were conducted to determine levels of plasm a corticosterone (CS), 3,5,3'-triiodo-thyronine (T-3), and thyroxine (T-4). The results indicated that strain did not affect percentage hen-day egg pro duction (%HDP). Strain and cage density did not affect H:L, T-3, T-4, or CS . However, these parameters were affected by bird age, which was related to the egg production cycle. Plasma CS significantly (P less than or equal to 0.001) increased during peak %HDP at 26 wk and 64 wk during the molt (fast ), and I-E:L significantly (P less than or equal to 0.001) increased during the molt (fast) at 64 wk. The sequence in which blood samples were obtaine d, from hens within a cage in sampling order, also increased plasma CS. The CS was significantly (P less than or equal to 0.001) elevated in the third , fifth, and sixth hens from which blood samples were drawn. Plasma T-3 and T-4 changed during die production cycle. The T-3 was signifi cantly (P less than or equal to 0.0001) depressed during peak egg productio n at 26 wk and during the molt (fast) at 64 wk when compared with the other time periods. Plasma T-4 was depressed (P less than or equal to 0.0001) at 51 wk and was elevated (P less than or equal to 0.0001) at 64 wk during th e molt (fast). The physiological and metabolic parameters of the different hen strains and cage densities were similar during egg production. However, CS, T-3, T-4, and H:L changed with age in relation to the egg production cycle. In additi on, the physiological demands of peak egg production and molt (fast) appear ed to be similar.