Tonic immobility, heterophil to lymphocyte ratio, and organ weights in slow-growing chickens

Abstract

Effects of sex and strain of bird on factors influencing welfare in chickens were investigated in 288 21-week-old Potchefstroom Koekoek (PK), Ovambo (OV), and Naked Neck (NN) chickens. The birds were allocated to 4 paddocks of Chloris gayana and a house subdivided into 4 pens. There were 12 males per strain and 12 females per strain in each paddock. Similarly, 12 males per strain and the same number of females were placed in each pen in the house. Twelve birds, 4 each of NN, OV, and PK, were randomly selected per paddock and per pen, and tonic immobility (TI) was determined. Ambient temperature and humidity were recorded and fitted into a PROC MIXED model as random effects, with strain and sex as main effects. On the last d of the trial, blood samples were collected from 9 (3 of each strain) randomly selected birds per paddock, via brachial venepuncture, using 5 mL syringes and 22 gauge needles. At slaughter, the liver and spleen were recovered and weighed immediately. Organ weights were expressed relative to body weight (BW) of each bird. Sex of bird influenced (P < 0.05) BW, spleen, and relative liver weights and heterophil to lymphocyte (H/L) ratio. Strain did not influence (P > 0.05) TI but affected (P < 0.05) all other parameters. Rearing system (P < 0.05) influenced all parameters. Strain × sex interactions were significant (P < 0.05) on all organ weights. Males appeared more stressed than females. The free-range system could minimize stress in birds though mechanisms should be devised to prevent attacks by predators. Perhaps future research could be designed to study similar parameters over a longer observation period using younger slow-growing chickens.