Behaviour, physiological responses, meat yield and gut morphology of free-range chickens raised in a hot environment.

Abstract

It is vital to minimise thermal stress and associated welfare problems for birds reared in hot environments and behaviour is a good indicator of thermal stress. The broad objective of the study was to investigate behavioural, physiological and gut morphological responses of free-range slow-growing chickens raised in a generally hot environement. A total of 488 Naked Neck (NN), Ovambo (OV) and Potchefstroom Koekoek (PK) chickens were used in the study. The experiments conducted in this study explored effect of strain, sex, rearing system and provitamin A bio-fortification of maize on gut development in chickens in the context of thermal stress. Two hundred and eighty-eight NN, OV and PK chickens were separated by sex and reared in either intensive or extensive rearing pens, with twelve birds from each strain per pen. Time budgets on free-ranging and related behavioural activities were determined at 3 different observation periods (0800 h, 1200 h and 1600 h) for 3 weeks. Body weight (BW), random environmental effects; ambient temperature (Ta) and relative humidity (RH) as well as bird stress indicators; rectal temperature (RT), heart rate (HR), breathing rate (BR), tonic immobility (TI), spleen and liver weights were determined for the free-ranging and confined flocks. On the last day of the trial, blood samples were collected from randomly selected birds via brachial venepuncture. Body weight (BW), carcass weight (CW), dressed weight (DW), portion and giblet yields were determined. Gut organs were recovered and weighed on a digital scale within 10 min of slaughter. Intestinal length, weight, ileal villus parameters; villus height (VH), villus density (VD), villus width (VW) and muscularis externa (ME) thickness and apparent villus surface area (aVSA) were assessed. Ambient temperature (Ta) and RH were used to compute a temperature humidity index (THI) and data were subjected to ANOVA with strain, sex and rearing system as the main effects. Time of day influenced (P < 0.01) free-ranging-related behaviours namely; foraging, drinking and preening. Females spent more time compared to males on the same activity and also appeared, generally, more stressed than males. Physiological responses of PK, OV and NN were generally comparable under similar rearing conditions and none of the factors studied had an effect (P > 0.05) on RT. Sex influenced (P < 0.05) VH, aVSA, VW and gizzard weight. Villi were taller, wider, hence greater aVSA in males than females on WM and PABM while ME thickness decreased (P < 0.01) between 18 and 21 weeks of age. Strain influenced (P < 0.05) VW, aVSA, ME thickness, intestine length, liver, gizzard, pancreas and heart weights. Sex of bird influenced (P < 0.05) carcass weight (CW), heart, proventriculus and abdominal fat pad (AFP) weight. The heart, liver and pancreas weights were significantly higher in OV than PK and NN chickens. Strain influenced (P < 0.05) BW, H/L ratio, spleen, relative liver weights, thigh, neck, pancreas, gizzard and crop weights but not TI (P > 0.05). Sex of bird affected (P < 0.05) BW, spleen, relative liver weights, H/L ratio, shank, drumstick and abdominal fat pad (AFP) and pancreas weight. Strain × sex interactions were observed (P < 0.05) on spleen and liver weights. There was negative correlation between time spent foraging and THI. Higher BW and heavier portions were obtained with OV than with NN and PK chickens. Generally, males yielded heavier portions than females of the same strain. Free-range birds experienced crop and gizzard hypertrophy and pancreas atrophy. Free-range males yield heavier cuts and females were fattier than males. It was concluded that rearing system, strain and sex of bird influence gut morphology, physiological responses, meat and fat yield in free-range slow-growing chickens. While free-ranging could minimise stress in birds, mechanisms should be devised to prevent predation in outdoor rearing of birds. Endo- and ecto-parasite infestation, behavioural studies using more elaborate techniques and evaluation of fatty acid profiles are possible areas of future research to help understand, hence improve bird welfare for slow-growing chickens in outdoor systems.