Cold stress adversely affects the run of the digestive and metabolic processes and other biological functions in chickens.
This may occur to a varying extent depending on factors such as:
In most cases, however, chickens under cold stress conditions often exhibit reduced performance and impaired health due to changes in the internal body systems. The following is a review of these effects and the ways of keeping them at minimum levels.
In one study, broiler chickens were exposed to cold stress using cold air conditioning for 8 hours daily from the 3rd week to the 6th week of age. Average feed intake (g/bird) was 3,407 in cold-stressed birds compared to 3,082 for the control group at normal temperature, with the average body weight remaining constant at 1,340 grams in both groups.
In another study with male chickens obtained from a commercial strain of broiler breeders, cold stress has increased feed intake (4,131 g/bird compared to 3,904 g/bird in the control group) while decreasing body weight at 6 weeks of age (2,260 and 2,300 grams in low and normal temperatures, respectively).
The decreased feed utilisation for growth may be attributed to 1 or more of the following factors:
The cold stress increases the pH of jejunum and caecum content, which favours the multiplication of C. perfringens, the major aetiological agent of necrotic enteritis. Necrotic enteritis is described as a disease with a high economic impact on the health and welfare of broilers. It also leads to haemorrhage and epithelial damage in the intestinal mucosa with resulting villous atrophy which reduces nutrient absorption and growth of chickens. The intestinal damage also results in the release of plasma proteins into the lumen of the intestinal tract. This can provide a necessary growth substrate for the extensive proliferation of Eimeria and coccidial species which, in combination with C. perfringens, can lead to various clinical signs such as mucus-like or bloody diarrhoea, dehydration, anaemia, listlessness, ruffled feathers, and stunted growth, with a mortality percentage of 50% or more.
During cold stress, a substantial amount of energy is diverted from immune functions to thermoregulation. Thus, immunosuppression will ensue to compensate for heat production. Besides, cold stress affects the function of the hypothalamic-pituitary axis, which results in the alteration of serum adrenal and thyroid hormone levels. As these hormones have immune-modulating effects, changes in their levels may affect the immune system either directly or indirectly. When a chicken’s immune system is suppressed, it can’t fight off harmful pathogens or keep parasite level in balance. Cold stress also leads to oxidative damage to different organ tissues when the anti-oxidation is out of balance.
Only a few studies have reported a decreased dressing percentage in cold-stressed chickens (69%) compared to those kept under normal temperature (73%) with darker colour carcasses arising from the lower glycogen concentration in muscles of cold-stressed chickens. In other studies, however, no such effects have been observed under cold stress conditions.