Thermoregulation at high ambient temperature in genetically fat and lean broiler hens fedad libitumor on a controlled‐feeding regime

Abstract

1. Hens of lines divergently selected for fatness and leanness, fed either ad libitum or on a controlled regimen, were compared for susceptibility to heat stress. 2. The rate of increase in deep-body (rectal) temperature during exposure to 32°C was used as the index of thermoregulatory ability. Comb and foot surface temperatures were measured as indicators of peripheral vasomotor tone. 3. Because body temperature control depends on the balance between heat production and heat loss, heat production was measured to determine whether fat line hens had a higher heat production, which they would then have to dissipate. 4. During the first hour of heat exposure, rectal temperature in the ad libitum-fed birds increased twice as rapidly as in the corresponding lean line sample and 6 times more rapidly than in the control-fed fat-line group. 5. Surface temperatures of comb and foot increased from 27°C to 37°C within the first hour at 32°C, with no effect of either genotype or feeding regimen on rate of increase. 6. Heat production was unrelated to genotype but was reduced by 23% by controlled feeding, largely because of the reduction in body weight. 7. The results demonstrated that ad libitum-fed fat-line birds are susceptible to heat stress and that this is related not to increased heat production, but to a decreased ability to lose heat. Elevation of blood viscosity by plasma triglycerides is suggested as a causal factor.

Welfare assessment of broiler breeders that are food restricted after peak rate of lay

“…Welfare during the breeding period of ad libitum and restricted birds was previously assessed, on the basis of a limited number of indices, by MacLeod and Hocking (1993), Hocking et al (1994) and Zuidhof et al (1995). Compared with restricted birds, ad libitum-fed broiler breeders had high rates of mortality throughout their life in all studies.…”

“…Zuidhof et al (1995) reported that birds fed a ration that was diluted with 15% of oat hulls had improved welfare as judged by changes in behaviour and heterophil-lymphocyte ratios. The ability of adult broiler breeding birds that were fed ad libitum to lose heat was shown to be greatly diminished (MacLeod and Hocking, 1993;Hocking et al, 1994) and is associated with higher mortality in ad libitum compared with food-restricted birds (Katanbaf et al, 1989;Hocking et al, 1992;Hocking and Bernard, 1997). Whilst high rates of mortality in birds fed generously during rearing are assumed to reflect poor welfare, to our knowledge, the welfare of broiler breeders that were food restricted during lay has not been systematically determined.…”

Thermoregulatory capacity and muscle membrane integrity are compromised in broilers compared with layers at the same age or body weight

“…A reduced resistance to heat stress in broiler birds may be attributable to a decreased ability to lose heat (MacLeod and Hocking, 1993) or an inappropriately increased heat production during exposure to high thermal loads (Sandercock et al, 1995). Comparative studies that characterise the responses to thermal challenge in broiler chickens and in lines not selected for enhanced growth and feed conversion efficiency at the same age may be confounded by disparities in body size and geometry (Monteith, 1973).…”

Effects of genetic group × ambient temperature interactions on performance and physiological responses of Naked-neck chickens and their F₁ crosses with Lohmann White and New Hampshire laying hens

“…A decreased heat-tolerance observed in high performing hens may be attributable to a reduced ability to lose heat (MacLeod and Hocking, 1993) or an inappropriately increased heat production during exposure to high thermal loads (Sandercock et al, 1995). In agreement with the findings of Davis et al (2000), the levels of circulating T 3 in the current study varied with respect to the age and egg production cycle of the hens.…”