Most modern highly productive poultry strains have been developed in temperate
countries, with little opportunity for heat tolerance to be a selection factor. When these
birds are moved to tropical or subtropical countries their egg production, egg quality, food
intake and growth rate decrease at high ambient temperature. Varying reports on the
effects of heat stress on egg and yolk production may be attributable to the differences of
the range and duration of thermal loads employed, different bird strains, ages, diets and
food intake responses. There are, however, only a few studies which attempt to explain
the actual physiological mechanisms involved in the adaptation of laying hens to thermal
loads, especially those relating to the changes in egg yolk precursors including vitellogenin
and very low density lipoprotein (VLDL) synthesis, secretion and deposition in the
oocytes.
The major yolk precursors are vitellogenin, a metalobinding protein, and VLDL, a
specialised type of triglyceride-rich lipoprotein found in abundance in the plasma of laying
hens. Both vitellogenin and VLDL are synthesised in the liver in response to oestrogen
stimulation, transported to the ovary and transferred and deposited into the growing
oocytes by a selective mechanism. Chronic heat stress in laying hens reduces yolk size and
total yolk production considerably. Any changes in yolk precursor synthesis mediating
such a
response may involve altered oestradiol secretion or reduced sensitivity of the
hepatocytes to oestrogen stimulation. Other possible mechanisms of reduced yolk
accumulation include disturbances in the transport of vitellogenin in the blood stream and
changes in the level of ovarian uptake of egg yolk precursors by the growing oocytes.
The present project examined the changes in egg yolk precursor levels in the
plasma of laying hens chronically exposed to different degrees of thermal stress (32 and
35°C with various relative humidities) and precursor uptake by the oocytes. The study
also addressed the role of oestrogen in yolk precursor responses to chronic heat stress. In
addition, the role of nutritional vitamin E supplementation as a strategy for alleviating the
effects of heat stress has been examined. The studies have confirmed that heat stress
reduces yolk and egg production concomitant with reduced circulating vitellogenin and
VLDL in the plasma. The decreased plasma concentrations of vitellogenin and VLDL
resulting from exposure to high ambient temperatures are not simply a consequence of
reduced food intake. Studies applying some degree ofrestriction on feeding the birds (85,
70 and 55%) compared to the control group {ad libitum; 100%) and a paired-feeding
study, where a control group was fed with the same amount as the heat stressed group
consumed were also undertaken.
The present project examined the changes in egg yolk precursor levels in the
plasma of laying hens chronically exposed to different degrees of thermal stress (32 and
35°C with various relative humidities) and precursor uptake by the oocytes. The study
also addressed the role of oestrogen in yolk precursor responses to chronic heat stress. In
addition, the role of nutritional vitamin E supplementation as a strategy for alleviating the
effects of heat stress has been examined. The studies have confirmed that heat stress
reduces yolk and egg production concomitant with reduced circulating vitellogenin and
VLDL in the plasma. The decreased plasma concentrations of vitellogenin and VLDL
resulting from exposure to high ambient temperatures are not simply a consequence of
reduced food intake. Studies applying some degree ofrestriction on feeding the birds (85,
70 and 55%) compared to the control group {ad libitum; 100%) and a paired-feeding
study, where a control group was fed with the same amount as the heat stressed group
consumed were also undertaken.
It was also demonstrated by oestrogen administration in vivo under different heat
loads (30°C and 80% relative humidity [RH]; 35°C and 55% RH), that oestrogen increases
the availability of both vitellogenin and VLDL in the circulation of heat stressed and
control (thermoneutral) hens although their responses were not identical quantitatively.
There was a doubling of the concentration of VLDL in response to oestrogen treatment
during the second week of heat stress. The results of this exogenous oestrogen treatment
suggest that the liver is capable of synthesising vitellogenin and VLDL and this also means
that hepatocyte sensitivity is not altered during heat stress.
Other experiments in this study were also carried out to determine whether the
mechanism mediating reduced egg production in heat-stressed hens involved a change in
the rate of uptake of yolk precursors by the oocytes. This led to the establishment and
validation of a new technique which allows characterisation of oocyte yolk precursor
uptake in vivo. The labelling of natural vitellogenin present in high concentrations in fresh
laying hen plasma by incubation with 65Zn proved extremely effective. The findings
indicate that the novel technique is appropriate for the measurement of oocyte vitellogenin
uptake in vivo and therefore for the elucidation of the mechanisms controlling oocyte
development and yolk accumulation in response to environmental challenges.
Further studies have been carried out in an attempt to alleviate the effects of heat
stress in laying hens using supplementation of the diet with a-tocopherol (vitamin E), a
biological antioxidant. Supplementation of the diet with 500mg/kg vitamin E increased
circulating egg yolk precursor concentrations during exposure to high heat loads and this
may support the observed improvement in yolk and egg production. It is suggested that
vitamin E supplementation represents a possible strategy for reducing the effects of heat
stress. The study provides important evidence of a possible physiological basis for the
changes in yolk and egg production induced by chronic heat stress in laying hens.
