Heat stress in laying hens
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The combination of high ambient temperatures and high humidity is a well-known external stressor in livestock production. Layers are particularly sensitive to heat stress, because of daily egg and eggshell production.
Consequence of heat stress for layers
During heat stress, laying hens start panting (increased and deepened respiration rates) to increase evaporative heat loss. Panting leads to decreased blood carbon dioxide (CO2) levels and increased blood pH, which hampers blood bicarbonate (HCO3-) and calcium (Ca) availability for eggshell mineralization (Figure 1). On top, blood flow is redirected from internal organs towards the skin, reducing nutrients supply to the reproductive organs.
Ca availability
A laying hen deposits 2.2 g of Ca per egg: 2/3 of which is directly supplied via feed, and 1/3 by mobilization of Ca from medullary bone. Any disruption in Ca intake, absorption, transport, and/or mobilization can be detrimental to eggshell quality, whereas during heat stress, all are affected.
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Reduced feed intake leads to reduced Ca intake
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Panting leads to an excessive loss of CO2 and increasing blood pH (respiratory alkalosis)
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To control blood pH, HCO3- is excreted via the kidneys whereas the remaining H+ is used to lower blood pH
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The increased blood pH lowers the concentration of ionized Ca, which in combination with a reduced HCO3- concentration, lowers the availability of these crucial molecules to the shell gland for the formation of CaCO 3
Activation of vitamin D3
Vitamin D3 is key in Ca metabolism. It is either produced in the skin or supplemented in the diet. In the liver, vitamin D3 is converted to 25-hydroxyvitamin D3 (25(OH)D3), which must be converted in the kidney to 1,25-dihydroxycholecalciferol (1,25(OH)2D3), the metabolic active form of vitamin D3.
Role of 1,25(OH)2D3
Among other functions, 1,25(OH)2D3 increases the expression of calbindin, an intracellular Ca-binding protein, that enables active Ca transport through the cell (Figure 2). During heat stress the density of calbindin in the gut is reduced. This further reduces the amount of available Ca for the hen, as not only Ca intake (feed intake) is reduced, but also Ca transport across the intestine.
Furthermore, heat stress reduces the production of estrogen. Estrogen is one of the factors influencing the production of 1α-hydroxylase, the enzyme that converts 25(OH)D3 into 1,25(OH)2D3 in the kidney.
Therefore, a reduction of estrogen leads to a reduction of 1,25(OH)2D3 in the body
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Panbonis® (1,25(OH)2D3 – glycosides) is the natural source of the bioactive form of vitamin D3 from dried and ground Solanum glaucophyllum leaves.
Panbonis® helps the laying hen to better cope with the negative effects of heat stress on Ca-metabolism, increasing the Ca availability for eggshell formation.
