Maintaining electrolyte and water balance to alleviate heat stress in broiler chickens
The acid-base balance is further disrupted by increased electrolyte excretion through urine and faeces.
Electrolyte supplementation to the diet has been shown to restore the acid-base balance and to improve birds' performance. Heat stressed broilers lose more water through urine and panting which decrease the heat dissipation capacity by evaporation and increase the osmotic stress on body cells.
Organic osmolytes such as betaine have been shown to protect the cells from high osmotic pressure and to control intracellular water, suggesting their use in maintaining water balance under heat stress conditions.
Electrolytes provided in diets are of great importance in maintaining acid-base balance, osmotic pressure and electrical potential of cell membranes; and are also essential for intracellular-extracellular homeostasis (Borges et al., 2003).
Among these electrolytes, the monovalent ions (sodium, potassium, and chlorine) are the key minerals involved in acid-base balance of the body fluids (Mongin, 1981), because they have higher permeability and greater absorption than divalent ions (calcium and magnesium) (Borges et al., 2004).
To maintain homeostasis, water intake plus that formed by oxidative metabolism should equal water lost by evaporation and through urine and faeces. However, birds exposed to high ambient temperatures lose more water in urine (>60 percent) than those maintained in the thermoneutral zone (Belay and Teeter, 1993).
Plasma expansion under high temperatures facilitates heat dissipation by peripheral blood and evaporation. However, this expansion leads to a lowering of arginine vasopressin concentrations in blood resulting in a rise in urine flow. Although birds consume more water to overcome these consequences, water retention is reduced due to increased electrolyte excretion (Belay et al., 1992) and continuous loss of water through panting.
Cells must accumulate ions and osmolytes to maintain intracellular water against the extracellular osmotic gradient. The osmotic pressure of intestinal fluid is hypertonic to plasma (650 mOsm in the jejunum versus 300 mOsm in the plasma; Mongin, 1976).
To withstand this osmotic pressure, intestinal epithelia controls water and ion transport via ion pumps and water channels (Rao, 2004). However, these ion pumps use adenosine triphosphate as an energy source to operate (Moeckel et al., 2002), which means energy would be diverted from growth to regulate osmotic pressure.
Under hyper tonicity conditions, cells respond quickly by accumulating inorganic ions to prevent water flux. However, these ions can perturb the structure of macromolecules such as proteins and enzymes. Therefore, cells increase synthesis of either organic osmolytes (such as betaine, sorbitol, inositol) or transporters for these osmolytes (betaine aminobutyric acid transporter) and replace the inorganic ions with the organic osmolytes (Alfieri et al., 2002).
Few studies have been conducted to investigate the effect of betaine supplementation on maintaining water balance during heat stress challenge, particularly to examine the effect of betaine in combination with electrolytes in diets and/or water. Therefore, the authors of this research paper hypothesise that betaine along with electrolyte supplementation could assist broilers during heat stress.
Article made possible through the contribution of Australian Poultry Science Symposium (APSS) 2007.