Heat stress is known to lead to poor performance and mortality. However, the reasons for that are varied and not so clear-cut. Acid-base imbalance may play a significant role here. Taking care of it can thus help us to deal with the problem, besides being important on its own.
By Dr Rogério G. T. da Cunha, Sao Paulo, Brazil
Keeping birds in a thermal comfort state is now widely known to have a significant impact on performance. Less acknowledged, though, is that an adequate internal pH is equally important for productivity. Even less known is that the two may have a significant link. Interestingly, the management of electrolytes in the diet can help us to "kill two birds with one stone". Dr Sebastião Aparecido Borges, professor at Universidade Tuiuti do Paraná, and André Favero (Master’s student at UFPR – Federal University of Paraná, Brazil) dissected these issues in a paper they presented at a poultry event.
Acids, bases and salts
Borges explains that the body attempts to maintain its pH, or acid-base balance, through the control of the ions H+ and HCO3-. These are produced by the dissociation of carbonic acid (H2CO3), which, on its turn, is formed by the combination of carbon dioxide (CO2) and water (H2O). The body also needs to keep electrolytes at appropriate and reasonably constant levels, the so-called electrolytic balance. For that end, he reminds that the sum of positively charged ions, like sodium and potassium, needs to be the same as the sum of the negative ones, such as chloride. Besides, each one of the ions has a narrow range of optimal values for its bodily concentration.
However, the two aspects are closely interconnected. "An acid-base imbalance makes the control of electrolyte levels by the body more difficult, and can lead to an imbalance of them as well, and the other way round," says Borges, adding that heat stress plays a role as it leads to an acid-base imbalance. "In very simple terms the panting, which is an attempt of the bird to cool itself, leads to the loss of water and carbon dioxide. This takes H2CO3 from the body, which then removes H+, thus increasing the pH, a process called respiratory alkalosis."
On a cascade of consequences, there is an increase in water consumption, a decrease in feed intake, prostration, and an impact on the electrolytic balance, with some ions being lost in the urine.The electrolytic and acid-base imbalancesmake internal conditions for the entire physiological machinery of the body far from ideal. All of this has an obvious impact on zootechnical indexes and may even lead to death.
Spicing them up
Given the relationship between electrolytic and acid-base balance, scientists began to consider ways to play around with them. Even more interesting, they began to wonder if they could not get round heat stress concerns. According to Borges, the main idea is to correct acid-base imbalance through the administration of salts, which break up in electrolytes. "This way we fight the loss of some ions, and also stimulate water consumption. This reduces the temperature of the bird, which in turn decreases panting, thus stabilising the pH. Both electrolytic and acid-base balances are maintained," Borges explains.
Nonetheless, things are not as simple as merely throwing some salts in the feed. Borges recalled that it was found that giving too much salt or giving them at wrong concentrations has adverse effects. The concept of interest here is the electrolytic make-up of the feed. In theory, we should take into consideration all the ions for its calculation. However, working with the most important ones in terms of bodily concentration (sodium, potassium and chloride) is enough for our purposes. In that case, a simple formula (mEqNa + mEqK – mEqCl) describes the electrolytic make-up of the feed. He explains that if such relation is too high, the birds end up drinking too much water. This leads to excreta that are more fluid and a wet bed. "It also causes alkalosis, while a very low relation leads to metabolic acidosis, which is a decrease in bodily pH. Furthermore, too much or too little of the component salts also causes problems," Borges details. In addition, the recommendations vary with the birds’ age and breeding purpose. Reviewing dozens of papers on the issue, and a few of his own team, he arrives at the recommendations in Table 1.
Cautions and precautions
One also needs to pay attention to individual ions. Borges recommends avoiding extreme levels of Na+ (below0.15% and above 0.45% in the formulation of the feed) and Cl- (above 0.70%). Among the most commonly used salts we have potassium chloride (KCl), sodium chloride (NaCl), sodium bicarbonate (NaHCO3), and potassium carbonate (K2CO3).
An alternative to the addition of salts to the feed is to provide them through the drinking water. Borges considers it a simple and effective way to give electrolytes to the birds, especially in heat stress situations. "It is also an interesting strategy to consider for stimulating water consumption in the pre-slaughtering fast. This decreases heat stress and dehydration, and their concurrent losses, also improving meat quality. One should not forget, though, that the total amount of electrolytes given to the birds must respect the limits above," he completes.
Despite being a strong advocate of the use of electrolytes, given that they do increase viability, improve zootechnical indexes, and fight the adverse effects of heat stress, Borges adds some words of caution. "Before considering the use of salts, it is of utmost importance that one knows the exact quantity that is already present in the other feed components, not to run the risk of going over the limits. We should pay close attention to our own results and tune or review our actions accordingly. Besides, this field is still under development. Topics that are yet to be more fully worked include: incorporating more ions to the recommendations; understanding the interplay between them; developing finer considerations about individual concentrations, and the form of administration through different salts. Thus, recommendations may change or new precautions may arise," Borges concludes.