Animal stress is a critical factor that directly influences performance and production in poultry and swine farming. It can be caused not only by heat, but also by cold, handling, vaccination practices, disease, and nutritional and environmental variations, significantly affecting welfare and productivity. 

Over the past few decades, genetic advances have led to a remarkable increase in productivity, generating more meat and eggs in less time. However, this progress has brought with it greater stress in production animals, especially those with high metabolic rates and lower resistance.  

With this in mind, we will explore why today’s animals are more susceptible to heat stress and other environmental factors. We will also understand how this affects animal welfare in poultry and swine. In addition, we will address oxidative stress, its causes, and how it negatively impacts productivity. 

The role of genetics in modern animal production 

Over the past three decades, poultry and swine production has evolved rapidly. Genetic improvement has focused on high-efficiency traits: 

• Daily weight gain;  

• Feed conversion (both in meat and eggs);  

• Number of eggs produced per unit of time.  

These achievements come from more intense metabolisms, raising the metabolic rate of animals compared to previous generations. However, this intensification also increases susceptibility to heat stress, nutritional deficiencies, and variations in management and environment. 

Another important aspect is decreased resistance. High-efficiency animals are more sensitive, requiring very precise feeding, environmental, and management conditions to express their full potential. Any imbalance can trigger stress, thus affecting animal welfare in pigs and poultry. 

Key concepts: antioxidants and animal stress 

To understand how animal stress affects productivity, it is essential to review some concepts: 

• Antioxidant 

Any natural or synthetic substance capable of neutralizing and protecting a biological system against free radicals (oxygen, nitrogen, lipids, etc.).  

• Animal stress  

Broom and Johnson (2000) define stress as an environmental effect that overloads control systems and reduces performance. In other words, any stimulus (heat, cold, handling, vaccination) that disrupts homeostasis can be considered a stressor.  

• Free radicals   

These are atoms, molecules, or ions with one or more unpaired electrons in their outermost layer. In seeking stability, they “steal” electrons from other molecules, generating chain reactions that damage lipids, proteins, and nucleic acids.  

• Oxidative stress  

When the generation of free radicals exceeds the capacity of the body’s antioxidant systems, causing functional changes and cellular damage. In high-yield poultry and swine, oxidative stress becomes relevant because metabolic demands are higher and there is an increase in the production of free radicals.

Factors that increase oxidative stress in animal production  

• Increased production of free radicals due to animal stress  

Whatever the cause of stress (heat, cold, handling, disease), it has been shown that animals in stressful conditions generate more free radicals (Jahanian; Mirfendereski, 2015).  

• Higher metabolic rate  

Modern high-efficiency genetic lines have a very intense metabolism. This high metabolic activity favors the formation of reactive oxygen species.  

• Limited antioxidant systems  

Genetic improvement has not been directed toward increasing the endogenous capacity to produce antioxidant enzymes. In addition, these systems depend on cofactors (riboflavin, cysteine, manganese, selenium, etc.) that can become limiting, especially in situations of high demand or nutritional imbalance. 
 
Young animals 

Animals in their early stages, with very fast metabolisms, have not yet fully developed their antioxidant systems. 

Adult animals 

With advancing age, the effectiveness of enzymatic antioxidant systems tends to decrease, making individuals more vulnerable to oxidative stress. 

Presentation on farms 

Oxidative stress usually manifests itself mainly in: 

• Broiler chickens (rapid growth); 

• Pigs in the growth and finishing stages; 

• Laying hens at an advanced age (from 40 weeks); 

• Sows from the fourth farrowing onwards; 

• Light and heavy breeding birds.  

In high-efficiency genetic lines subjected to technified breeding, exposure to multiple stressors (handling, heat, cold, vaccinations, diseases) is common. This puts animals in a state of constant stress, increasing the production of free radicals and leading to oxidative stress.  

When combined with an intensive production environment – which includes temperature variations, higher nutritional requirements, and constant handling – this creates the ideal scenario for oxidative stress to impair the health and performance of animals.  

In this first part, we have seen how genetics, accelerated metabolism, and exposure to environmental factors contribute to triggering oxidative stress on technified farms.  

Don’t miss our next article, as we will delve deeper into the specific consequences that arise from this imbalance, as well as strategies and practical measures to mitigate its negative effects on animal production.