Malvern, United Kingdom - Heat stress is predictable. So why does it keep catching producers off guard? Hannah Elliott, monogastric technical manager at Lallemand Animal Nutrition, explains why viewing heat stress as a seasonal problem is increasingly out of sync with modern production reality.
Rising global temperatures and higher metabolic heat output in modern genetics mean heat stress is showing up more often, more intensely, and in more stages of production than before. It is a multidimensional issue that demands a systemic resilience strategy.
The thermal conflict in the farrowing room
One of the most persistent challenges to heat management is structural. In farrowing rooms, producers must balance two opposing thermal needs: the warmth required by newborn piglets and the cool comfort required by the sow.
"This unavoidable contrast often places the sow under significant heat stress even in well-managed facilities," Hannah says.
When this balance tips, the consequences are rarely isolated. Heat stress can undermine reproduction, gut integrity, feed efficiency and animal welfare. It is a systemic strain that triggers inflammation and oxidative stress, leading to a drop in performance which effects the entire production cycle.
Where the performance losses begin
Lactation is often the stage where heat stress hits the hardest and where losses can remain silent until later.
"The most damaging early, silent losses include feed intake reduction and immediate drop in milk output, sow body reserve mobilisation, and litter growth slowdown, which is often only seen at weaning," Hannah comments.
When sows reduce feed intake during lactation, they fall into a negative energy balance that weakens subsequent fertility outcomes. This is why heat stress in lactation is not a single-cycle issue; it shapes the current litter and compromises the next reproductive cycle.
Figure 1. Heat stress affects the productive performance of the current cycle and the next one
The 'silent' delayed costs
In breeding herds, heat stress damage isn't always immediate and there is often a lag between the event and the economic impact. Heat stress disrupts ovarian function and hormonal balance, yet the consequences often come 60 to 120 days later.
"These effects often manifest as drops in piglet supply, empty farrowing spaces and irregular production flow". Hannah states that feed intake reduction may be the most overlooked economic loss because it silently drives poorer growth and weaker lactation. "These hidden inefficiencies accumulate across the herd and are easily underestimated because they show up months after the heat has subsided."
To catch these losses early, "silent" indicators to monitor before farrowing rates decline are: longer weaning-to-oestrus intervals, irregular oestrus expression and reduced semen quality in boars.
The gut: The new lens for heat stress
While many simplify heat stress to "pigs eating less and irregularly," the modern understanding focuses on gut integrity. When a pig is heat-stressed, blood flow is redirected from internal organs toward the skin to facilitate cooling. This leaves the gut lining oxygen-deprived and compromised.
This triggers a cascade: the gut barrier weakens (commonly known as "leaky gut"), leading to oxidative stress, villus damage and endotoxin leakage. "This makes the gut a major driver of performance losses, health challenges and inflammation, even beyond the effects on feed intake," Hannah says. On the farm, this impairment shows up as poorer nutrient digestibility and inconsistent growth.
Moving from temperature to heat load
To manage this risk, we need a shift in how we measure the environment. Temperature alone is an insufficient metric. Humidity prevents evaporative cooling, meaning the physiological threshold for stress often arrives much earlier than you might expect.
Temperature-humidity index (THI) and stage-specific thresholds are far more reliable than temperature alone. High-performing lactating sows and large finishing pigs have massive body heat loads and limited cooling capacity, meaning they may tip into stress at temperatures that seem safe on a standard thermometer.
Building a resilience strategy
So, what does a resilience-focused approach look like in practice? Saornil prioritises three pillars:
The bottom line
Heat stress is a predictable risk to reproduction, lactation and gut health. The most successful farms are moving away from reactive cooling, towards a proactive system. By reducing heat load, protecting intake during high-risk stages and tracking early indicators, producers can address small shifts before they turn into expensive, long-term setbacks.
Resilience isn't about surviving the summer; it's about maintaining stability across the entire production flow, every month of the year.
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