Signs of heat stress in finishing pigs results from wide night and day temperature fluctuations, which can cause stress, lower performance and trigger health problems. 

Problems tend to occur in houses when pigs are nearing their end weights, and this is exacerbated in buildings run on an all-in, all-out system and can result in a high average live weight for the house. "The heat output from large numbers of pigs nearing 100 kg is enormous," says Tim Miller, environmental specialist with ARM Buildings.

Badly insulated older houses are more vulnerable, especially if they are poorly ventilated, but siting has an effect, too, especially with ACNV buildings which are sheltered from the wind.

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"We've recorded temperatures as high as 33.5 ⁰C and as low as 20⁰C inside houses within a 12-hour period but in autumn fluctuations can be even more severe," he said. "You can have 30⁰C at 4.00 pm and 16⁰C at 9.00 pm - a 14 degree drop in just five hours. There is evidence to show that big swings in temperature have a greater impact on pig performance than high temperature alone."

Keeping pigs in their thermo-neutral zone between their upper and lower critical temperature is the aim and ventilation is the key, says Miller. However, in the absence of air-conditioning it can't lower the temperature below the prevailing ambient temperature. 

"We are trying to avoid a lift of more than three degrees over ambient," he points out. "Some continental suppliers install systems with half the capacity of those in the UK, leading to temperature lifts as high as 6 degrees, but we try to limit the temperature lift." In cooperation with control specialists, Farmex, ARM Buildings is developing a fan-control program which will moderate the rate of temperature drop in piggeries during the evening, making conditions more comfortable for the pigs.