High temperature in growing pigs: how heat cuts growth and feed efficiency

Why the question mattered to Australian pig producers#

Farrell opens by noting that pig nutrition research in Australia had been limited by a lack of financial support, and argues that many existing studies were of restricted value because ambient temperature swings so widely and has such a strong influence on performance. He makes the case that very high temperatures deserved far more research attention than they had received, pointing out that pig sheds are often modified with insulation, exhaust fans or evaporative coolers, but that such installations can be costly and may not always be economical.

The paper was presented as part of the Recent Advances in Animal Nutrition in Australia series, within the wider record kept by the RAAN Conference Proceedings and the broader Recent Advances in Animal Nutrition community. Farrell stresses that despite the practical importance of heat stress to pig producers, very little research had been carried out anywhere in the world on this question, and that the few published studies often disagreed with one another.

How pigs cope physiologically with heat#

The pig is described as poorly suited to warm conditions. Its moisture loss through the skin is lower than for any other domestic species measured, chiefly because it lacks sweat glands, and in humid, hot conditions the skin can even gain water rather than lose it. This is why pigs given access to the outdoors wallow in mud or water, to shed body heat. At the highest temperatures described in the paper, pigs depend heavily on breathing out moisture as vapour to cool themselves, a process that consumes 2.5 kJ for every gram of water converted, and because pigs can only change their breathing pattern to a limited degree compared with sheep, this cooling route is itself restricted.

Pigs kept for long periods in hot conditions develop anatomical adaptations. Fuller observed that pigs raised in a hot environment grew less hair, larger ears and longer limbs than pigs raised in the cold, changes that appear to help with sensible heat loss. Subcutaneous back-fat matters here too, since it slows the transfer of heat from the body core outward; as pigs age and fatten, they become progressively less heat tolerant. The paper also reports a theoretical thermoneutral zone, the band of temperature within which pigs need not alter heat production, calculated by Holmes and Close for pigs of different liveweights fed at maintenance, twice maintenance and three times maintenance, underlining that both body size and feeding level shift the point at which heat becomes a problem.

Growth, feed intake and the upper critical temperature#

Upper critical temperature, the point above which pigs begin showing clear strain through changes in body heat balance, is shown to depend on the plane of nutrition, the age or weight of pigs, and how many pigs share a pen. Data summarised from Verstegen and colleagues, using 15OC as a standard reference temperature, indicated that feed intake and feed conversion ratio reach their best values only above 20OC, while in a draughty, uninsulated shed with wet floors the critical temperature could run as high as 29OC.

Across the studies reviewed, feed intake tended to fall as shed temperature rose above 15OC, while rate of gain and feed conversion often improved, except in heavier pigs where this pattern reversed at one of the higher temperatures tested. Much of this picture rests on New Zealand work by Holmes, conducted up to experimental temperatures of 32 to 34OC. A recurring complication is that feeding scales suited to pigs in warm sheds can lead to feed being left uneaten once pigs exceed about 50 kg liveweight in hot conditions.

Feeding level interacts strongly with temperature. Tonks and colleagues found that lighter pigs fed on a restricted basis performed similarly regardless of house temperature, and Holmes observed a comparable pattern in pigs between 31 and 78 kg: at 33OC, growth rate fell by 14% under a high feeding level but rose by 24% under a lower feeding level. Feed conversion followed a similar pattern, though it barely differed between temperatures when pigs were fed ad libitum. The effect of heat on performance, in other words, depends heavily on how much pigs are fed, and becomes more pronounced as pigs near slaughter weight.

Can dietary protein offset the effect of high temperature in growing pigs?#

Very few experiments had tested whether adjusting diet composition could blunt the impact of heat. Seymour and colleagues fed pigs ad libitum diets containing 20% or 16% crude protein at 16OC and 32OC. Growth responded to the extra protein at both temperatures, but there was no sign that the higher-protein diet gave any extra advantage specifically at the high temperature. Feed conversion, however, improved more on the high-protein diet at the high temperature than at the low temperature, when compared over the 7 to 57 kg range.

Farrell is openly sceptical of this experiment, noting that the 16% crude protein diet was probably too low to let pigs perform at their best, so growth rate and feed conversion should not have differed between the two diets at 16OC if both had supplied adequate nutrients. He concludes that the question of dietary protein concentration under heat stress remains effectively untested and needs to be revisited with better-designed trials.

Energy and nitrogen metabolism under heat stress#

The review reports that digestible energy of pig diets tends to rise with ambient temperature, with Fuller and Boyne recording a 0.12% increase per degree Celsius of temperature rise between 5OC and 23OC for the same diet, although Holmes's own data ran counter to this in places. On maintenance energy requirement, Close and Mount found that for growing pigs in a stated weight range, the requirement at 20OC was 5% higher than at 25OC, which was itself 14.5% lower than the requirement at 30OC, confirming that heat raises the energy cost of simply maintaining the animal.

Net efficiency of using metabolisable energy, normally reported in a range the paper describes as acceptable, was examined by Close and Mount between 20 and 25OC and judged reasonable by Farrell, while he regards the Holmes figures for this measure as less reliable. Close and Mount also calculated partial efficiencies of forming protein and fat, finding these declined between the 20 to 25OC range and 30OC, showing that heat makes both maintenance and the productive use of energy less efficient. Nitrogen retention, which depends on diet quantity and quality as well as feeding level and liveweight, was found to decline in hot conditions compared with a thermoneutral environment of 20 to 25OC, with feeding level having a more marked effect once temperatures turned hot.

Effects on carcass quality and other nutrient needs#

High temperature generally increased carcass length compared with cooler conditions, though its effect on fatness was inconsistent across studies, ranging from increased fatness, to no clear effect, to a leaner carcass, differences the paper attributes partly to feeding level, sex and age at slaughter. Straub and colleagues, working with pigs between 70 and 110 kg fed ad libitum, found that high temperature significantly reduced back-fat thickness and eye-muscle area, a pattern that might point to an inferior carcass, although Farrell cautions that this is hard to judge objectively.

Holmes reported that at a high feeding level, more of the retained energy went toward fat relative to protein in hot conditions than in milder warm conditions, across two separate experiments, meaning relatively more fat was being laid down in the hotter environment; at lower feeding levels the temperature-related differences were less clear. On other nutrients, water intake relative to feed dry matter normally runs at two parts water to one part feed in thermoneutral conditions, rising to a ratio of 5 to 1 at 34OC, partly inflated by pigs spilling water deliberately to cool themselves. Thiamin requirement was suggested, though not convincingly shown, to increase in heat, while potassium and calcium excretion in urine rose in heat-exposed pigs, a change that may have been linked to lameness often seen in these animals.

What the evidence does and does not support#

Farrell is candid that the review ends without being able to specify exactly how temperature affects many aspects of pig performance, because so few studies exist and because those available often used different ages, weights, feeding levels and temperature ranges, making direct comparison difficult. He identifies specific gaps: no published work had tested whether lifting dietary energy concentration, for instance through added dietary fat, might reduce the heat generated by digestion and so help pigs cope with hot conditions, and the roles of feeding frequency, daily temperature fluctuation and the timing of feeding relative to temperature changes remained unexplored.

Given this patchwork of studies using different pig ages, housing and feeding regimes, we think the review is best read as a clear statement of what remained unknown in 1978 rather than as a settled set of recommendations; it is a scoping of the problem more than a solution to it. What does come through consistently is that older, heavier pigs tolerate heat less well, and that pigs eating more at any given age are themselves less heat tolerant. Maintenance energy requirement rises and the efficiency of using metabolisable energy for gain falls as temperature climbs into the hot range, a combination that squarely poses a nutritional and management challenge for producers working in Australia's variable climate. Readers may also find a separate catalogue record of interest, titled getting the most out of the feed given to livestock, by the same authors, alongside another titled sourcing livestock feed ingredients by 2007.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Meat & Livestock Australia: red meat industry research and marketing body
• Feedipedia animal feed database: an open-access database of animal feeds
• Australian Meat Processor Corporation: research for red meat processors

Questions#

— The Livestock Library team