Pigs are warm-blooded animals, and their internal body temperature must stay within a narrow range around 39°C to function properly. Since pigs are unable to sweat, they will use a variety of different means to try in stay cool when the ambient temperature is too high. These include increasing water intake, laying on a cool surface, wallowing in the mud or water, panting, and reducing their feed intake. For farm animals the reduction of feed intake is a highly important issue as it is directly related to production.
The effect of temperature on feed intake is not linear and at higher temperatures, the higher the temperature, the greater the reduction in feed intake will occur with an increase in temperature. According to Quiniou N. from 18-25°C feed intake drops 150g/d/°C, from 25-17°C this reduction is increased to 390g/d/°C, and to 950g/d/°C from 27-29°C. (Quiniou et al., 2000) The reduction in feed intake is primarily related to a decrease in meal size. At higher temperatures the pigs will eat less food per feeding as well as reduce the number of feedings per day; the sows restricting the majority their feed intake to the morning and evening when temperatures are lower. The effects of temperature are compounded by high levels of humidity as this interferes with the sows’ ability to dissipate heat through respiratory evaporation, forcing the sow to rely on more on other means of staying cool. This effect was demonstrated by Gourdine et al. in 2004 showing a decrease in ME intake of 5.7MJ ME/d/°C from 24.5- 27°C at 85% relative humidity. Though not directly related to feed intake due to body reserve mobilization, sows have impaired milk production at ambient temperatures above. This not only has a negative effect on piglet performance, but can also lead to issues with returning to estrus, conception, and overall fertility.
The above mentioned reduction in feed intake has a direct result on the body production of heat. The extra heat associated with feed intake is called the thermic effect of feed (TEF), and originates from the activities involved with eating: digestion of the feed, absorption of nutrients, fermentation, and utilization of the nutrients. The TEF is represented by the difference between Metabolizable energy (ME) and Net energy (NE). NE is the final step of energy utilization, represented as the energy stored in protein deposition (muscle), fat deposition, milk production, etc. The extra heat can be measured in a respiration chamber, where the exchanges between the animal and its environment can be analysed and recorded. In these conditions: The quantification of the heat production (HP) during fasting (FHP) provides an estimate of the maintenance energy requirements, and appears to be proportional to the feed intake for maintenance. Beyond that point, i.e. for feeding to cover the needs for growth, milk production, etc., the Extra Heat varies according to numerous factors:
Because of the variability of the heat production, and the natural adjustment of pigs to feed intake, taking into account heat production, by using NE, gives a better prediction of growth performance and body composition of animals then ME.
The heat increment (percent of energy converted to heat) of fat is approximately 10% of the ME content, while it is 18% for carbohydrates (starch), and 42% for protein (Noblet et al., 1994). In correspondence, the conclusion of most data from INRA, AJIMOMOTO is that less protein (supplemented by free amino acids), and more lipids will decrease total heat production. The pigs will in response eat more, and retain more NE for growth or milk production regardless of being in a thermos neutral or heat-stressed environment (Noblet et al., 1987; Le Bellego et al., 1999).
Dietary fibres are primarily Non Starch Polysaccharides (NSP) resistant to the digestive process (resistant to carbohydrases) and potentially fermentable by the bacteria in the large intestine of monogastric animals (Resistant starch is considered dietary fibre as well). The Volatile fatty acids produced by intestinal fermentations benefit the animal by:
Apart from fermentable properties, dietary fibre physical properties like, particle size, water holding capacity (WHC), viscosity and solubility will influence the digestibility of nutrients, satiety (i.e. behavior and feed intake), and intestinal transit. Having sufficient dietary fibre and a good balance of fibre types is necessary to support both proper digestive processes, and good gut health. The effect of dietary fibre on heat production differs depending on the nature of the source and the properties of the fibre:
During heat stress, lactating sows have a reduced appetite, impacting their feed intake. This has negative consequences on milk yield, body weight loss, and future reproductive performance. Return to estrus and conception rate are partial related to the amount and the nature of body reserve losses, particularly protein loss, during the lactation period. Therefore, the management goal during lactation will be to maximize nutrient intake and minimize deficiencies by increasing the nutrient density of the feed.
Fat has a gross and net energy content (high digestibility >80%), this coupled with the low impact on heat production makes fat the most efficient solution to increase the energy density of sow diets. Limiting starch from cereals, primarily corn, and increasing fat (animal fat or oil), to accomplish this leads to a need for supplementation fiber to ensure proper peristalsis in the gut of the sow. This will also have the effect of promoting beneficial bacterial fermentation in the large intestine, ensuring an extended energy generation.
Crude fibre (de Weende) or ADF (Van Soest) measure the non-fermentable DF components. The less fermentable fraction of the fibre has a negative effect on heat production (42% of ME content) and acts as an energy and amino acid diluent in the diet. So under heat stress and to reinforce the feed concentration, one must consider a concentrated source of DF to (1) keep diet high nutrient density (2) limit possible side effects of iNSP. Increasing the fermentable fraction of NSP will improve the general digestive process, specifically the fermentation in the hind gut, which results in natural acidification and improves the microflora development.
Apart from these feed formulation requirements; it is of high importance to pay attention to sow herd conditions:
Under heat stress, feeding lactating sow with a corn-SBM diet based on high starch and high crude protein levels, will increase the natural TEF. The NE system, which takes into account the heat increment of the diet and differentiates between the digestibility of the different fractions of the vegetal fibre, is more precise than the ME system, and will limit the adverse effect of heat on feed intake and feed efficiency for the lactating sows. Limiting crude protein and formulating to provide a precise ratio of free amino acids (SID), will limit heat production without adverse effect on animal performances. It will also help reduce the nitrogen excretion which is important for odor control and the environment. The use of fat and partly fermentable DF improves the feed density, and improves the global feeding management of sow: better sow condition, feed intake and improved gut health status under hot conditions.
Gilles LANGEOIRE is a French agronomist who dedicated 45 years of his career to feed industry. He works today as a Consultant and swine nutritionist for feed compounders, home mixer farmers, and feed additives companies.
Gilles LANGEOIRE worked as a technical manager for the major nutrition companies in France focusing on the national pig feed market and abroad, mainly South Europe and Asia. His major contributions to animal nutrition concern net energy supply for growing pigs and sows, new fiber approaches in all stages of pig nutrition, specific feeding management of Chinese blood sow breed, environmental issues (protein and phosphorus residues), and the use of different feed additives.
Specialist in pig nutrition Gilles LANGEOIRE was a member of the Scientific Committee of IFIP (French Institute for pig production), and is still in close contact with the main INRA and IFIP Researchers.