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Environmental factors
Ambient temperature
An ambient temperature above that of the thermoneutral environment exercises a negative influence on the appetite, theirs being a non-linear relationship. The level of reduction in ingestion can be more or less intense depending on other circumstances like the time of day or the feed presentation, aspects that will determine intake behaviours.
This influence is more pronounced as the weight of the pig goes up. Globally, feed consumption decreases 1g for every degree the temperature exceeds the level of comfort and for each kilogram of live weight.
However, pigs have a great ability to adapt to the heat, in the sense that after an initial period of thermal stress, they are capable of adapting and increasing their feed intake.
On the other hand, when the pigs are subjected to lower temperatures, we note an increase in feed intake, together with a worsening in conversion.
Humidity
High relative humidity has a much more negative effect on consumption and conversion in high temperatures than in low ones, due to the difficulties in thermo regulation.
Air speed
A low level of ventilation leads to an accumulation of toxic gasses (CO2, SH4, CH4 o NH3) and dust, reducing feed intake. On the other hand, high ventilation levels increase consumption in thermal stress conditions.
Feed consumption decreases in a quadratic manner while the ambient temperature rises.
Physical factors
Animal density
When the pig is faced with temperatures that are higher than the thermoneutral environment, an increase in area per animal causes an increase in consumption and an improvement in conversion, in that as density decreases, the velocity of ingestion increases (g/min).
Troughs: number and lineal spacing.
As the number of troughs increases, so does the feed intake of pigs that are housed in large groups. Therefore, the reduction of lineal trough space per pig diminishes consumption, though there is no noted interaction between the trough space and the size of the group.
Social factors
Group size
The size of the group doesn’t have a very consistent affect, there being contradictions among authors, though it is capable of altering ingestion behaviour, influencing consumption. Also, it is noted that conversion worsens as the group size increases.
Batch mix
Regrouping pigs reduces consumption and worsens conversion, although this negative effect is diluted in time, since the batch mix is a transitory stress factor.
Physiological factors
Health state
During infectious processes, anabolic hormones are found to be inhibited, exhibiting a reduction in consumption and conversion in pigs with highly active immune systems.
Age
The growth curve doesn’t only influence nutritional needs, but also, the capacity of the animal to ingest, digest, and metabolize the nutrients. As the pig grows, the feed consumption increases and conversion worsens.
Genetics
When genetic selection is done stressing obtaining lean carcasses and improving conversion, consumption reduces. In effect, pigs with an improved genotype, consequence of selection against the fat of the carcass, have less appetite than those pigs without it. This is due, in part, to a decrease in intestinal capacity and to the variation of hormone levels related with satiety, like CCK-8.
Gender
The females present a better level of consumption than the males, although these males consume 13% less than castrated males when they are fed ad limitum. This situation, the heavier they are, causes decreased conversion.
The genetic improvements of the last decades has resulted in pigs with decreased ingestion capacity.
Nutritional Factors
Feed volume
The volume of the diet can exercise a certain effect on performance, through limitations of the appetite, with ED values less than 3350 kcal/kg for 50kg fattening pigs.
Characteristics like viscosity or the water retention capacity of certain raw materials should be used to predict feed consumption.
Content and balance of the nutrients in the diet
Only when the nutrients in the diet coincide with the nutrients needed does the appetite reflect a rationed selection as far as those needs are concerned. When the availability of energy in the diet is reduced, pigs try to compensate by eating more feed, but only up to a certain point, since their intake is actually restricted by their physical ability to ingest, or rather, by the negative feedback of the rest of the nutrients that are consumed in excess.
Likewise, pigs that consume low protein diets, or diets deficient in some amino acids, respond by ingesting more feed in order to maintain the requirements for essential amino acids, although this is not always the case.
When the imbalance is due to the excess of a nutrient, appetite can diminish, for example, an excess of minerals provokes a decrease in consumption.
Contaminants in the diet
The presence of mycotoxins in feed reduces consumption.
Acidifiers in the diet
Short-chain organic acids (SCOA) cause an increase in feed ingestion and improves the conversion index. The increase in ingestion is favoured by the stimulation of the taste buds and the increase of saliva production.
Feed presentation
Granulation reduces consumption but improves conversion. Wet feed improves ingestion with respect to dry feed, in granulated as well as powdered form.
Organoleptic Characteristics
Pigs demonstrate a preference for sweet feeds, laminated cereals, oils, fresh or powdered milk; while those that cause a decrease in appetite are meat on-the-bone, some fish, grape seeds, rapeseed seed or cotton seed. The use of flavour enhancers masks unpalatable ingredients, and therefore favours the animals’ appetite.
Availability of water
In conditions of thermal stress, water ingestion contributes to the reduction of the negative effects of the high temperatures on voluntary consumption.
Table 1. Factors that influence voluntary consumption, or the appetite, of pigs.
| Environmental factors | Physical factors |
| Temperature Humidity Air speed |
Density Trough size Number of troughs |
| Physiological factors | Nutritional factors |
| Health Age and live weight Genotype Gastrointestinal capacity Gendr |
Volume Nutrient density Ingredients Additives Contaminants Feed presentation Water ingestion |
| Social factors | |
| Group size Regrouping, or mixing batches |
