By Dr Francois Siebrits
Voluntary feed intake is an important aspect of nutrition. It is possible and relatively straight forward to determine (or calculate) the nutrient requirements (in grams or kg per day) of a pig at any stage of life. The problem is to know how much the pig will consume of that diet so that one can determine what the concentration of each of the nutrients must be in that diet. The convention (and possibly the convenience) is to say that intake is regulated by energy content so that nutrient content must be expressed as a ratio to energy. Now this is only partially true and there are many more factors that play a role in voluntary feed intake.
This article is based on two recent review articles on this topic and I am going to extract some of the interesting facts from them. The first one is Factors involved in the regulation of feed and energy intake of pigs by Qingyun Li and John F Patience in Animal Feed Science and Technology Vol 233 (2017) p22–33. The second one is titled, Taste, nutrient sensing and feed intake in pigs (130 years of research: then, now and future) by E. Roura and M Fu in Animal Feed Science and Technology Vol 233 (2017) p3–12.
There are several hormones released by the gastro intestinal tract and the pancreas in response to carbohydrates, proteins and fats, as well as microbial fermentation end products which then regulates voluntary feed intake. Some may control the rate of movement through the intestinal tract, which affects intake. Others have an effect on the appetite control in the brain.
There are two main long-term energy monitoring systems that are situated in the hypothalamus of the brain. They act by releasing messenger peptides in the brain, which then acts in a complicated way to either stimulate intake or reduce it in the short or the long term. The gastro intestinal tract can also send messengers in the form of peptides when increased amounts of undigested nutrients or microbial fermentation end products are detected in the hind gut and which then result in a long term depression of intake.
Dietary energy concentration
The primary nutrient involved in intake regulation is energy. Dietary energy manipulation is usually done by varying fat or fibre content. Pigs can adjust their voluntary intake to compensate for low or high energy diets. When the energy content is low, pigs will consume more feed to the point when capacity becomes limiting or when environmental factors such as high temperatures come into play. However, research has shown that the pig is not fully able to compensate for low energy diets. When high energy diets, which usually contains high fat levels are fed, Apo-AIV is released, which slows down motility and ultimately the passage rate of digesta and thereby decreasing voluntary feed intake. At the same time high levels of long chain fatty acids released during fat digestion also acts in the brain to decrease intake. When progressively diluting a highly digestible diet with lower energy constituents, intake increases to maintain a constant DE (digestible energy) intake up to a critical point, presumed to be physical gut capacity. Up to about 50 kg live weight this point is at a DE content of 14.02 MJ/kg and at 13.43 MJ/kg for heavier pigs. When high fibre (lower energy) diets are fed, pigs increase intake to the point when capacity becomes limiting. Furthermore, another factor comes into play: the end products of fibre fermentation in the hind gut includes acetate, which also limits intake. It is unknown which is the main contributor to the lack in further intake adaptation.
Amino acid balance
As early as 1959 Harper reported that in addition to energy content intake is also regulated by amino acid profile of the diet. It is well known that intake is negatively affected by a severe deficiency in essential amino acids or an excess of certain essential amino acids or total protein. Some amino acids such as tryptophan act directly in the brain to control appetite. Others may be antagonistic, for example excess leucine may induce a valine deficiency which reduces appetite. It is therefore important to keep a proper balance between all essential dietary amino acids and available energy to maintain intake.
Feed processing and pelleting
The reduction of grain particle size is associated with a reduction in intake in weaner and grower-finisher pigs. However, this reduction in intake may be as a result of gastric ulcers caused by fine feed or some other form of discomfort.
Pelleting causes a reduction in feed intake as well as an improved feed conversion. This is mainly due to a reduction in feed wastage and also an improved digestibility. Wet feeding may improve intake and as a consequence also growth performance and feed conversion ratio compared to dry feeding in growing-finishing pigs. Feed conversion may not be improved by liquid feeding for weaner pigs.
Gender and bodyweight
Generally barrows grow 6-10% faster than gilts and therefore consume more feed to satisfy their daily needs. Feed intake is also associated with body weight. As the pig grows, the need for nutrients increases so that feed intake also increases.
Health status and infection
Feed intake depression happens in response to infection so that health status is an important determining factor of feed intake and growth performance. When the immune system is activated by infection it causes stress to the animal and is associated with inflammation and a reduced feed intake. In a research study piglets were challenged by an oral dose of E. coli K88 (serotype 0139:K88, resistant to oxytetracycline) and had a 9.3% lower feed intake during the first week after the challenge compared to the control group which received a saline water placebo.
Feed intake is influenced by the heat exchange between the pig and its environment. Generally the pig performs best in the so-called thermoneutral zone (TNZ). The definition of TNZ is “the range of ambient temperature at which temperature regulation is achieved only by control of sensible (dry) heat loss, i.e., without regulatory changes in metabolic heat production or evaporative heat loss”. Pigs tend to increase their voluntary intake below the lower critical temperature (cold stress) and reduce intake above the upper critical temperature (heat stress).
In hot temperatures pigs tend to change their posture i.e. they limit their body contact with other pigs. Voluntary feed intake is reduced by 40g for every °C above the TNZ. This reduction is accompanied by behavioural adaptations such as reduced eating time and meal size. Pigs under heat stress will eat more early in the morning when it is cooler. Fortunately pigs adapt to heat stress conditions so that feed intake recover somewhat over time.
The effects of heat stress on voluntary feed intake is somewhat mitigated by feeding a low protein diet supplemented with synthetic amino acids. Such a diet has a lower heat increment (heat generated by digestion and metabolism of a diet).
Feeder space is the first aspect of the physical environment that affects feed intake. This can be expressed as number of pigs per feeder space, which sounds logical, but doesn’t take into account the shoulder width of the pigs. Space allocation is often expressed as the number of pigs per trough length but research results are inconsistent.
The feeder gap has an effect on feed flow, reducing the number of pigs per feeder width due to a decreased feeding speed. Increasing the feeder gap leads to higher feeding speeds and consequent average daily gains and feed intakes, but with poorer feed conversions due to increased wastage. Therefore younger pigs may need an increased feeder gap to ensure adequate intakes, which should be adapted for older pigs to minimise wastage and improve feed conversion ratios.
A recent study compared the effect of feeder spaces of 2.1, 2.5 or 2.9 cm per nursery pig and of 4.1, 4.9, or 5.7 cm per growing-finishing pig in a commercial system. The results showed no effect on voluntary feed intake over the entire period from weaning to market. There was, however, a linear decline in growth rate and feed efficiency with declining feeder space during the last stages of production (96.1–122.6 kg).
Floor space affects voluntary feed intake. Several studies showed that restricted space is associated with lower feed intakes and growth rates. Chronic stress may be induced by restricted space which then decrease feed intake and performance. Reduced space may also increase interaction such as aggressive behaviours and competing for space at the feeders. High pen densities and competing interactions may also aggravate the effects of heat stress. The negative effects of high densities could not be rectified by increased feed energy, lysine and vitamins. Group size is important during the period of rapid growth when the traditional 20-25 pigs per group is compared to large groups (100-120 pigs). Average daily gain and feed intake was reduced in large groups of weaner pigs. In grower pigs (31-68kg) growth rate was reduced but not feed intake so that poor feed conversions were obtained. There were no differences found for the finisher group (>68kg).
Pigs are being fed on balanced diets without any choice so that taste is completely ignored in the commercial setup as if taste doesn’t have any effect on voluntary feed intake. However, electron microscopy showed that pigs have relatively large tongues with one of the highest numbers of taste buds (19 904) among mammalian species. The first studies on taste were conducted in the 50’s and 60’s where sucrose (ordinary sugar) was used as an intake enhancer in creep feed. So-called human high intensity sweeteners (saccharin) were also tested. The use of electrophysiological procedures found that pigs have a poor perception of human high intensity sweeteners. Later work identified taste receptors for glucose in the upper gastro intestinal tract (GIT) which stimulate the release of certain hormones which control short-term feed intake.
The advent of genomic research introduced a whole new field of study and led to the discovery of the molecular mechanisms of taste and nutrient sensing. Results indicated that the pig has a wider umami sensing ability than humans. A pig can sense eight different amino acids as umami taste, which drive an appetite for these nutrients, while humans can only taste two of which one is glutamate (MSG). The pig’s taste receptors are not limited to the mouth, but also occur in the stomach and gut.
Voluntary feed intake is controlled by a whole range of factors, ranging from feed factors to environmental factors to physiological factors and even genetic factors. It would therefore be still some time before all of these factors can be integrated into a single model to predict intake accurately at any stage of life in any type and gender of pig. It would however soon be possible to manipulate intake for instance to increase it during the early growth phase or during lactation and then to limit intake during finishing or during pregnancy.