Today I wanted to share with you a review paper –-- It is not a most recent publication , but as I’ve been reading more about pigs and feed intake regulation, I thought this is a very well-written review. It is a work from Dr. Qingyun Li and Dr. John Patience at Iowa State. I had the pleasure to meet Qingyun at the ASAS meeting this year, who certainly is a bright young mind in the swine world!
The paper is titled "Factors involved in the regulation of feed and energy intake of pigs" and was published on Animal Feed Science and Technology in 2016.
This paper can be accessed for free through Research Gate, so I won’t try to repeat everything discussed on it. Just as a brief summary, I think this review helped answer the following 10 key questions.
1). HOW IS FEED INTAKE CONTROLLED PHYSIOLOGICALLY?
Answer: Through the AMPK and mTOR pathways.
In addition, increased accumulation of undigested nutrients in the distal intestine → gut releases certain peptides → long-term depression of feed intake
2). ARE PIGS ABLE TO ADJUST FEED INTAKE IN RESPONSE TO ENERGY CONCENTRATION?
Answer: Yes...but in young pigs (< 20kg), "the pig’s ability to adjust feed intake is not capable of fully compensating for the reduction in dietary [energy] concentration".
Yet despite the gut capacity limit, other factors such as dietary fat or fiber content can still alter feed intake. (Example: a high fiber diet can suppress appetite through hormones released in gut or directly affecting the hypothalamus via the fermentation product of acetate) → So, "lowering dietary energy density by increasing fiber content may not result in feed intake compensation as expected".
3). WHAT ENERGY SYSTEM TO USE WHEN LOOKING AT RELATIONSHIP BETWEEN ENERGY DENSITY AND FEED INTAKE?
DE or ME systems would underestimate the available energy concentration of fat, while overestimate that of fiber and protein → biased conclusions on energy density vs. feed intake. Net Energy system is more accurate.
4). HOW DO AMINO ACIDS AFFECT FEED INTAKE?
Feed intake can be depressed by a severe deficiency in limiting amino acids or by an excessive supply of total protein or some essential amino acids or by imbalanced amino acid profile (such as imbalanced Branched Chain amino acids)
Tryptophan is a special one...it can enhance feed intake in young piglets, because of its influence on serotonin and ghrelin secretion, which are both appetite stimulating hormones. But balance of Large Neutral AA and Trp important because they share the same transport route through the blood brain barrier.
5). PARTICLE SIZE? PELLETED FEED? LIQUID FEED?
Inconsistent results as to particle size and pelleted feed, the latter might be due to differences in pellet quality, such as durability and hardness between studies.
On the contrary, liquid feeding has been consistently shown to stimulates feed intake after weaning, partially through alleviating weaning stress of switching from milk to solid feed.
6). SICK ANIMALS EAT LESS?
Of course. Immunological stress is associated with feed intake depression, which is an adaptive response to infection and is mediated by inflammatory cytokines --- these cytokines convey a message from the immune system to the endocrine and central nervous systems to reduce feed intake. ('Lemme focus on fighting the disease...')
7). DIFFERENT TEMPERATURE ON FEED INTAKE
Hot --- Feed intake is reduced by approximately 40 g for every °C above the thermo-neutral zone. This reduction is associated with changes in feeding behavior such as eating time and meal size and is more severe during the initial period of heat stress. Over time, the anorexic response to thermal stress would gradually reduce.
Cold --- Because heat loss to the environment increases, additional heat production is needed, which then increases AMP:ATP and actives the AMPK pathway, as a result, pigs increase feed intake to support the extra heat production.
("Extra feed consumed for each °C below the Lower Critical Temperature has been estimated at 25 and 39 g/day for growing and finishing pigs, respectively".
Again, here, young pigs may have limited ability (i.e. limited gut size) to compensate that need of extra feed intake.
8). FEEDER GAP AND FEEDER SPACE...WHAT'S BEST?
Increasing feeder gap can help increase feed intake in weaning piglets & grow-fin (but more waste in grow-fin pigs). If the feeders are tighter, pigs tend to eat slower, and that can lead to reduced feeder capacity.
"Younger pigs may need a larger feeder gap to ensure all pigs obtain adequate feed for optimum performance. As pigs move from placement (25 kg) to market weight, feeders should be adjusted accordingly to minimize wastage and improve feed efficiency."
As to feeder space, naturally, increasing number of pigs/feeder space would reduce feed intake
9). HOW DOES FLOOR SPACE AFFECT FEED INTAKE?
If floor space is restricted, that will lead to a decline in feed intake possibly due to induced chronic social stress and heat stress.
Notably, "the negative effects caused by reduced space allowance could not be overcome by increasing dietary energy, lysine and vitamin levels"
10). HOW DOES GROUP SIZE AFFECT FEED INTAKE?
Still inconsistent results...some found large group size can be a cause for reduced feed intake; but research showed that "housing growing-finishing pigs in groups of up to 80 pigs was not detrimental to performance if space allowance is adequate".
TAKE HOME MESSAGE
✅ So how is feed intake regulated in pigs? The answer is ---- It's complicated... 😆
FULL ARTICLE ACCESS👇
Li, Q. and Patience, J.F., 2017. Factors involved in the regulation of feed and energy intake of pigs. Animal Feed Science and Technology, 233, pp.22-33.
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