Feed Additives on Health Status of Grow–Finish Pigs: Comparison
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There are numerous feed additives that can be used to enhance grow–finish pig growth performance and carcass characteristics, which can potentially lead to a higher economic return. Feed additives have shown benefits throughout the literature in improving grow–finish pigs’ growth performance and carcass characteristics. 

  • carcass
  • feed additive
  • feed efficiency

1. Acidifiers—Mechanism of Action

Acidifiers have been used in animal diets for their beneficial effects on antimicrobial activity and nutrient digestibility coefficients. The most used acidifiers are organic acids in the form of short-chain fatty acids (fumaric, citric, malic, formic, lactic, acetic, butyric, and propionic acid), medium-chain fatty acids (sorbic, capric, and caprylic acid), and benzoic acid. Acidifiers lower the pH of the digestive tract, which provides an acidic environment (pH < 4.5) that inhibits the growth of acid-sensitive bacteria [1]. The low pH also assists the digestibility of protein and minerals by stimulating the secretion and activity of enzymes in the small intestine [2][3]. Moreover, in an acidic environment, non-dissociated organic acids can freely penetrate the bacterial cell wall and reduce the pH of cytoplasm [1]. The increased H+ requires bacteria to spend energy on removing these H+ and, therefore, retards the growth of acid-sensitive pathogens [1]. With these mechanisms, acidifiers can potentially improve growth performance and carcass characteristics by enhancing the gut health and digestibility of the pig [1][2][3].

2. Acidifiers—Results

There were 68 comparisons for ADG (average daily gain) between pigs fed a control diet or diets with added acidifiers with an average of a 1.7% increase (range between −14.9 and 11.4%) in pigs fed acidifiers, and for G:F (gain-to-feed ratio), there were 65 comparisons between pigs fed a control diet or diets with added acidifiers with an average of a 3.1% increase (range between −9.7 and 11.3%) in pigs fed acidifiers. For carcass data, there were 24 comparisons evaluating backfat thickness (BF) change between pigs fed a control diet or diets with added acidifiers with an average of a 0.6% decrease (range between −15.3 and 14.4%) in pigs fed acidifiers. For percentage lean, there were 24 comparisons between pigs fed a control diet or diets with added acidifiers with an average of a 0.5% decrease (range between −3.6 and 4.2%) in pigs fed acidifiers. There were 11 comparisons for loin muscle area (LMA)/loin muscle depth (LD) between pigs fed a control diet or diets with added acidifiers with an average of a 1.6% improvement (range between −7.2 and 8.1%) in pigs fed acidifiers. These results could be expected because the mechanisms do not directly affect protein and lipid metabolism. In summary, feeding acidifiers has the potential to improve growth performance but only minor effects on carcass characteristics.

3. Essential Oils (EO)- Mechanism of Action

Essential oils (ethereal oils) are classified as phytogenic feed additives. Essential oils are a mixture of volatile and non-volatile compounds extracted from plants (approximately 1% of the wet weight of plants), such as oregano, thyme, rosemary, and garlic [4]. The primary active ingredients in essential oils (EO) are phenols (thymol, carvacrol, eugenol, ρ-cymene). These phenolic components have been widely used for antibacterial, antiviral, antifungal, insecticidal, and antiparasitic activities in humans and animals [5]. For the antibacterial effects, the lipophilic structures of EO can penetrate and disrupt the cell wall and cell membrane of the pathogens, which causes alterations in the cell functions [6], which is similar to the antimicrobial mechanism of the organic acids. The phenolic OH group can also act as an antioxidant by donating hydrogen to free radicals [4]. Moreover, EO may potentially enhance the immune system by interacting with the microbiota of the pigs and altering the lymphocyte population and distribution in the gut [4]. These beneficial mechanisms suggest that EO may potentially improve grow–finish pig growth performance and carcass characteristics.

4. Essential Oils—Results

For ADG, there were 20 comparisons between pigs fed a control diet or diets with added EO with an average of a 5.8% improvement (range between −2.9 and 18.8%) in pigs fed EO. There were 17 comparisons for G:F between pigs fed a control diet or diets with added EO with an average of a 5.8% improvement (range between −2.6 and 19.9%) in pigs fed EO. Fourteen comparisons evaluated BF between pigs fed a control diet or diets with added EO with an average of a 2.7% decrease (range between −14.2 and 6.3%) in pigs fed EO. For percentage lean, there were 9 comparisons with an average of a 0.9% improvement (range between −2.5 and 2.8%) in pigs fed EO. For LMA/LD, there was an average of a 1.9% improvement (range between −6.3 and 12.3%) in pigs fed EO.
Overall, the results suggest that EO positively affected ADG and G:F. Adding EO alone or in combination with acids has the potential to improve growth performance. However, there was only a small amount of research on EO’s effect on growth performance, and only three studies were conducted in the US; therefore, using EO may not be beneficial in US-based conditions. More experiments are needed to determine the effect of including EO in the diets of grow–finish pigs.

5. Direct-Fed Microbials (DFM) -Mechanism of Action

Direct-fed microbial (DFM) or probiotic products are defined as feed additives that contain live (viable) microorganisms (bacteria and/or yeast) that are beneficial to the host. The most used DFM strains added in grow–finish pig diets are yeast (Saccharomyces cerevisiae), and Bacillus and Lactobacillus species either as a single strain or blend (the effect of the single addition of yeast in diets was discussed in the yeast section). Adding DFM aims to achieve a healthy and balanced intestinal microbial composition [7]. These beneficial microorganisms may improve the digestibility of nutrients and reduce the adverse effects of pathogens in the gastrointestinal tract by competitive exclusion, modulation of the immune response, and/or the production of bacteriocins [8]. The inclusion of DFM has been used as an alternative to antibiotics and has shown beneficial effects in research when fed mainly in weaned pig diets.

6. DFM—Results

There were 71 comparisons for ADG between pigs fed a control diet or diets with added DFM with an average of a 3.3% improvement (range between −6.2 and 20.3%) in pigs fed DFM. For G:F, there were 66 comparisons between pigs with an average of a 3.3% improvement (range between −7.2 and 13.1%) in pigs fed DFM. There was an average 1.5% decrease (range between −18.1 and 20.3%) in BF for pigs fed a DFM vs. a control diet across 21 comparisons. For percentage lean, there were 13 comparisons between pigs fed a control diet or diets with added DFM with an average of a 1.0% improvement (range between −2.0 and 3.6%) in pigs fed DFM. There were 19 comparisons evaluating added DFM for LMA/LD, with an average of a 1.5% improvement (range between −5.8 and 10.9%) in pigs fed DFM.
In summary, DFM can potentially improve the growth performance of grow–finish pigs. However, the small effects and lack of statistical differences of DFM on carcass characteristics may suggest that the mechanisms of DFM do not directly affect pigs’ protein and lipid metabolism. It is worth mentioning that there were relatively few US-based studies for DFM; therefore, the effects of DFM in US-based conditions may not be the same as what has been observed to date.

7. Yeasts—Mechanism of Action

Yeast is a single-cell fungus used in the food industry, ethanol production, and animal feed for its nutritional and health benefits. The most used yeast strain in animal feed is Saccharomyces cerevisiae, while Phaffia rhodozyma (red yeast) is rarely used. Yeast products are added as live yeast (as a DFM additive), yeast cell wall extracts, or a combination of both. Yeast converts substrates (carbon and nitrogen sources) into carbon dioxide, ethanol, and yeast cell contents through fermentation [9]. The fermented yeast cell culture contains vitamin B, β-glucan, α-mannans polysaccharides, and microbial protein, which can serve as a protein source for animals. The yeast cell wall extracts mainly consists of β-glucan and α-mannan polysaccharides, which have shown prebiotic effects on improving nursery pigs’ immune system and gastrointestinal health [9]. Mannan oligosaccharides (MOS) are the side chains of mannan polysaccharides and have been widely studied as an antimicrobial feed additive for their positive effects on microbiota and intestinal morphology in nursery pigs [10]. In addition, MOS reduces the colonization of pathogens by binding to the pathogens and improves gut morphology by increasing the villus height:crypt depth ratio [10].

8. Yeasts—Results

There were 36 comparisons for ADG between pigs fed diets with added yeasts with an average of a 1.6% improvement (range between −13.7 and 10.3%) in pigs fed yeasts. For G:F, there were 33 comparisons between pigs fed a control diet or diets with added yeasts with an average of a 2.7% improvement (range between −11.7 and 17.7%) in pigs fed diets containing yeasts. There were 21 comparisons evaluating pigs fed diets with added yeasts on BF with an average of a 3.1% decrease (range between −30.7 and 11%) in pigs fed yeasts. For percentage lean, there were 8 comparisons between pigs fed a control diet or diets with added yeasts with an average of a 1.0% improvement (range between −1.7 and 6.6%) in pigs fed yeasts. Lastly, for LMA/LD, there were 17 comparisons with pigs fed added yeasts having an average of a 1.4% improvement (range between −4.3 and 16.6%).
In summary, yeasts can be a potential feed additive with a relatively large magnitude of improving the growth performance of grow–finish pigs, especially for growth performance.

9. Copper (Cu)—Mechanism of Action

Copper is an essential trace mineral for several metalloenzymes that play roles in oxidation–reduction reactions, transport of oxygen and electrons, and protection against oxidative stress [11]. Feeding pharmacological levels of Cu has shown growth-promoting effects in weaned and growing pigs by reducing diarrhea frequency and increasing feed efficiency [12][13]. These improvements may be because of Cu’s effects on the enzymes (lipase, phospholipase A, lipoprotein lipase) involved in lipid digestion and metabolism [11]. Copper also showed bacteriostatic and bactericidal properties that improve weaned pigs’ microbiota, gastrointestinal structure, and immune status [14][15]. However, because Cu accumulates in the liver and other organs when fed above requirement estimates, toxicity should be a concern when provided above 250 mg/kg in pig diets. Feeding excess levels of Cu resulted in hemolysis and organ damage in pigs [11].

10. Copper—Results

There were 155 comparisons of ADG between pigs fed a control diet or diets with added Cu with an average of a 2.5% improvement (range between −12.2 and 15.2%) in pigs fed pharmacological levels of added Cu. For G:F, there were 149 comparisons between pigs fed a control diet or diets with added Cu with an average of a 1.8% improvement (range between −8.0 and 17.6%) in pigs fed Cu. Seventy-three comparisons evaluated BF between pigs fed diets with added Cu with an average of a 1.4% decrease (range between −17.0 and 11.5%) in BF of pigs fed Cu. For percentage lean, there were 25 comparisons with pigs fed added Cu having an average improvement of 1.6% (range between −2.7 and 34.7%). For LMA/LD, there were 62 comparisons between pigs fed diets with added Cu with an average of a 2.3% improvement (range between −7.5 and 14.5%).
Most studies used Cu additions of 125 to 250 mg/kg (137 comparisons), and increasing Cu addition did not generally further improve pig performance. The growth-promoting effects of Cu can potentially improve growth performance (2.5 and 1.8% improvement for ADG and G:F); however, with carcass characteristics, the effects were relatively small, with most comparisons finding no evidence of difference.

11. Zinc (Zn)—Mechanism of Action

Zinc is an essential trace mineral in several important metalloenzymes for the growth and development of animals. High levels (1500 to 4000 mg/kg) of dietary zinc oxide (ZnO) have been widely used as a growth-promotive feed additive in weaned pig diets to improve growth performance and gastrointestinal health [16]. However, the mechanisms of the growth-promotive effect of ZnO are still not fully understood. Zinc oxide may regulate the secretion of ions in the intestine, reduce the inflammatory reaction, stabilize the microbiota, prevent the attachment of pathogens, and improve the gastrointestinal structure [16]. Moreover, for grow–finish pigs, whether high Zn inclusion (above 100 mg/kg) can provide a growth-promotive effect is also unclear.

12. Zinc—Results

For ADG and G:F, there were 30 comparisons between pigs fed a control diet or diets with added Zn with increases of 0.6% (range between −14.4 and 18.7%) and 1.2% (range between −7.6 and 14.4%), respectively. There were 19 comparisons for BF and pigs fed diets with added Zn had an average of a 0.6% decrease (range between −7.6 and 13.1%). For percentage lean, there were 14 comparisons that observed an average 0.9% improvement (range between −0.4 and 3.9%) in pigs fed Zn. All the comparisons (15) found a 0.2% improvement (range between −2.9 and 2.7%) in LMA/LD.
Overall, the results suggest that Zn had positive but relatively small effects on ADG, G:F, and carcass characteristics. Moreover, there were insufficient data to support whether different types of basal diets and inclusion levels affected the response to added Zn.

References

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