Potential use of essential oils as an alternative to feed grade antibiotics in pork production
Can essential oils essentially replace antibiotics for pigs?
Following implementation in the U.S. of the Veterinary Feed Directive in January, 2017, which bans the use of medically-important antibiotics (i.e., those also used in human medicine) in livestock except for treatment or prevention of disease, researchers have intensified their search for alternative agents that promote gut health, especially in early post-weaned piglets. A wide variety of products are being tested, including organic acids, enzymes, probiotics, antimicrobial peptides, medium-chain volatile fatty acids, spray-dried plasma products and essential oils (also known as phytogenic plant products), as alternatives to antibiotics in swine rations. This review focuses on results from studies testing selected essential oils, and describes evidence suggesting that these products could become viable alternatives for antibiotics because of their potential for consistency, high safety factors for pigs and consumers, cost-effectiveness, and the fact that they are environmentally-friendly. Essential oils have been used by pig producers in the E.U. for several years, with mixed results reported.
Essential oils are defined as natural bioactive compounds that are derived from plants. They include aromatics, volatile, oily liquids extracted from materials such as seeds, flowers, leaves, buds, twigs, herbs, bark, woods, fruits, and roots. Essential oils that have been fed to pigs in multiple research studies include carvocrol, thymol, citral, eugenol, and cinnamaldehyde which are derived from thyme, lemongrass, clove, nutmeg, cinnamon, basil, oregano, and hay leaf.
The oily and evaporate nature of essential oils leads to challenges in their effectiveness within diets and absorption to the pig’s gut. Although the mechanisms underlying essential oil effects on intestinal function remain to be determined, researchers think the mechanisms have to do with the anti-oxidant and anti-inflammatory effects on the intestinal lining of mammals. These effects positively interfere with the processes by which E. coli may disrupt the pig’s immune system cause post-wean diarrhea (Li et al., 2012).
In the United States, the amount of research with essential oils for sows, nursey pigs and grow-finishers is increasing (discussed in greater detail below). A clear path to their widespread adoption by pork producers has not been delineated. In addition to lack of definitive information around the pharmacodynamics effects (i.e., relationship between dose and the mechanistic beneficial actions), key challenges facing the use of essential oils in pork production include: some unexpected off-target/undesirable effects (odor prevents pigs from eating feeds containing some essential oils), potential regulatory concerns, high inclusion costs, formulation and effective delivery methods.
Essential oils have been tested in sow diets in effort to increase overall reproductive performance; key performance indicators typically measured in these studies include sow feed intake, number piglets born alive, and sow milk production. Sows provided essential oils in their feed have shown small but significant indications of improved gut health, when compared to untreated controls, in terms of intestinal lining changes (especially microvilli density and length), lymphocyte proliferation, and various blood parameters. However, significant improvements in sow health or performance have not accompanied these changes in gut morphology (Ariza-Nieto et al., 2011; Miller et al., 2009; Allan and Bilkei, 2005). Still, some important secondary effects have been observed in pre-weaned piglets coming off of treated sows; piglets have been healthier and shown higher weaning weights. For example, Miller et al. (2009) reported that supplementation with 2 g/kg of a blend of essential oils (Biomin P. E. P., BIOMIN), from 10 days before the estimated farrowing date through weaning, improved early lactation feed intake in sows, decreased sow weight loss during the first week of lactation and enhanced piglet body weight at weaning. In a study involving 2100 sows, Allan and Bilkei (2005) reported that sows fed diets containing 1 g/kg oregano had higher voluntary feed intake, lower annual mortality rate (4.0 vs. 6.9 percent), reduced sow culling rate during lactation (8 vs.14 percent), increased farrowing rate (77.0 vs. 69.9 percent), increased number of live born piglets per litter (10.49 vs. 9.95) and decreased stillbirth rate (0.91 vs. 0.81). However, Ariza-Nieto and others (2011) noted that in their study of 70 second-parity sows, feeding 250 mg/kg oregano essential oil blend during gestation and farrowing did not result in increased growth or immune responses in the piglets.
Most research on essential oils in pigs has been directed toward nursery pigs, due to the dietary changes and other stresses they present at this crucial time, which often negatively impacts health and performance. Based on numerous studies, it appears that feeding essential oils during this period results in changes to the gut environment favoring a healthier bacterial population (Li et al., 2012; Franz et al., 2010; Huang et al., 2010). This proliferation of healthier bacteria appears, in some cases, to over-ride the harmful bacterial pathogens that cause diarrhea and decreased feed intake and performance within the first few weeks of weaning. Li and others, (2012) noted that encapsulated essential oils (thymol and cinnamaldehyde tested in these studies) improved performance, immunity and gut microflora in 240 piglets that were 36 days old (at start of study) over a 35-day period; results showed a reduced E. coli counts in feces, increased lymphocyte transformation, and reduced occurrence of diarrhea. Huang and others (2010) reported that dietary supplementation of blended essential oils fed 6 weeks to 90 weaned nursery pigs resulted in an improvement in post-weaning final ADG (487g vs 476g, P < 0.1) without any apparent negative effects on health or other performance indicators. However, Neill et al. (2006) showed that in-feed antimicrobials increased growth performance more effectively than a diet with essential oils in a piglet study conducted over a 28-day period after weaning at day 21. In that study, 210 piglets were fed either an oregano essential oil diet or a neomycin and oxytetracycline-supplemented diet. The antimicrobial diet slight improved body weight (17 kg vs 15.4 kg, P = 0.09) significantly more than the essential oil diet. Neill and others (2006) noted that ADG, ADFI, G:F, and 28-day weights of pigs fed oregano essential oil diet (25, 50, or 100 g per ton) were similar to those of pigs fed the control diet (P > 0.05), and there was no effect on growth parameters of increasing dose of essential oil (P > 0.05).
The addition of essential oils to grow-finish pig diets has impacted growth performance and carcass merit (Janz et al., 2007; Yan et al., 2010). Feed intake increases from 9 to 12 percent with dietary supplementation of essential oils according to a review of European essential oil use of Franz et al., (2010). Furthermore, Zeng et al., (2015b) reported the same impact on feed intake; ranging from 3 to 19 percent in their review of essential oil use in Europe. While most research has found that adding essential oils to grow finisher diets increases feed intake, interestingly Janz et al., (2007) and Yan et al., (2010) failed to observe any improvement in performance generated by essential oil blends in finisher pigs in the United States. Yan and others (2010) noted that for 96 grow finish pigs starting around 24 kg to market, essential oil diets increased the longissimus muscle area. Janz and others (2007) concluded that carcass and meat quality attributes were unchanged when comparing oregano essential oil diets to conventional diets in 64 finisher pigs. There are concerns if the concentration of the essential oils within the diet could alter the flavor of the final pork product, which is now being studied. It was also noted in the same study that sensory panelists were unable to detect a flavor or aroma differences between the conventional-fed and essential oil diets (Janz et al., 2007).
Yang et al. (2015) and others have noted that the cost effectiveness of essential oils is generally not achieved in pigs when products are used at concentrations required to affect health or performance. As interest in alternatives for in-feed antibiotics in pig production grows, however, and more research and information becomes available regarding the most effective products and dose regimens, it is reasonable to speculate that economies of scale in their production and formulation will be achievable, leading to wider use of essential oils in pork production.
Essential oils may become useful alternatives to feed-grade antibiotics. They are being studied for their health and performance benefits for swine in all phases of production. To this point, however, none of the essential oils tested in pigs has provided the same level of consistent positive benefits in disease prevention or performance that is achievable using antibiotics. Knowledge around how these molecules lead to improvements in gut health and growth parameters in pigs is emerging from research underway on a global basis, but especially in the E.U. and Asia. However, expanded use of essential oils in pork production will likely depend on more research focused on cost of production, formulation, and effective dosing/presentation.
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