Since the United States initiated the Veterinary Feed Directive (VFD) in January 2017, which limits the use of crucial antibiotics in farm animals, researchers have been exploring alternative ways to maintain the health of young pigs, particularly after weaning. Various products, including organic acids, enzymes, probiotics, antimicrobial peptides, medium-chain volatile fatty acids, spray-dried plasma products and essential oils (also known as photogenic plant products), are being investigated as substitutes for antibiotics in swine rations. This review consolidates the findings from studies that tested selected essential oils and dissects evidence suggesting that these products could emerge as viable alternatives for antibiotics. They offer the potential for consistent availability within the industry, high safety factors for pigs and consumers, cost-effectiveness and environmental friendliness. Despite mixed results, the use of essential oils by pig producers in the European Union for several years underscores the need for further research and understanding. However, the potential of essential oils to revolutionize pork production is a promising prospect.
Essential oils are defined as natural bioactive compounds that are derived from plants. They include aromatics, volatile (change in chemical structure and overall usefulness) and 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 carvacrol, thymol, citral, eugenol and cinnamaldehyde, which are derived from thyme, lemongrass, clove, nutmeg, cinnamon, basil, oregano and hay leaf.
While essential oils are natural bioactive compounds derived from plants, their oily and volatile nature presents challenges in pig diets. These challenges include their effectiveness within diets and their absorption into the pig’s gut. While scientists don’t completely understand how essential oils work in the intestines, they believe they help reduce inflammation and protect the gut. This means essential oils might help lessen some of the problems caused by E. coli, which can upset a pig’s immune system and cause diarrhea after weaning. The need for further research to fully understand the potential of essential oils in pig diets is not just critical; it’s urgent and of utmost importance.
In the United States, research with essential oils for sows, nursery pigs, and grow finishers is increasing (discussed in greater detail below). Although some studies have noted improved feed palatability and intake, contributing to overall growth performance, a clear path to their widespread adoption by pork producers has not been delineated. In addition to the lack of definitive information around the pharmacodynamics effects (i.e., the relationship between dose and the mechanistic beneficial actions), key challenges facing the use of essential oils in pork production include some potential regulatory concerns, high inclusion costs, formulation, and effective delivery methods. The current gaps in knowledge underscore the need for further research on the use of essential oils in pork production.
Sows
Essential oils have been tested in sow diets to increase overall reproductive performance and key performance indicators typically measured in these studies include sow feed intake, number of piglets born alive, and sow milk production. Sows with essential oils in their feed have shown small but significant indications of improved gut health compared to untreated controls regarding intestinal lining changes (especially microvilli density and length), lymphocyte proliferation and various blood parameters. Significant improvements in sow health or performance have accompanied these changes in gut morphology in some studies. For instance, a study in 2009 reported that supplementation of a blend of essential oils 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. A study involving 2,100 sows reported that they had higher voluntary feed intake, lower annual mortality rate (4.0% vs. 6.9%), reduced sow culling rate during lactation (8% vs. 14%), increased farrowing rate (77.0% vs. 69.9%), increased number of live born piglets per litter (10.49 vs. 9.95) and decreased stillbirth rate (0.91 vs. 0.81). However, another study noted that 70 second-parity sows fed oregano essential oil blend during gestation and farrowing did not increase piglet growth or immune responses.
Nursery Pigs
Most research on essential oils in pigs has been directed toward nursery pigs due to the dietary changes and other stressors they encounter at this crucial time of their development, which often negatively impact health and performance. Numerous studies show that feeding essential oils during this period changes the gut environment, favoring a healthier bacterial population. In some cases, this proliferation of healthier bacteria appears to override the harmful bacterial pathogens that cause diarrhea, decreased feed intake and reduced performance within the first few weeks of weaning. In one specific case, I encapsulated essential oils (thymol and cinnamaldehyde tested in these studies) to improve performance, immunity and beneficial gut microflora in 240 piglets that were 36 days old (at the start of the study) over 35 days; results showed reduced E. coli counts in feces, increased lymphocyte transformation and reduced occurrence of diarrhea. A study done by a South Korean University reported that dietary supplementation of blended essential oils fed six weeks to 90 weaned nursery pigs resulted in an improvement in post-weaning final average daily gain (ADG) (487g vs. 476g, P < 0.1) without any apparent adverse effects on health or other performance indicators. However, a 2006 study showed that in-feed antimicrobials increased growth performance more effectively than a diet with essential oils in a piglet study conducted 28 days 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 slightly improved body weight (17 kg vs 15.4 kg, P = 0.09) compared to the essential oil diet. Studies 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).
Grow-Finish
Adding essential oils to grow-finish pig diets has impacted growth performance and carcass merit, potentially decreasing the stress response during management practices. According to a review of European essential oils, feed intake increased from 9% to 12% with dietary supplementation. Furthermore, some studies have reported the same impact on feed intake, ranging from 3% to 19% 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, another study in 2010 failed to observe any improvement in performance generated by essential oil blends in finisher pigs in the United States. It has also been noted that essential oil diets increased the longissimus muscle area for 96 grow-finish pigs, starting at around 24 kg to market, increasing the hogs’ muscle mass. One study concluded that carcass and meat quality attributes were unchanged when comparing oregano essential oil diets to conventional diets in 64 finisher pigs. Concerns were noted that 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 could not detect flavor or aroma differences between the conventional-fed and essential oil diets. Certain essential oils have been researched and have shown promising results in decreasing stressful situations, such as moving and transporting pigs.
Cost
Researchers have noted that the cost-effectiveness of essential oils is generally not achieved in pigs when products are used at concentrations high enough to affect health or performance. As interest in alternatives for in-feed antibiotics in pig production grows and more research is conducted, it is reasonable to speculate that economies of scale in production and formulation of the essential oils will be achievable, leading to broader use of crucial oils in pork production.
Essential oils may become practical alternatives to feed-grade antibiotics. They are being studied for their health and performance benefits for swine in all production phases. However, none of the essential oils tested in pigs have provided consistent, positive benefits in disease prevention or performance compared to antibiotics. Knowledge about how these molecules lead to improved gut health and growth parameters in pigs is emerging from global research, especially in Europe and Asia. However, the expanded use of essential oils in pork production will likely depend on additional research focused on the cost of production, formulation, and effective dosing and presentation of essential oils.
