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Show me the data: Is oregano oil beneficial for calves?

Zach Janssen for Progressive Dairy Published on 12 January 2022

The words “essential oil” can elicit a strong emotional response. For some this may be positive, and for others it may carry a negative connotation equivalent to that of “snake oil.”

Often I am asked, “What is the research behind this?” Or, I may be bluntly told, “Show me the data!” Perhaps you would be surprised to learn that there is a growing body of scientific evidence around the benefits of a certain essential oil, oregano (oregano essential oil – OEO). The amount of evidence continues to grow as researchers look for alternatives to traditional pharmaceutical antibiotics for treatment of disease or an alternative to feeding antimicrobials in ruminant diets to enhance digestion and lower methane production.



When it comes to animal health and welfare, science should outweigh emotion when making informed decisions on which products to use, while consumers demand more non-pharma options on our farms.

First, let us start with a discussion of functional components. Plants produce phenolic compounds as a defense mechanism against infectious agents (bacteria, fungi, pests) and for cellular protection against ultraviolet radiation. These phenolic compounds are the bioactive compounds in essential oils, providing potential therapeutic applications. There are two main bioactives in OEO: carvacrol and thymol. Carvacrol is a phenol that is a natural inhibitor of bacterial growth, is used as a food additive and is a botanical anti-fungal agent. Thymol is a phenol used as a stabilizer in pharmaceutic preparations. It has been used for its antiseptic, antibacterial and antifungal actions and was formerly used as a vermifuge (dewormer).

Many studies have demonstrated that carvacrol and thymol are potent antibacterial agents against both gram-positive and gram-negative bacteria. The most frequently reported mechanism of antibacterial action of both phenols involves the disruption of bacterial membranes, leading to bacterial lysis and leakage of intracellular contents resulting in death. Other proposed mechanisms of antibacterial action include the inhibition of efflux (outflow) pumps, prevention of the formation and disruption of preformed biofilms, inhibition of bacterial motility and inhibition of membrane ATPases (energy-producing enzymes).

Published literature concludes that OEOs were effective in combatting calf scours. A 2017 study demonstrated that calves fed OEO for the first 10 days of life had better average fecal scores, lower incidence of diarrhea, and significantly lower duration and severity of diarrhea than controls. Against naturally acquired diarrhea under field conditions, OEO possess a preventive effect versus neonatal diarrhea syndrome. This evidence supports the incorporation of oregano into a daily fed scours prevention strategy.

Research also shows that calves fed OEO daily during the preweaning period can have enhanced growth characteristics. This was demonstrated in a 2020 Journal of Dairy Science paper showing greater average daily gain (ADG) through weaning on day 56 compared with calves fed other feed additives or a control diet. Body-length and wither-height gains were also greater. While hip-width gains were similar among the different diets, hip-height gains were increased in the group receiving a specific amount per day of OEO. These results reveal the optimal feeding rate to enhance growth.


Can essential oils such as oregano be used as adjuvants to traditional antimicrobial therapies? Merriam-Webster defines an adjuvant as: 1) serving to aid or contribute, 2) enhancing the effectiveness of a medical treatment. Adjuvants can be additive or synergistic. Additive describes the phenomenon of two separate treatments, when used together, equal the sum of their effects when used separately. Essentially, 1+1=2. Synergism occurs when the end result is greater than the sum of the effects of individual treatments. Or, 1+1=3. An in vitro study looking at common pneumonia-causing bacteria showed that carvacrol and thymol presented an additive effect when one of them was combined with a common respiratory antibiotic. Additive effect was also observed when a second antibiotic was combined with thymol. Synergism was observed when carvacrol was combined with the second antibiotic or with thymol. This evidence is especially intriguing for the subject of antimicrobial stewardship by allowing smaller amounts of pharmaceutical antibiotics to be used in treatments.

Cryptosporidium parvum (Crypto) is a parasite that commonly causes diarrhea in calves around 10 days old. Researchers at the University of Illinois found that treatment with OEO bioactives after invasion reduced relative C. parvum infectivity in a dose-dependent manner. Their findings indicate that oregano is an all-natural way to counteract C. parvum infection in children.  While no direct evidence has been shown for the effectiveness of OEO against eimeria (coccidia – another intestinal parasite), this study may infer benefit for the prevention or treatment of cryptosporidiosis in calves.

Oregano is a phytonutrient that is gaining popularity in animal production as producers search for alternatives to traditional antibiotics. Ultimately, reducing total antibiotic use in animal production will foster stewardship and alleviate fears of bacterial resistance. Research is building on the effectiveness of this non-pharma, plant-based compound and how it can influence growth, enhance immune status and prevent or treat disease. With this evidence, it makes sense to evaluate OEOs for incorporation into your farm’s protocols, where they can improve animal health and welfare. As always, work with your trusted advisers and choose products that have science and research supporting the recommended application.  end mark

ILLUSTRATION: Illustration by Corey Lewis.

Zach Janssen
  • Zach Janssen

  • Bovine Technical Services Veterinarian
  • TechMix LLC