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Similarly, the weight gain and feed conversion were not sta- tistically better for broccoli extract compared to the rest of the essential oils. The order for weight gain was as follows: thyme oil > broccoli extract > rosemary oil > oregano oil > turmeric oil. The authors concluded that since both weight gain and feed conversion are usually positively associated with general animal health, it can be suggested that phyto- genic feed additives do not merely act as growth promoters. Their suggested that the exclusive addition of phytogenic substances as growth promoters to animal feed may be less effective than their combinations with feed enzymes or pro- biotics. In their study, broilers were not subjected to any pathogenic or toxic substance challenge. However, under practical feeding conditions which usually involve high stock- ing rates and sub-optimal health conditions, it can be as- sumed that the threat of infections and the challenge of feed contaminants will increase; in that case, the antimicrobial na- ture of most phytogenic feed additives would prove benefi- cial. For example, in another study, performance parameters improved when turmeric oil and oregano oil were added to broiler diets when the birds were challenged with aflatoxin B1 and Eimeria tenella, respectively.


Antioxidant capacity The antioxidant activity of broccoli and essential oils was sim- ilar, but higher compared to the non-additive diet; this was seen through higher activities of superoxide dismutase and glutathione peroxidase enzymes. The mRNA overall expres- sion of xenobiotic and antioxidant enzymes in the GIT was higher for both the broccoli and the essential oil diets compared to a diet without any additive.


Since the main active compound of broccoli, isothiocyanate sulforaphane, has no direct antioxidant properties, the


antioxidant activity of broccoli extract can be assumed to be through its impact on the induction of intestinal ARE (antioxi- dant response element)-regulated xenobiotic and antioxidant enzymes. On the other hand, essential oils are shown to have antioxidant properties as they contain compounds such as polyphenolics, alkaloids, lignans, flavonoids and terpenoids. These compounds neutralise superoxide, hydrogen peroxide and nitric oxide by scavenging radicals or by increasing the production of enzymes such as catalase, superoxide dismutase and glutathione peroxidase.


Xenobiotic enzyme regulation In the jejunum, broccoli extract, turmeric and rosemary oils increased the expression of xenobiotic enzymes (epoxide hydrolases 1 and 2 and aflatoxin B1 aldehyde reductase) more than oregano and thyme oils.


Remarks • The exclusive addition of phytogenic substances as growth promoters to broilers may be less effective than combina- tions with feed enzymes or probiotics. However, in practical conditions where there are likely to be sub-optimal health conditions or presence of pathogens, phytogenic additives may be beneficial, owing to their antimicrobial properties.


• Although the mechanisms may differ, the antioxidant ca- pacity of essential oils (thyme oil, rosemary oil, oregano oil and turmeric oil) is no better when compared to broccoli extract. Broccoli extract acts as a modulator of ARE-regu- lated genes, inducing intestinal ARE-regulated xenobiotic and antioxidant enzymes.


• With regard to antioxidant capacity and its effect on the performance of broilers, broccoli extract can replace the use of essential oils in broiler diets without suppressing performance.


▶ ALL ABOUT FEED | Volume 28, No. 3, 2020 13


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