Effects of the different levels of pennyroyal powder (mentha pulegium L.) on production performance and quality traits of the eggs maintained under different storage conditions in old laying hen

Increased concerns about antibiotic resistance due to increased growth-promoting antibiotic inclusion in animal feed over the therapeutic dosage have been made scientists look for a proper alternative with similar benefits and without negative impacts on animal and human communities. Pennyroyal (Mentha pulegium L.) has been considered as a possible alternative for both antibiotics and synthetic antioxidants (Mohammadi, 2020). Although few studies have investigated the effects of this herbal plant on the production performance of laying hens and quality traits of eggs stored under different environmental conditions and storage times (Nobakht et al., 2011, Paymard et al., 2013), previous studies have been reported confusing information in this case (Aydın and Bölükbaşı, 2020). Most of these studies have reported reduced egg production due to the supplementation of pennyroyal powder to laying hens’ diet. Also, it has been reported that pennyroyal supplementation has less or no impact on egg quality traits. However, it has been suggested that pennyroyal might have strong antioxidant properties due to its composition of essential oils and/or phenolic compounds, such as pulegone as a dominant compound, which may affect the quality traits of eggs, especially those stored for a long period of time (Ruberto and Baratta, 2000). Therefore, the present study was designed to evaluate the effects of different levels of pennyroyal supplementation on the performance and egg quality traits of old laying hens that are highly prone to oxidative stress. In this experiment, one-hundred and twenty laying hens (Hy-line W36) with 53 weeks of age were provided farm the nearest commercial layer farm and distributed in three treatments, four replicate and eight birds per replicate with similar body weights (±5%) on a completely randomized design. Experimental treatments include 1) control diet (without pennyroyal supplementation), 2) 0.5%, and 3) 1% pennyroyal supplementation. In this study, two weeks were considered as an adaptation period and the birds fed with the control diet. The experiment lasted for eight weeks and birds fed the experimental treatments. The aerial parts of the pennyroyal were purchased fresh from a local store in West Azerbaijan, and their essential oil contents were measured by GC-mass spectrometry after air-drying and grinding. Chemical composition analysis has indicated that pulegone was the dominant component of the pennyroyal powder used in this study (17.11%). The present study consisted of two experiments; the first one for evaluating the production performance and egg quality traits, and the second one for evaluating the internal quality traits of the eggs stored under different storage times (7 or 30 days) and temperatures (4 or 25°C). Experimental diets based on the corn-soybean meal were formulated according to the nutritional recommendations of the Hy-line W36 strain and fed for eight weeks. The egg numbers and mean egg weight were recorded daily, and the feed conversion ratio, feed intake, and egg mass were calculated weekly. Two eggs were collected from each replicate to evaluate their quality traits at both experiments. Serum samples were obtained by blood sampling from the wings vein to evaluate serum biochemical parameters. The results of the present study indicated that pennyroyal supplementation has significantly reduced egg production and egg mass, and increased FCR (P<0.05). These findings are in agreement with the previous studies (Arjomandi et al., 2011, Nobakht et al., 2011, Paymard et al., 2013). According to the findings of the present study and previous investigations, the negative impacts of the pennyroyal supplementation might be due to the high content of pulegone (in this study 17.11%), which converts to menthofuran (a hepatotoxic composition) after being metabolized. Furthermore, pennyroyal supplementation significantly reduced the Haugh unit and increased albumen pH (P<0.05). These findings were in agreement with the study of Paymard et al. (2013) and disagreement with the other studies (Arjomandi et al., 2011, Nobakht et al., 2011); Since they have seen no impact of pennyroyal supplementation of egg quality traits. The probable reason for these negative impacts might be explained by the previous studies that investigated the other antioxidant alternatives, which have stated that polyphenolic compounds or essential oils of herbal plants could increase the albumen pH and therefore weakens the protein bonds between lysozyme and ovomucin of albumen (Ahn et al., 1999, Arpášová et al., 2014). Evaluation of the internal quality traits of the eggs stored under different times and temperatures indicated that the main effects of pennyroyal supplementation were not significant in desired parameters; however, the main effects of times on HU and ApH, and the main effects of different temperatures on YI, HU, ApH, and YH were significant (P<0.05). Additionally, significant interactions existed between pennyroyal levels and storage times in HU, YI, and YH (P<0.05). Also, Significant interactions were reported between pennyroyal levels and storage temperatures in ApH and YpH (P<0.05). Furthermore, significant interactions existed between storage times and temperatures in HU and YP (P<0.05). Though, no significant interactions were reported between all three factors. Almost no study has been conducted to evaluate the effects of pennyroyal supplementation on the internal quality traits of eggs stored under different times and temperatures. Therefore, according to the previous studies, oxidation of lipids in the egg's vitelline membranes, over time, increases the transfer of ions between the albumen and the yolk, which in turn increases the pH and ultimately reduces the Hough unit (Silversides et al., 2001). According to the findings of the present study, pennyroyal supplementation reduced production performance, egg quality traits, and internal quality traits of the stored eggs. Therefore, it is not recommended to use the pennyroyal powder supplementation in old laying hens’ diet to improve production performance and egg quality traits.