Introduction

In recent years, demand for livestock products has increased due to an elevation in world population growth (Thornton 2010). The regulation of feed intake in ruminants is a critical factor for livestock’s growth performance and economic returns. Therefore, there are several strategies to increase the energy intake in ruminants. Both central and peripheral nervous systems are engaged in feed intake, and studying the underlying mechanism affecting appetite in animals showed that leptin and ghrelin were two important hormones to be considered (Frederich et al. 1995). Leptin and ghrelin act as regulatory hormones for feed intake and are secreted peripherally but control the appetite through the central nervous system (CNS) (Schwartz 2001). Hypothalamus has several receptors for both leptin and ghrelin. The activation of these receptors results in various pathways demonstrated by a significant alteration in feed intake (Pralong and Gaillard 2001; Sahu 2003). Based on the vital roles of these two hormones in controlling appetite and feed intake, an agent with the ability to control these regulatory hormones could be beneficial. Antibiotics have proven to have some effect on the feed intake in livestock. They are not only used to control the appetite but also they have attracted considerable attention due to their effect on eliminating several disorders including metabolic diseases. Due to the high chances of antibiotic resistance, considerable effort has been devoted towards developing alternatives to antibiotics (Benchaar et al. 2008). In this respect, plant extracts with antibacterial effect have been suggested as a potential alternative. Using a plant which has the ability to control the appetite of ruminants and possesses biological properties, such as antibacterial and antioxidant activities, can be valuable.

Onion (Allium cepa L.) is one of these plants that have several biological activities such as antibacterial, anti-mutagenic, and antioxidant activities (Dini et al. 2008; Griffiths et al. 2002; Singh et al. 2009). Onions are also a rich source of polyphenols, anthocyanins, flavonoids, glycosides, quercetin, and allicin (Ye et al. 2013). Onion extract is a rich source of flavonoids and other phenolic compounds such as quercetin, allicin, camphorol, gallic acid, caffeic acid, vanillic acid, para-coumaric acid, and salicylic acid, which have a strong antioxidant property (Abdou Bouba et al. 2012; Gorinstein et al. 2008; Musekil et al. 2007). Although, excessive intake of onions leads to hemolytic anemia and poisoning in domestic animals (Aslani et al. 2005; Keyvanlou et al. 2011), but reports have shown that hemolytic anemia occurs in dogs, cats, and horses earlier than ruminants. Moreover, among livestock, cattle are more susceptible to the onion toxicity than horses, sheep, and goats (Radostits et al. 2007). A study looking at onion feeding in cattle showed that feeding up to 25% cull onions on a dry matter basis resulted in mild decreases in red blood cells (RBCs), hemoglobin (Hb), and packed cell volume (PCV) but did not result in clinical anemia (Lincoln et al. 1992). Sheep can also be maintained on diets containing up to 50% (DM) onion bulbs with no clinical abnormality, and onions can be fed safely to sheep with weight gains comparable with those from whole sorghum grain (Fredrickson et al. 1995). The biological activities of onion along with its high dose of safety make it a suitable option to be used in the diet of ruminants. Therefore, this study was conducted to evaluate the effects of onion extract on leptin, ghrelin, total antioxidant capacity (TAC), and performance in Torki Qashqai suckling lambs.

Materials and methods

Preparation of onion extract

To prepare a hydroalcoholic extract of onions, 500 ml of ethanol (70%) was added to 50 g of dried onion powder. The solution was placed in a dark room for 24 h. The extract was then filtrated with Whatman No. 2 paper. Solvent separation from the extract was performed by a rotary evaporator and a vacuum pump at 50 °C. The extract was then freeze-dried and stored at 4 °C.

Animals and blood sampling

Eighteen Torki Qashqai suckling lambs with 30 ± 3 days of age and with the body weight of 10.3 ± 0.3 kg were used in this experiment. This research was performed at the breeding station of Shiraz University, School of Veterinary Medicine, Shiraz, Iran, during the Iranian summer 2017. The animals were randomly divided into three groups and were treated with diets in 60-days trial as follows:

  1. 1.

    Control group; basement diet without onion extract + sheep milk

  2. 2.

    T150 group; basement diet containing 150 mg/kg onion extract + sheep milk

  3. 3.

    T250 group; basement diet containing 250 mg/kg onion extract + sheep milk

Basement diets were adjusted according to the requirements of NRC (1994) by the UFFDA software (Table 1). In order to adapt lambs, basement diet was provided to the lambs for a week prior to the experiment. The animals were placed in individual pens and fed twice a day (morning and afternoon) without limitation in feed and water. The amount of extract for each lamb was daily prepared and administrated through oral gavage.

Table 1 The ratio of food, nutrients, and energy of the starting concentrate used in the experiment (NRC 1994)
Full size table

Each lamb was weighed at the beginning of the study and then weighted every week before the morning feeding.

Lamb performance characteristics, including daily weight gain, daily feed intake, and feed conversion rates, were measured for 60 days.

Blood samples were collected in the morning before feeding the animals from the jugular vein on days 0, 30, and 60 of the trial. The samples were collected into tubes with EDTA and centrifuged at 750g for 10 min. Then, the plasma was pipetted into different aliquots and stored at − 20 °C for further studies.

Biochemical analysis

Leptin and ghrelin measurement

Plasma levels of leptin and ghrelin were measured by a quantitative sandwich enzyme immunoassay using commercial sheep-specific kits (Shanghai Crystal Day Biotech, Shanghai, China).

Total antioxidant capacity measurement

The commercial kit (Zell Bio Company, Germany) was used to measure TAC. The color product of the chromogenic substrate (tetramethylbenzidine) appeared and the change in the color was measured calorimetrically at 450 nm and expressed as millimoles per liter (mmol/l).

Statistical analysis

Data were analyzed by One-Way ANOVA, followed by Duncan multiple range test by SAS software version 2003. The significance level was considered (P < 0.05).

Results

Table 2 shows the results for leptin, ghrelin, and TAC in three different groups.

Table 2 The effect of onion extract on the level of leptin (ng/ml), ghrelin (pg/ml), and TAC (mmol/l) in the plasma of lambs
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The amount of leptin on days 30 and 60 was significantly lower in the onion extract treated groups (T150 and T250) compared with the control group (P < 0.05), but there was no significant difference between treatments of 250 mg/kg and 150 mg/kg of onion extract.

Plasma level of ghrelin on day 30 showed a significant increase in T250 group in comparison with the control group. However, there was no significant difference among T150 and control group. On the 60th day, the amount of ghrelin in T250 group was also significantly higher than that of the control treatments (P < 0.05).

The level of TAC showed no significant difference among groups after 30 days of intervention. On the 60th day, the level of TAC was significantly increased in the T250 group compared with the control group. However, there was no significant difference between group T150 and control after 60 days of intervention.

Table 3 shows the effect of onion treatments on the performance of lambs, including weight gain, feed intake, and food conversion ratio in different periods. The results showed that the use of 250 mg/kg onion extract significantly increased body weight during the flowering period (P < 0.05). However, no significant difference was observed between control treatment and T150 group. Feed intake of T250 group was significantly higher than that of both the control and T150 group (P ˂ 0.05). In addition, the diet intervention had no significant effect on food conversion ratio.

Table 3 The effect of onion extract on daily weight gain (gr), daily feed intake, and food conversion ratio on lambs
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Discussion

The control of feed intake is an important factor in formulating diets to increase health, productivity, and efficiency of nutrient utilization in ruminants (Benchaar et al. 2008). The application of antibiotics in animals feed represents a high risk to human health. Therefore, it is critical to recognize alternative agents as to minimize resistant to antibiotics (Landers et al. 2012). The use of plant extracts, as feed additives, has attracted considerable attention in recent years because of its capability to enhance body weight and feed intake in ruminants.

The results of this study showed that the consumption of onion extract reduced the level of serum leptin in lambs. Chan et al. (2003) stated that the decrease in serum glucose levels caused the lipid cells to produce less leptin. Polyphenols are considered as an anti-obesity compound and are able to reduce levels of lipids and glucose in the blood, leading to lower levels of leptin (Boque et al. 2013). Since onion is a rich source of polyphenols, it is suggested that the onion extract could actually decrease serum leptin levels by affecting serum glucose level (Stajner and Varga 2003). According to Table 3, the animals receiving onion extract showed a significant increase in their body weight (P < 0.05). Rouru et al. reported that the decrease in the secretion of leptin hormone could cause obesity (Rouru et al. 1999). Moreover, leptin hormone can reduce appetite by binding to specific receptors in the hypothalamus and inhibiting the neuropeptide Y secretion (Hebert et al. 2005). The results of the present study showed that adding onion extract to the diet of lambs increased the level of serum ghrelin. Ghrelin is a hormone that stimulates appetite by affecting neuropeptide Y and agouti-related peptide (AGRP) (Cummings and Shannon 2003; Henry 2003). Also, ghrelin hormone increases growth hormone, secretion of gastric acid, polypeptidase, and gastrointestinal movements (Ahmed and Harvey 2002; Bhanja et al. 2007). There are proven records which show that ghrelin hormone could increase the appetite by a semi-motilin effect in the gastrointestinal tract (Kitazawa et al. 2007). In the present study, the increase in the feed intake at the treatment groups (T150 and T250) could be due to the effect of onion extract in reducing the leptin hormone and increasing the secretion of the ghrelin hormone.

Oxidative stress is the primary cause of many metabolic diseases in ruminants. The oxidative stress status is an important factor to be considered in livestock since it directly affects growth performance and health issues (El-Far et al. 2014). Therefore, the addition of antioxidants to the diet of animals can be used as a palliative or preventive treatment (Castillo et al. 2013). In the present experiment, the consumption of onion extract was able to increase serum TAC in lambs. Several studies have reported the anti-oxidant effects of Liliaceae plants. Allium family plants are a great source of phenolic and flavonoid compounds (Tepe et al. 2005). The antioxidant properties of phenolic compounds are mostly due to their ability to interrupt free radical chain reactions (Rice-Evan et al. 1996). Moreover, various reports have shown the effect of polyphenols, such as green tea, on the improvement of oxidative stress status in animals (Serafini et al. 1994).

On the other hand, the results of the present study showed an improvement of livestock growth performance in lambs during the onions consumption. In the research done by adding onion powder (60 g/day) to sheep diet for 20 days significantly increased the body weight of animals (Mehlhorn et al. 2011). Moreover, it has been reported that adding onion to broiler diets could increase the feed intake and improve body weight (Goodarzi et al. 2013). In a study by Ghosh et al., the daily intake of 250 mg/kg of garlic extract significantly improved the body weight and feed intake in infant calves (Ghosh et al. 2010). In addition, Chaves et al. (2008) reported that some herbal extracts had the ability to inhibit the growth of undesirable bacteria, and by thinning, the intestinal wall, they improved the absorption of nutrients. Therefore, they could enhance the weight and growth in ruminants. Based on the results of the present study, it is suggested that adding onion extract to the diet of lambs may improve the digestive system by inhibiting the reproduction of pathogenic bacteria due to the presence of sulfur and allicin compounds (Goodarzi et al. 2014). Moreover, the increase in feed intake and daily weight in lambs can be attributed to the role of onion extracts in reducing leptin hormone and increasing ghrelin hormone, both of which contribute to an increased appetite. The use of dietary antibiotics has resulted in controversial problems for consumers such as development of antibiotic resistant bacteria (Nasir and Grashorn 2006), and drug residue in the final products (Frankic et al. 2009). Therefore, consumption of antibiotics as a growth promoter is no longer acceptable and it is forbidden in European Union countries. As a consequence, it has become necessary to develop alternative substances and strategies for animal growth promotion and disease prevention. Similar to antibiotics, herbs and phytogenic products could control and limit the growth and colonization variety of pathogenic and nonpathogenic species of bacteria in gut. The use of herbs and phytogenic product can lead to a greater efficiency in the utilization of feed, resulting in increased growth and improved feed efficiency (Bedford 2000). Onion can be used as natural preservatives to control infection due to its high antibacterial activity, (Pszczola 2002). Onion has been known as a source of bioactive phenolic compounds and contains high organic sulfur compounds (Corzo-Martinez et al. 2007). The latter act similar to antibiotics and have the ability to reduce the growth of some harmful bacteria in the gastrointestinal tract (Bedford 2000).

Due to the undesirable problems and side effects arisen from the consumption of antibiotics and artificial chemical compounds, the essential oils and extracts from various plant species, especially edible and medicinal ones have attained appreciable interest among the research community. The results of the present study indicate that onion extract may be able to use as natural alternative as an in-feed antibiotic. In conclusion, the onion extract has a promising effect as a feed additive in ruminant nutrition and has the ability to improve feed intake and growth performance while it also can help these animals to gain weight faster.