Salvia miltiorrhiza (S.m.) is a perennial herbaceous medicinal plant 30 to 80 cm in height. Recently, clinical studies have confirmed that S.m. crude extract or active components had the ability to treat diabetic, cardiovascular, and neurodegenerative diseases [1–3]. The action mechanism of S.m. relies on the amelioration of oxidative stress by regulating antioxidant systems, such as superoxide dismutase, catalase, and glutathione [4]. The popularity of S.m. phytochemicals lead to a strong market demand and consequent large artificial planted area for obtaining its root while aerial parts including stems, leaves, and flowers are also worthy of investigation.

The significant pharmacological components extracted from S.m. can be mainly divided into two categories: fat-soluble tanshinone compounds and water-soluble salvianolic acids which are representative active ingredients in herbal medicine [5,6]. Given the function against oxidation, inflammation, allergy, and cancer and regulation of tissue repair and regeneration, whether the aerial parts of S.m. can be a forage with a growth-promoting effect in farm animals is unclear. Moreover, no literature reports the relationship between S.m., nutrient digestibility, and digestive enzyme activity.

Therefore, for the exploration of new forage resources and maximum utilization of by-products from the human consumption, the present study aimed to investigate the effect of S.m. aerial parts as an alternative ingredient on growth performance, nutrient digestibility, and activities of digestive enzymes in farm rabbits compared to controls with/without a growth-promoting antibiotic.

Root plants including S.m. are well-known for their root utilization but more than half of photosynthesis products are deposited in aerial leaves and stems which can be a good forage source for herbivores [13]. Especially under artificial planting, aerial parts of root crops have a high yield [5]. In the present study, mowing of S.m. was arranged at the last week of the flowering period that lasts approximate 6 months, thereby leading to less lignification, higher nutritional values reflecting on crude fiber and crude protein and a non-significant negative effect on root growth and quality (data not shown) from the serial pilot studies of authors. The proximate nutrient contents primarily indicate that S.m. aerial parts can be a forage in the rabbit feed.

The S.m. aerial parts at the three doses did not cause differences in ADFI and ADG of rabbits among treatments. Literature showed that tanshinone IIA increased ADG of broilers with pulmonary arterial hypertension [14]; Salvia root improved egg performance of hens [15]; however, salvianolic acid B prevented ADG in high-fat diet-induced obese mice [16]. These inconsistencies in feed consumption and growth by feeding aerial parts, root, or bioactive compounds of S.m. to animals need further studies. Interestingly, the feed efficiency in the herb at 9.0% was greater, demonstrating its beneficially physiological regulation. This may be due to the bioactive components of S.m. because tanshinones and salvianolic acids are capable against oxidative stress and inflammation [3,17]. Furthermore, the comparable ADG between the herb and the antibiotic implies the potential growth-promoting effect of S.m. aerial parts.

In the present study, in contrast with the control, supplementation of the antibiotic and the herb resulted in significant increases in the digestibility of nutrients of rabbits. The improved feed efficiency and nutrient digestibility may also be attributed to health benefits of tanshinones and salvianolic acids of S.m. Indeed, S.m. root improved intestinal barrier integrity, physiological metabolism, and amino acids absorption [18,19]. Paradoxically, in a rodent model, tanshinones and salvianolic acids depressed nutrient deposition and modulated gut microbiota [16,20]. Information is however unavailable about the effect of S.m. aerial parts on the growth performance and nutrient digestion in farm animals. In the present study, significant increases in nutrient digestibility of diets with the herb addition indicate its beneficial regulation on the digestive physiology of rabbits.

Indeed, the activities of digestive enzymes including α-amylase, sucrase, maltase, lipase, and trypsin were increased in diets containing S.m. aerial parts. However, high doses of flavonoids from Salvia chlorolueca aerial parts showed inhibitory effects on α-amylase and α-glucosidase [21]. Similarly, cryptotanshinone or tanshinone IIA from S.m. at 1% inhibited pancreatic lipase activity [22]. Jordan indigenous Salvia spp. extracts depressed gastrointestinal carbohydrate and lipid digestion and absorption [23]. Besides the flavonoid concentration, the underlying reason why the extracted flavonoids and their raw materials showed reverse effects on carbohydrases and lipase deserves further study, and molecular docking studies will be needed to establish whether a direct interaction of the principal constituents of the S.m. with digestive enzymes can be evoked. Importantly, the herb also improved protein digestion, but literature is unavailable about S.m. or its extracts on the activity of proteases, which needs further study. Additionally, with increasing threats from the environment and pathogens, intestinal antioxidant activity of farm animals becomes especially important [24–27], whether the dietary aerial parts of S.m. influence intestinal health is also an interesting topic.

Supplementation of S.m. aerial parts at the dose 9.0% decreased feed/gain compared to the control. The antibiotic exhibited significant improvement in the digestibility of dry matter, crude protein, energy, fiber, and ash, and similar results were found for the herb for most nutrients except for fiber which was lower than the antibiotic but similar to the control. The antibiotic and the herb had similar effects on the activities of α-amylase, sucrase, maltase, lipase, and trypsin. The results suggest that S.m. aerial parts can be a versatile forage with nutritional and growth-promoting effects on herbivores.