Plant extracts refer to a class of active substances derived from plants, which have one or more biological functions. They have various functions when added to feedstuffs, including promoting growth, antioxidant, antiviral, and immune enhancement, etc. They have no residues in livestock products and drug resistance of pathogenic bacteria is not easily produced in animals.

Plant essential oil is a kind of aromatic oily liquid extracted from plant tissues and organs. Its main biologically active components include terpenoid mixtures and many low molecular weight aliphatic hydrocarbons. Essential oil can remove free radicals in vivo and play an antioxidant role. At the same time, it also has the functions of regulating intestinal flora balance, killing pathogens and promoting digestive juice secretion.

Flavonoids are a subclass of polyphenols, which can increase the turnover rate of proteins and have anti-inflammatory and anti-oxidative functions.

Plant tannins, also known as plant polyphenols, are a class of polyphenols widely existing in plants. Tannin has long been considered an antinutrient. Tannins in feed bind to protein, which reduces the degradation rate of protein in feed.

The growth-promoting mechanism of plant extracts may include the following aspects:

First, improve feed palatability to increase animal feed intake.

Secondly, plant extracts can increase the secretion and activity of endogenous enzymes, improve intestinal microflora, and improve feed nutrient utilization.

Thirdly, it affects the transformation and utilization of nutrients in vivo after absorption, and improves the transformation of energy for growth. Bravo et al. added 100 g/t mixture of plant extracts containing 5% carvacrol, 3% cinnamaldehyde and 2% capsaicin in corn-soybean meal diet could significantly increase weight gain of broilers, improve feed utilization, increase carcass energy retention and reduce calorie loss.

Last, regulate growth-related hormone secretion and promote animal production performance. Insulin-like growth factor (IGF) can promote faster intestinal development. Ariza-Nieto et al. found that adding 0.01% thymol and cinnamaldehyde (18%) in the diet of weaned piglets could significantly increase the content of IGF-1 in plasma of weaned piglets. In the meantime, the weight gain of weaned piglets was also significantly increased, and the production performance was improved.

Polyphenols in plants have antioxidant effects. There are many kinds of polyphenols, including hydroxybenzoic acid, hydroxycinnamic acid, anthocyanin, lignans, flavonols, flavonoids and so on.

Lahucky et al. found that adding 60mL oregano extract into pig diets can improve the antioxidant capacity of muscles. Placha et al. studied the effect of thyme essential oil on antioxidant performance of broilers. It was found that adding 0.5g/kg thyme essential oil to the diet could reduce the content of MDA in duodenal mucosa and kidney and improve the integrity of intestinal function.