Nutritive value of Adiantum capillus-veneris and Salvia officinalis L. forages and the effect of their dietary levels on in vitro digestibility, methane production, antioxidant capacity, and fermentation parameters

Introduction: Pastures provide an important part of ruminant fodder needs, and farmers store some pasture plants to use in winter-feeding of livestock. Therefore, having enough information about the nutritional value of such plants can help to optimize their consumption in animal feeding. Completing the information on the good-quality plants adapted to different environments will help to use and develop these valuable resources in a better way. In addition, it may be possible to improve rumen fermentation by aromatic pasture plants. Adiantum capillus-veneris (Adiantum) and Salvia officinalis L. (Salvia) are valuable genetic resources in pastures that are also used in livestock feeding. Due to the presence of plant secondary metabolites, they may improve rumen fermentation. However, there is little information on the nutritional value of Adiantum. Some information is available on the chemical composition of Salvia species (especially the leaves) and the effect of its essential oil on the rumen, but data on the use of the whole plant as forage are limited. Therefore, this study aimed to assess the nutritional value of Adiantum and Salvia compared to alfalfa, and the effect of including different levels of them in the diet on in vitro ruminal fermentation variables and digestibility.
Materials and methods: The experimental pasture plants (Adiantum and Salvia) were obtained in July, after flowering, from the pasture of Safrin village located in the rural district of Rajaeedasht in western Alamut (Qazvin, Iran). In experiment I, the chemical composition of the fodders was determined by the standard methods, considering alfalfa as the control forage. In experiment II, the effect of different dietary levels of the fodders was assessed using in vitro gas production technique with five treatments including 1. A diet without the rangeland plants (control diet), 2. A diet containing 15% of Adiantum, 3. A diet containing 30% of Adiantum, 4. A diet containing 15% of Salvia, and 5. A diet containing 30% of Salvia (on a DM basis). The in vitro digestibility, fermentation variables, microbial biomass production (MBP), total antioxidant power (TAP), protozoa numbers, and methane production were determined. The gas test was performed in three replicates and two runs, and data were analyzed using the GLM procedure of SAS.
Results and discussion: According to the results of experiment I, Adiantum and Salvia had higher ash compared to alfalfa (P<0.05). The crude protein and digestibility of Adiantum were lower than Salvia and alfalfa (P<0.05). The lower digestibility of Adiantum could be due to its higher lignin and ash and the lower crude protein concentration. Therefore, when consuming Adiantum in diets of high-productive ruminants, it is necessary to pay more attention to the use of energy supplements. Compared to alfalfa, the in vitro fermentation of Adiantum and Salvia resulted in higher in vitro ruminal MBP and TAP and lower ammonia-N and protozoa population (P<0.05), due to the presence of plant secondary metabolites such as phenolics, tannins, flavonoids, anthocyanins, and terpenes. The results of experiment II showed that the inclusion of Adiantum and Salvia in the diet had no significant (P>0.05) effect on in vitro gas production, organic matter digestibility, and metabolizable energy. Dietary use of Salvia, however, enhanced truly degraded substrate (P<0.05). Sometimes, high amounts of secondary metabolites in plants may have negative effects on the rumen but the amounts of metabolites in the pasture plants used in this study were not so high as to hurt rumen microbes and digestion. Dietary inclusion of Adiantum and Salvia improved in vitro TAP and decreased protozoa population, methane production, and acetate to propionate ratio (P<0.05). The lowest methane production was observed for the diets containing Salvia. The positive effect of the rangeland plants on in vitro TAP was because both plants contain significant amounts of various secondary metabolites (such as phenolic compounds, flavonoids, and terpenes) that are known to be favorable and high-capacity antioxidants. These plant metabolites can remove free radicals, bind transition metals (such as free iron), remove active oxygen from the environment, induce antioxidant enzymes, and reduce and moderate the conditions of oxidative stress and destruction. The reduction of methane release using Adiantum and Salvia in the diet was probably related to the fact that plant secondary metabolites destruct the protozoa or interfere with their metabolic pathways. In addition, they may inhibit methanogenic and hydrogen-consuming bacteria or hydrogen-producing Gram-positive bacteria. The dietary addition of Adiantum and Salvia improved in vitro ruminal MBP and decreased the ammonia-N concentration (P<0.05) compared to the control diet. The result could be due to the reduction of the protozoa population, causing a decrease in nitrogen recycling in the rumen and a decrease in bacteria predation. The formation of phenolic-protein complexes can also be another possible reason for reducing protein breakdown and ammonia concentration. Moreover, as rumen protozoa are reduced, the bacteria will have fewer natural predators and their population (reflected in the higher MBP) will increase, improving the degradation of feed ingredients. The inclusion of Adiantum and Salvia in the diet did not affect in vitro pH, total volatile fatty acids, and partitioning factor (P>0.05).
Conclusions: It is possible to include whole plants of Adiantum and Salvia in the diet, up to 30% DM, without adverse effects on digestibility. It also improves diet efficiency by increasing in vitro ruminal MBP and TAP and reducing methane and ammonia production.