Meta-analysis of the relationship between dietary condensed tannins and methane emissions by cattle

Abstract

Using condensed tannins (CT) in ruminant nutrition yields many benefits, especially regarding better dietary protein utilization and decreased methane (CH4) emission by altering ruminal fermentation kinetics. A meta-analysis was conducted to develop and analyze a database from published studies and verify the effects of CT inclusion on enteric CH4 emission by cattle, nutrient digestibility and ruminal parameters, and their interactions. The study aimed to develop regression equations based on the significant components and their interactions to target a specific ruminal CH4 mitigation when correctly associating CT inclusion level and diet composition. A database was developed from 40 published studies that measured cattle CH4 emission using in vitro or in vivo methods and reported the source and inclusion levels of CT as well as typical diet characteristics, including crude protein (CP), organic matter (OM), neutral detergent fiber (NDF), and ether extract (EE). Other related parameters were also included in the database when reported, such as dry matter intake (DMI) and digestibility, OM digestibility, NDF digestibility, and ruminal parameters, including pH, ruminal ammonia nitrogen, total volatile fatty acids (VFA), acetate, propionate, butyrate, and the acetate-to-propionate ratio. The meta-analysis was conducted by multiple linear regression, using the nlme package of R 4.2.1, in which studies were assumed as random variables affecting the intercept and slopes. The CH4 emission, expressed in L/kg of DMI, decreased with increasing levels of dietary CT when in vitro and in vivo were analyzed together (P < 0.001) or in vivo alone (P < 0.001). Total VFA concentration (P = 0.049), propionate (P = 0.042), and butyrate (P = 0.047) increased with increasing levels of dietary CT on in vitro + in vivo methods, suggesting improved fermentability. When analyzing all chemical components of the diet, OM (P = 0.043) and CP (P = 0.010) decreased CH4 production with in vitro methods, EE decreased CH4 production on in vivo method (P = 0.001), while dietary NDF increased CH4 production on in vitro + in vivo (P = 0.016), and also separately for in vivo (P = 0.058) and in vitro method (P = 0.022). There was an interaction between CT and CP for in vivo experiments (P = 0.010): CH4 = 35.64 – 6.87 ×CT – 0.49 ×CP + 0.33 ×CT*CP. Across many experiments, increasing the level of dietary CT led to a decrease in ruminal CH4 concentrations for in vitro and in vivo fermentation conditions and an increase in molar proportions of propionate, butyrate, and total VFA, indicating that it is an efficient natural alternative to reduce environmental impacts of ruminant production systems possibly. Our meta-analysis identified diet components that significantly affect enteric CH4 emissions by cattle and developed a prediction model based on these components, including OM, CP, NDF, and EE. The in vivo CH4 method showed that an interaction between CT and CP diet content affects CH4 emissions. Future research and meta-analysis must focus on CT biological activity along with percent content to dictate ideal diet inclusion to suppress CH4 emissions without depressing animal performance.