The effect of feeding a mix of condensed and hydrolyzable tannins to heifers on rumen fermentation patterns, blood urea nitrogen, and amino acid profile

•The use of mixed and hydrolyzable tannins reduced methanogen relative abundance compared to a control.•The use of mixed and hydrolyzable tannins increased the relative abundance of propionate concentration in the rumen.•The use of mixed and hydrolyzable tannins reduced blood urea nitrogen content compared to a control.•Reductions in both methanogen relative abundance and increased propionate concentration, as well as less blood urea nitrogen content, are likely to reduce heifers environmental effects.