Guanidinoacetic acid and betaine supplementation have positive effects on growth performance, nutrient digestion and rumen fermentation in Angus bulls

•Guanidinoacetic acid (GAA) or betaine (BT) increased growth performance and rumen microbial populations in bulls.•Supplementing BT changed rumen fermentation pattern with greater acetate production.•Supplementing GAA at 0.6 g/kg DM did not affect blood concentrations of folate, homocysteine and methionine.•No additional effect on performance was found with both GAA and BT compared to GAA or BT alone.