208 Effects of supplemental guanidinoacetic acid, creatine, and choline on protein deposition and methyl group metabolites in growing steers

Methyl group donors (e.g., methionine, choline) can improve performance in transition dairy cows. Our objective was to determine how modulation of methyl group status would affect protein deposition and plasma and urine metabolites associated with methyl group metabolism. Six ruminally cannulated Holstein steers (200 kg) were used in a 6×6 Latin square design with 10-d periods. Factorial treatments, which were continuously infused abomasally, included 3 methyl group modulators (MGM: control; 15 g/d guanidinoacetic acid [GAA]; or 16.8 g/d creatine) and 2 levels of choline (0 or 5 g/d choline ion). Providing GAA or creatine increases body creatine supply; however, GAA consumes methyl groups to synthesize creatine, whereas supplemental creatine directly spares methyl groups that would be used for its synthesis. Steers received 4 kg/d of a corn-based diet. Total collection of urine and feces occurred from d 7 to 9 to measure N retention. Jugular blood was collected on d 10. No interactions between MGM and choline were observed. GAA increased N retention (P = 0.04), whereas creatine did not. Urinary creatine excretion was increased by GAA and creatine supplementation (P < 0.0001). Urinary GAA output was greater for GAA than creatine and control (P < 0.0001). Plasma creatine concentration was increased by GAA and creatine supplementation (P ≤0.01), with GAA leading to a larger increase. Plasma GAA concentration increased with GAA provision (P < 0.0001) but tended to be decreased by creatine (P = 0.09). Plasma urea-N and urinary urea-N output were not affected by MGM (P ≥ 0.25). Choline did not affect N retention (P = 0.69) or urinary GAA or creatine excretions (P ≥ 0.14). Choline increased plasma creatine (P = 0.04), but not plasma GAA. Choline did not affect plasma urea-N or urinary urea-N output (P ≥ 0.68). These data demonstrate that GAA has potential to improve protein deposition in growing cattle. Additionally, choline provision may allow for greater creatine synthesis in the body.