Efficacy of guanidinoacetic acid supplementation to growing cattle and relative bioavailability of guanidinoacetic acid delivered ruminally or abomasally

Master of ScienceDepartment of Animal Sciences and IndustryEvan C. TitgemeyerTwo experiments were conducted to assess the value of guanidinoacetic acid (GAA) as a feed additive for growing cattle. The first experiment utilized 7 ruminally cannulated Holstein steers (280 ± 14 kg) in a 5 × 5 Latin square design and evaluated relative bioavailability of GAA. Treatments were continuous ruminal or abomasal infusion of 0, 10, or 20 g/d GAA, and blood and urine samples were collected on the final day of each period for analysis of creatine, creatinine, and GAA concentrations. Plasma and urinary creatine concentrations were used as the primary response criteria to calculate ruminal bypass value of GAA by slope-ratio methodology; values were 47% and 49%, respectively. In Exp. 2, effects of GAA supplementation on N retention and methionine (Met) methyl group flux in steers fed corn-based diets were determined utilizing 6 ruminally cannulated Holstein steers (256 ± 14 kg) in a 6 × 6 Latin square design. Factorial treatments were 2 levels of Met (0 or 5 g/d) and 3 levels of GAA (0, 7.5, or 15 g/d) delivered by continuous abomasal infusion. Periods were 10 d in length and included 3 d for total fecal and urine collections; blood samples were collected and Met flux was measured on the final day of each period. Nitrogen retention increased (P < 0.01) with Met supplementation and tended to decrease linearly (P = 0.12) with increasing amounts of GAA. The response in N retention suggests dietary Met was limiting. Methyl group flux tended to increase linearly (P = 0.10) with GAA provision and significantly increased (P < 0.01) with Met supplementation. A tendency (P = 0.10) for a GAA × Met interaction was also observed, because methyl group flux only increased in response to GAA when Met was supplemented. Plasma and urinary creatine concentrations linearly increased (P < 0.05 and P = 0.06, respectively) when GAA was supplemented. Urinary GAA concentrations increased (P < 0.01) with GAA supplementation. Plasma urea N also increased linearly (P < 0.05) with GAA supplementation, and no differences in plasma total amino acid concentrations were observed (P ≥ 0.29) across treatments. It can be concluded from these studies that GAA is degraded approximately 50% in the rumen and supplementation of GAA alone or with Met as a methyl donor in a corn-based diet did not improve protein deposition in growing steers