PS2-25. Effects of Dietary Guanidinoacetic Acid Supplementation on Growth Performance and Carcass Traits of Feedlot Nellore Cattle.

Abstract Guanidinoacetic acid (GAA) is a key precursor of creatine in skeletal muscle, where it plays an important role in ATP regeneration and protein synthesis. Therefore, the objective of this study was to evaluate the effects of dietary GAA supplementation on growth performance and carcass traits of feedlot Nellore cattle. The experiment was conducted at the COMIGO Technological Center (Rio Verde, Goiás, Brazil). Ninety-six Nellore bulls (330.1 ± 2.37 kg initial BW) were randomly allocated to 16 feedlot pens (6 animals per pen; experimental unit) in a completely randomized design. Pens were assigned to 1 of 2 treatments, with 8 replicate pens per treatment: control (CON; no GAA) or GAA supplementation (625.2 mg/kg DM). The 100-d experimental period was preceded by a 21-d step-up dietary adaptation period, during which no treatments were applied. Experimental diets were offered twice daily (0800 and 1500 h) and consisted of 25% corn silage and 75% commercial concentrate (Rações COMIGO) on a DM basis. Monensin was included in all diets at 24.6 mg/kg DM. Animals were weighed at 20-d intervals, and ADG was estimated by regressing BW on time. Feed offered and refusals were recorded daily to determine DMI. Ultrasound measurements of longissimus dorsi muscle area (LMA) and backfat thickness (BFT) were obtained between the 12th and 13th ribs of each animal at the beginning and end of the experiment. All data were analyzed using the MIXED procedure of the SAS software. Performance data were subjected to analysis of variance, whereas carcass data were evaluated by analysis of covariance, with baseline values used as covariates. Final BW was similar for animals of both treatments (P = 0.225; 490.8 and 503.7 ± 5.15 kg for CON and GAA, respectively), resulting in similar ADG (P = 0.230; 1.60 and 1.70 ± 0.042 kg/d for CON and GAA, respectively). Moreover, although GAA inclusion did not affect DMI (P = 0.765; 10.14 and 10.06 ± 0.131 kg/d for CON and GAA, respectively), GAA-supplemented bulls had greater G:F (P = 0.032; 0.158 and 0.170 ± 0.0028 kg for CON and GAA, respectively). Regarding carcass traits, LMA did not differ between treatments (P = 0.445; 91.6 and 93.1 ± 0.98 cm2 for CON and GAA, respectively), whereas BFT was greater in bulls not supplemented with GAA (P = 0.011; 6.14 and 5.43 ± 0.140 mm for CON and GAA, respectively). In conclusion, although dietary GAA supplementation did not affect ADG or DMI, it improved feed efficiency of feedlot Nellore cattle, likely because of a shift in the composition of gain toward reduced fat deposition.