Effect of Guanidinoacetic Acid Supplementation on Growth Performance, Rumen Fermentation, Blood Indices, Nutrient Digestion, and Nitrogen Metabolism in Angus Steers

Simple Summary This study investigated the effects of guanidinoacetic acid (GAA) supplementation on the growth performance, rumen fermentation, blood indices, nutrient digestion, and nitrogen metabolism of Angus steers. In a 130-day feeding experiment, steers receiving GAA at a conventional dose (0.8 g/kg) and a high dose (1.6 g/kg) exhibited significantly higher average daily weight gain and improved feed conversion efficiency compared to the control group without GAA. GAA supplementation also influenced rumen fermentation, with lower acetate levels, higher propionate levels, and higher acetate: propionate ratio in GAA groups. Blood analyses revealed elevated concentrations of urea, blood ammonia, GAA, creatine, and catalase, indicating improved creatine metabolism and antioxidant activity. A subsequent 3-day digestive metabolism experiment confirmed that the GAA-supplemented groups had increased dry matter and crude protein digestibility, as well as increased N retention and improved organismal nitrogen metabolism. In summary, dietary GAA supplementation at a 0.8 g/kg and 1.6 g/kg DM basis positively influenced growth performance, rumen fermentation, blood indices, nutrient digestion, and nitrogen metabolism in Angus steers, emphasizing its potential as a nutritional strategy. Abstract Guanidinoacetic acid (GAA) functions as a precursor for creatine synthesis in the animal body, and maintaining ample creatine reserves is essential for fostering rapid growth. This study aimed to explore the impact of GAA supplementation on growth performance, rumen fermentation, blood indices, nutrient digestion, and nitrogen metabolism in Angus steers through two experiments: a feeding experiment (Experiment 1) and a digestive metabolism experiment (Experiment 2). In Experiment 1, thirty-six Angus steers (485.64 ± 39.41 kg of BW) at 16 months of age were randomly assigned to three groups: control (CON), a conventional dose of GAA (CGAA, 0.8 g/kg), and a high dose of GAA (HGAA, 1.6 g/kg), each with twelve steers. The adaptation period lasted 14 days, and the test period was 130 days. Weighing occurred before morning feeding on days 0, 65, and 130, with rumen fluid and blood collected before morning feeding on day 130. Experiment 2 involved fifteen 18-month-old Angus steers (575.60 ± 7.78 kg of BW) randomly assigned to the same three groups as in Experiment 1, with a 7-day adaptation period and a 3-day test period. Fecal and urine samples were collected from all steers during this period. Results showed a significantly higher average daily gain (ADG) in the CGAA and HGAA groups compared to the CON group (p = 0.043). Additionally, the feed conversion efficiency (FCE) was significantly higher in the CGAA and HGAA groups than in the CON group (p = 0.018). The concentrations of acetate and the acetate:propionate ratio were significantly lower in the CGAA and HGAA groups, while propionate concentration was significantly higher (p < 0.01). Serum concentration of urea (UREA), blood ammonia (BA), GAA, creatine, and catalase (CAT) in the CGAA and HGAA groups were significantly higher than in the CON group, whereas malondialdehyde (MDA) concentrations were significantly lower (p < 0.05). Digestibility of dry matter (DM) and crude protein (CP) and the nitrogen retention ratio were significantly higher in the CGAA and HGAA groups than in the CON group (p < 0.05). In conclusion, dietary addition of both 0.8 g/kg and 1.6 g/kg of GAA increased growth performance, regulated rumen fermentation and blood indices, and improved digestibility and nitrogen metabolism in Angus steers. However, higher doses of GAA did not demonstrate a linear stacking effect.