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.

Guanidinoacetic acid (GAA) has been used in ruminant feeding, but it is still unclear whether the exogenous addition of methyl donors, such as methionine (Met), can enhance the effects of GAA. This study investigated the effects of dietary GAA alone or combined with Met on beef cattle growth performance and explored the underlying mechanisms via blood analysis, liver metabolomics, and transcriptomics. Forty-five Simmental bulls (453.43 ± 29.05 kg) were assigned to three groups for 140 days: CON (control), GAA (0.1% GAA), and GAM (0.1% GAA + 0.1% Met), where each group consisted of 15 bulls. Compared with the CON group, the average daily gain (ADG) and feed conversion efficiency (FCE) of the two feed additive groups were significantly increased, and the digestibility of neutral detergent fiber (NDF) was improved (p < 0.05). Among the three treatment groups, the GAM group showed a higher rumen total volatile fatty acids (TVFAs) content and digestibility of dry matter (DM) and crude protein (CP) in the beef cattle. The serum indices showed that the contents of indicators related to protein metabolism, lipid metabolism, and creatine metabolism showed different increases in the additive groups (p < 0.05). It is worth noting that the antioxidant indexes in the serum and liver tissues of beef cattle in the two additive groups were significantly improved (p < 0.05). The liver metabolites related to protein metabolism (e.g., L-asparagine, L-glutamic acid) and lipid metabolism (e.g., PC (17:0/0:0)) were elevated in two additive groups, where Met further enhanced the amino acid metabolism in GAM. In the two additive groups, transcriptomic profiling identified significant changes in the expression of genes associated with protein metabolism (including PIK3CD, AKT3, EIF4E, HDC, and SDS) and lipid metabolism (such as CD36, SCD5, ABCA1, APOC2, GPD2, and LPCAT2) in the hepatic tissues of cattle (p < 0.05). Overall, the GAA and Met supplementation enhanced the growth performance by improving the nutrient digestibility, serum protein and creatine metabolisms, antioxidant capacity, and hepatic energy and protein and lipid metabolisms. The inclusion of Met in the diet was shown to enhance the nutrient digestibility and promote more efficient amino acid metabolism within the liver of the beef cattle.

This study investigated lipid metabolism differences between low and high residual feed intake (LRFI and HRFI) bulls. Fifty-five Simmental bulls (449.84 ± 8.49 kg) were fed the same diet for 161 d to determine RFI values. At the end of experiment, back fat thickness (BFT) and rib eye area (REA) were measured via ultrasound, and serum samples were collected for lipid metabolism indicators. Post-slaughter, subcutaneous adipose tissue was analyzed for lipid metabolism indicators, histology, transcriptomics, and lipid metabolomics. Results 1) Ultrasound showed LRFI bulls had lower BFT (P = 0.031) but higher REA (P = 0.021). 2) Serum total cholesterol (P = 0.028) and low-density lipoprotein cholesterol (P = 0.015) were lower in LRFI bulls. Leptin levels were lower in both serum (P = 0.007) and adipose tissue (P = 0.001) in LRFI bulls, as were acetyl-CoA carboxylase levels in serum (P = 0.032) and adipose tissue (P = 0.043). Conversely, adipose triglyceride lipase levels were higher in both serum (P = 0.045) and adipose tissue (P = 0.043), while adipose tissue fatty acid synthase levels were lower (P = 0.035) in LRFI bulls. 3) Transcriptomics identified two pathways, adenosine monophosphate-activated protein kinase (AMPK) and cyclic adenosine monophosphate (cAMP), involving five lipid metabolism-related genes. Among these, adenylate cyclase activating polypeptide 1 receptor type 1 (ADCYAP1R1), adenylate cyclase 10 (ADCY10), and adenylate cyclase activating polypeptide 1 (ADCYAP1) were significantly upregulated (P < 0.05), while leptin receptor (LEPR) was downregulated (P = 0.047) in LRFI bulls. Additionally, leptin (LEP) showed a downward trend (P = 0.098) in LRFI bulls. 4) Lipid metabolomics revealed that the LRFI group had significantly higher levels of wax ester (WE) (P = 0.040) and significantly lower levels of monogalactosyldiacylglycerol (MGDG) (P = 0.040) compared to the HRFI group. Additionally, sphingomyelin (SPH) showed a trend towards lower abundance (P = 0.054), and C12:0 levels tended to be higher (P = 0.089) in the LRFI group. In conclusion, differences in the AMPK and cAMP signaling in adipose tissue are associated with reduced lipid synthesis and enhanced lipid degradation in LRFI bulls, which may contribute to reduced lipid deposition.