The aim of this study was to investigate the effects of different combinations of molasses levels and slow-release urea on Holstein fattening bulls. Sixty Holstein fattening bulls of a similar age, weight, and health status were randomly divided into four groups of fifteen Holstein fattening bulls each. All of the treatments were as follows: (1) basic diet group (CON); (2) 2% molasses slow-release urea group (LMU); (3) 4% molasses slow-release urea group (MMU); and (4) 6% molasses slow-release urea group (HMU). The results of the study showed that the MMU had an outstanding performance, with a 13.3% increase in average daily weight gain compared with the control group, a significant decrease in feed conversion ratio (p < 0.05), and a significant increase in apparent digestibility of crude protein (p < 0.05). In terms of serum biochemical indices, blood ammonia and alanine aminotransferase (ALT) concentrations were significantly higher in the MMU than in the CON (p < 0.05). The rumen pH of all treatment groups was lower than that of the CON (p < 0.05), whereas the concentrations of microbial crude protein (MCP), as well as acetic acid, propionic acid, and total volatile fatty acids (TVFA) were significantly higher in both the MMU and HMU (p < 0.05). The dominant phyla in each group were Bacteroidetes, Firmicutes, and Patescibacteria, and the relative abundance of Bacteroidetes in the MMU increased by 5.47% compared with that in the CON. In the MMU, Prevotellaceae, Lachnospiraceae, and Ruminococcaceae increased by 9.03%, 0.67%, and 3.43%, respectively, compared with the CON. The economic benefit analysis showed that the daily feeding cost of fattened cattle in the MMU was reduced by RMB 1.62 yuan, and the daily farming benefit of each cow was increased by RMB 7.19 yuan. In conclusion, the MMU was effective in improving the growth performance of fattening cows, optimizing rumen fermentation, reducing cost, and increasing profit, which is a nutritional strategy with great application value.

This study aimed to evaluate the effects of eight plant essential oils on in vitro ruminal fermentation. The oils tested were verbena (VE), cinnamon (CE), rosemary (RE), Torreya grandis (TG), ginger (GE), fennel (SF), thyme (TE), and oregano (OE). Each was supplemented at five graded doses (0, 0.75, 1.5, 3, and 6% of the substrate DM) during 24 h incubations. The results demonstrated that adding 3% CE significantly increased the in vitro dry matter degradation rate (IVDMD) while enhancing microbial protein (MCP) synthesis efficiency and acetate concentration. The addition of 1.5% SF increased MCP and total volatile fatty acid (TVFA) concentrations without affecting ruminal ammonia nitrogen levels, and 0.75% TE enhanced nitrogen utilization efficiency while maintaining primary fermentation pathways and ammonia nitrogen levels. Although 0.75% OE did not alter overall fermentation intensity or nitrogen metabolism, it specifically regulated volatile fatty acid metabolic pathways, significantly promoting butyrate production. Through preliminary screening, four essential oils (CE, SF, TE, OE) that positively influenced rumen fermentation were selected and combined into five distinct blends: T1 (CE + SF + TE), T2 (CE + SF + OE), T3 (CE + TE + OE), T4 (SF + TE + OE), and T5 (CE + SF + TE + OE). In vitro 24 h fermentations with varying inclusion levels (0, 0.75, 1.5, 3% of the substrate DM), in comparison with the CON, showed that the T2 group significantly outperformed the other four formulations in nitrogen metabolism and IVDMD, while substantially reducing methane content. Simpson’s index of microbial diversity was markedly elevated, alongside an increased relative abundance of key bacterial genera—Selenomonas and Succinibacter. Considering comprehensive performance metrics and economic viability, the 1.5% T2 was selected as the optimal formulation.

This study investigated the effects of supplementing rumen-protected lysine (RPL; 60 g/d) and methionine (RPM; 30 g/d) on nutrient digestibility, rumen fermentation, and microbial composition in 30 early-lactation Holstein dairy cows under both heat stress (HS) and non-heat stress (NHS) conditions. Cows were paired based on parity, milk yield, and body weight, and then assigned to one of two dietary treatments: the control group (CON), receiving the basal diet only, or the RPLM group, which received the basal diet supplemented with 60 g/d of RPL and 30 g/d of RPM (n = 15 per group). All animals underwent a transition from HS to NHS conditions during the experimental period, allowing for within-animal comparison across thermal environments. Results demonstrated that RPLM supplementation significantly improved the apparent digestibility of dry matter (DM), crude protein (CP), and neutral detergent fiber (NDF) (p < 0.05), with a significant treatment × environment interaction observed for DM digestibility (p < 0.01). Under HS conditions, RPLM reduced ruminal NH₃-N concentration (p < 0.05) and increased total volatile fatty acids (TVFA), acetate, and butyrate levels (p < 0.01), suggesting enhanced nitrogen utilization and energy metabolism. No significant effects were observed on ruminal pH or the acetate-to-propionate ratio. 16S rRNA sequencing revealed that RPLM supplementation increased the relative abundance of fiber-degrading and butyrate-producing taxa, including NK4A214_group, Christensenellaceae_R-7_group, norank_f__Muribaculaceae, and norank_f__F082. These microbial taxa exhibited strong positive correlations with DM and CP digestibility (p < 0.001). LEfSe analysis further confirmed their status as key microbial biomarkers in the RPLM-NHS group. Collectively, concurrent supplementation of RPL and RPM modulates the ruminal microbiota, alleviates HS-induced reductions in digestive efficiency, and enhances overall nutrient utilization in dairy cows.