Milk is rich in a variety of essential nutrients, including fats, proteins, and trace elements that are important for human health. In particular, milk fat has an alleviating effect on diseases such as heart disease and diabetes. Fatty acids, the basic units of milk fat, play an important role in many biological reactions in the body, including the involvement of glycerophospholipids and sphingolipids in the formation of cell membranes. However, milk fat synthesis is a complex biological process involving multiple organs and tissues, and how to improve milk fat of dairy cows has been a hot research issue in the industry. There exists a close relationship between milk fat synthesis, genes, and microbial functions, as a result of the organic integration between the different tissues of the cow’s organism and the external environment. This review paper aims (1) to highlight the synthesis and regulation of milk fat by the first and second genomes (gastrointestinal microbial genome) and (2) to discuss the effects of ruminal microorganisms and host metabolites on milk fat synthesis. Through exploring the interactions between the first and second genomes, and discovering the relationship between microbial and host metabolite in the milk fat synthesis pathway, it may become a new direction for future research on the mechanism of milk fat synthesis in dairy cows.

Consumers are increasing their daily demand for beef and are becoming more discerning about its nutritional quality and flavor. The present objective was to evaluate how the ration energy content (combined net energy, Nemf) impacts the slaughter performance, carcass characteristics, and meat qualities of Honghe yellow cattle raised in confinement. Fifteen male Honghe yellow cattle were divided into three groups based on a one-way design: a low-energy group (LEG, 3.72 MJ/kg), a medium-energy group (MEG, 4.52 MJ/kg), and a high-energy group (HEG, 5.32 MJ/kg). After a period of 70 days on these treatments, the animals were slaughtered and their slaughter performance was determined, and the longissimus dorsi muscle (LD) and biceps femoris (BF) muscles were gathered to evaluate meat quality and composition. Increasing the dietary energy concentration led to marked improvements (p < 0.05) in the live weight before slaughter (LWBS), weight of carcass, backfat thickness, and loin muscle area. HEG also improved the yield of high-grade commercial cuts (13.47% vs. 10.39%) (p < 0.05). However, meat quality traits were not affected by treatment except for shear force, which was affected by dietary energy. A significant improvement (p < 0.05) in the intramuscular fat (IMF) content was observed in the HEG. Little effect on the amino acid profile was observed (p > 0.05), except for a tendency (p = 0.06) to increase the histidine concentration in the BF muscle. Increasing dietary energy also reduced C22:6n-3 and saturated fatty acids (SFAs) and enhanced C18:1 cis-9 and monounsaturated fatty acids (MUFAs, p < 0.05). Those results revealed that increasing energy levels of diets could enhance slaughter traits and affect the meat quality and fatty acid composition of different muscle tissues of Honghe yellow cattle. These results contribute to the theoretical foundation to formulate nutritional standards and design feed formulas for the Honghe yellow cattle.

The present study was conducted to analyze the correlation between the milk fat content of Binglangjiang buffaloes and their microbial and host metabolites. The 10 buffaloes with the highest milk fat content (HF, 5.60 ± 0.61%) and the 10 with the lowest milk fat content (LF, 1.49 ± 0.13%) were selected. Their rumen fluid and plasma were collected for rumen microbiota and metabolome analysis. The results showed that the rumen bacteria abundance of Synergistota, Quinella, Selenomonas, and Fretibacterium was significantly higher in the HF buffaloes. The abundance of 14 rumen fungi, including Candida, Talaromyces, Cyrenella, and Stilbella, was significantly higher in the HF buffaloes. The analysis of the metabolites in the rumen and plasma showed that several metabolites differed between the HF and LF buffaloes. A total of 68 and 42 differential metabolites were identified in the rumen and plasma, respectively. By clustering these differential metabolites, most of those clustered in the HF group were lipid and lipid-like molecules such as secoeremopetasitolide B, lucidenic acid J LysoPE (0:0/18:2 (9Z, 12Z)), and 5-tetradecenoic acid. Spearman’s rank correlations showed that Quinella, Fretibacterium, Selenomonas, Cyrenella, and Stilbella were significantly positively correlated with the metabolites of the lipids and lipid-like molecules in the rumen and plasma. The results suggest that rumen microbiota such as Quinella, Fretibacterium, Selenomonas, and Cyrenella may regulate milk fat synthesis by influencing the lipid metabolites in the rumen and plasma. In addition, the combined analysis of the rumen microbiota and host metabolites may provide a fundamental understanding of the role of the microbiota and host in regulating milk fat synthesis.

Lowering dietary protein content is one of the effective ways to reduce nitrogen (N) emissions and conserve protein feed resources. However, it is unclear how reducing dietary protein levels affects milk quality and the efficiency of N utilization in lactating goats. It is therefore difficult to determine exactly how much reduction in dietary protein levels is optimal. The objective of this study was to evaluate the effects of low-protein diets on milk quality, N balance and rumen microbiota in lactating goats. A total of 50 lactating goats were enrolled in a completely randomized design and maintained on either a diet with 15.82% protein level as the control group (CON) or reduced protein levels with 13.85% (R2 group), 11.86% (R4 group), 9.84% (R6 group) and 7.85% (R8 group), respectively. The results showed that the dry matter intake, milk yield, fecal and urinary N excretion and utilization efficiency of N of lactating goats decreased linearly with reduced dietary protein levels. Specifically, the milk yield was reduced by the R8 group (p < 0.05). Furthermore, the R8 group reduced the contents of protein, fat and lactose (p < 0.05), but R2 and R4 have no influence (p > 0.05). The R6 group decreased protein content only at the 4th week. Fecal and urinary N excretion and utilization efficiency of N reduced linearly with decreasing dietary protein levels (p < 0.05). The R8 group affected the relative abundance of rumen microbiota including Christensenellaceae_R-7_group, NK4A214_group and UCG-005 (p < 0.05). In conclusion, lowering dietary protein levels decreased milk quality and N excretion by altering rumen microbiota in goats during lactation. This phenomenon was most pronounced when the dietary protein level was reduced by 8 percentage points. Nevertheless, dietary protein levels should not be reduced by more than 6 percentage points to ensure normal performance of the goat during lactation.