Ruminal background of predisposed milk urea (MU) concentration in Holsteins

Multi-tissue gene expression profiling of cows with a genetic predisposition for low and high milk urea levels

Milk urea (MU) concentration is proposed as an indicator trait for breeding toward reduced nitrogen (N) emissions and leaching in dairy. We selected 20 German Holstein cows based on MU breeding values, with 10 cows each having low (LMUg) and high (HMUg) MU genetic predisposition. Using RNA-seq, we characterized these cows to unravel molecular pathways governing post-absorptive body N pools focusing on renal filtration and reabsorption of nitrogenous compounds, hepatic urea formation and mammary gland N excretion. While we observed minor adjustments in cellular energy metabolism in different tissues associated with different MU levels, no transcriptional differences in liver ammonia detoxification were detected, despite significant differences in MU between the groups. Differential expression of AQP3 and SLC38A2 in the kidney provides evidence for higher urea concentration in the collecting duct of LMU cows than HMU cows. The mammary gland exhibited the most significant differences, particularly in tricarboxylic acid (TCA) cycle genes, amino acid transport, tRNA binding, and casein synthesis. These findings suggest that selecting for lower MU could lead to altered urinary urea (UU) handling and changes in milk protein synthesis. However, given the genetic variability in N metabolism components, the long-term effectiveness of MU-based selection in reducing N emissions remains uncertain.

Mammary gland, kidney and rumen urea and uric acid transporters of dairy cows differing in milk urea concentration

The milk urea concentration (MUC) serves as indicator of urinary nitrogen emissions, but at comparable crude protein (CP) intake, cows with high (HMU) and low (LMU) MUC excrete equal urea amounts. We hypothesized that urea and uric acid transporters and sizes of the kidney, mammary gland, and rumen account for these phenotypes. Eighteen HMU and 18 LMU Holstein dairy cows fed a low (LP) and normal (NP) CP diet were studied. Milk, plasma and urinary urea concentrations were greater with NP feeding, while plasma and urinary urea concentrations were comparable between phenotypes. Milk and plasma uric acid concentrations were higher with LP feeding but not affected by phenotype. The milk-urine uric acid ratio was greater in HMU cows. The mRNA expressions of the ruminal urea transporter SLC14A1 and AQP10, the mammary gland and rumen AQP3, and the mammary gland uric acid transporter ABCG2 were not affected by group or diet. Renal AQP10, but not AQP3, AQP7, and SLC14A2 expressions, and the kidney weights were lower in HMU cows. These data indicate that renal size and AQP10 limit the urea transfer from blood to urine, and that MUC determines if uric acid is more released with milk or urine.

Microbial signature inferred from genomic breeding selection on milk urea concentration and its relation to proxies of nitrogen-utilization efficiency in Holsteins

Increasing the nitrogen-utilization efficiency (NUE) of dairy cows by breeding selection would offer advantages from nutritional, environmental, and economic perspectives. Because data collection of NUE phenotypes is not feasible in large cow cohorts, the cow individual milk urea concentration (MU) has been suggested as an indicator trait. Considering the symbiotic interplay between dairy cows and their rumen microbiome, individual MU was thought to be influenced by host genetics and by the rumen microbiome, the latter in turn being partly attributed to host genetics. To enhance our knowledge of MU as an indicator trait for NUE, we aimed to identify differential abundant rumen microbial genera between Holstein cows with divergent genomic breeding values for MU (GBVMU; GBV_HMU vs. GBV_LMU, where H and L indicate high and low MU phenotypes, respectively). The microbial genera identified were further investigated for their correlations with MU and 7 additional NUE-associated traits in urine, milk, and feces in 358 lactating Holsteins. Statistical analysis of microbial 16S rRNA amplicon sequencing data revealed significantly higher abundances of the ureolytic genus Succinivibrionaceae UCG-002 in GBV_LMU cows, whereas GBV_HMU animals hosted higher abundances of Clostridia unclassified and Desulfovibrio. The entire discriminating ruminal signature of 24 microbial taxa included a further 3 genera of the Lachnospiraceae family that revealed significant correlations to MU values and were therefore proposed as considerable players in the GBVMU-microbiome-MU axis. The significant correlations of Prevotellaceae UCG-003, Anaerovibrio, Blautia, and Butyrivibrio abundances with MU measurements, milk nitrogen, and N content in feces suggested their contribution to genetically determined N-utilization in Holstein cows. The microbial genera identified might be considered for future breeding programs to enhance NUE in dairy herds.

The Benefits of Adding Sulfur and Urea to a Concentrate Mixture on the Utilization of Feed, Rumen Fermentation, and Milk Production in Dairy Cows Supplemental Fresh Cassava Root

Fresh cassava roots that contain hydrocyanic acid (HCN) can be hazardous to animals when consumed. Prior literature has shown that adding sulfur may eliminate HCN without harming the health of animals. Additionally, adding urea is advised if sulfur was utilized since it helps with microbial protein synthesis. We thus proposed that supplementing the fresh cassava root diet with a high sulfur and urea in concentrate diet would be advantageous for rumen fermentation and milk production in animals. The purpose of this study was to see how high sulfur and urea levels in concentrate combinations affected feed utilization, rumen fermentation, and milk production in dairy cows fed diets including fresh cassava root. Four Holstein Friesian cows with 480 ± 50.0 kg BW, 10 ± 2 kg/head/day of milk yield, and 90 days in milk (DIM) were assigned at random in a 4 × 4 Latin square design with a 2 × 2 factorial design. Factor A was the concentration of sulfur in the concentrate diet at 10 g/kg and 20 g/kg dry matter (DM), while factor B was the concentration of urea in the concentrate diet at 10 g/kg and 20 g/kg DM. Fresh cassava root was given to each cow on a daily basis at a rate of 15 g DM/kg of BW. According to the findings, sulfur and urea had no interaction impact on feed intake, rumen fermentation, or milk production. Sulfur supplementation at 20 g/kg DM improved sulfur intake and digestibility of DM and organic matter much more than 10 g/kg sulfur. Additionally, sulfur supplementation at a dose of 20 g/kg DM in concentrate markedly increased blood and milk thiocyanate concentrations while lowering the somatic cell count. When compared to 10 g/kg DM urea, 20 g/kg DM urea significantly enhanced crude protein digestibility, ammonia-nitrogen concentration, blood urea nitrogen, and total volatile fatty acid concentration. Sulfur might detoxify hydrogen cyanide toxicity and be added at 20 g/kg DM in concentrate without harming the animals, whereas urea at 20 g/kg DM could increase feed digestion and rumen fermentation.