Antimicrobial use (AMU) is the major driver of antimicrobial resistance (AMR) among bacteria in dairy herds. There have been numerous studies on AMU and AMR in dairy cows; however, studies on AMU and AMR in dairy calves are limited. A comprehensive overview of the current state of knowledge of AMU and AMR among pathogens in dairy calves is important for the development of scientifically supported and applicable measures to curb antimicrobial use and the increasing risk of AMR. Therefore, we performed a systematic review of research on AMU and AMR in dairy calves. A total of 75 publications were included, of which 19 studies reported AMU data for dairy calves and 68 described AMR profiles of the four most prevalent bacteria that are associated with calf diarrhea and calf pneumonia. Large variation in AMU was found among herds across different regions. There seems to be a positive association between exposure to antimicrobials and occurrence of resistance. Most AMU was accounted for by treatment of diseases, while a small proportion of AMU was prophylactic. AMU was more common in treating calf diarrhea than in treating pneumonia, and the resistance rates in bacteria associated with diarrhea were higher than those in pathogens related to pneumonia. Organic farms used significantly fewer antimicrobials to treat calf disease; however, the antimicrobial resistance rates of bacteria associated with calf diarrhea and pneumonia on both types of farms were comparable. Feeding waste or pasteurized milk was associated with a higher risk of AMR in pathogens. Altogether, this review summarizes AMU and AMR data for dairy calves and suggests areas for future research, providing evidence for the design of antimicrobial use stewardship programs in dairy calf farming.

Targeted sequencing, a pivotal branch of next-generation sequencing (NGS), enables the selective enrichment of specific genomic regions and has demonstrated significant advantages in the detection of animal pathogens. This review systematically explores the underlying principles of targeted sequencing, various enrichment strategies—including PCR amplification, probe hybridization, and CRISPR-Cas systems—and their key applications in veterinary pathogen diagnostics. Due to its high throughput, sensitivity, and cost-effectiveness, targeted sequencing has been successfully applied in the multiplex detection of pathogens in economically significant livestock, such as cattle, as well as in the surveillance of antimicrobial resistance (AMR) genes, pathogen typing, and source tracing. It is particularly effective in identifying mixed infections and low-abundance pathogens. Nonetheless, wide application is restricted by some factors, like incomprehensive reference databases, cost-effectiveness, and limited application in primary-level laboratories. Further development directions are AI-based panel design, multimodal diagnostic platform integration, standard workflow construction, and introduction of a multi-omics method. Such progress focuses on enhancing the targeted sequencing scalability and precision consistent with the “One Health” initiative objective.

Introduction: Dietary fiber plays a crucial role in maintaining gastrointestinal health. However, its protective effects on the intestinal health of calves remain to be fully elucidated. This study aimed to investigate the impact of dietary fiber supplementation on the intestinal microbiota of pre-weaned calves and its potential role in modulating microbial metabolic pathways.Methods: A randomized controlled trial was conducted, enrolling 135 calves that were randomly assigned into three groups: (1) inulin supplementation, (2) psyllium husk powder (PHP) supplementation, and (3) a control group receiving no dietary fiber. Fecal microbiota samples were collected from calves without diarrhea at five time points (0, 7, 14, 28, and 56 days of age). Metagenomic sequencing was performed to analyze microbial composition and functional pathways. Additionally, a differential analysis of carbohydrate-active enzymes (CAZymes) was performed to evaluate the effect of dietary fiber on carbohydrate metabolism enzyme activity within the intestinal microbiota.Results: Calves supplemented with dietary fiber exhibited a significant increase in the abundance of Bifidobacterium and Prevotella compared to the control group. These bacterial genera contributed to intestinal protection by modulating secondary bile acid metabolism and flavonoid metabolism pathways. CAZymes differential analysis revealed an increased abundance of carbohydrate metabolism enzymes in response to dietary fiber supplementation, with distinct microbial community compositions observed among different fiber treatments. Notably, at 56 days of age, calves fed PHP harbored intergeneric symbiotic clusters comprising Clostridium, Prevotella, and Bacteroides, suggesting a cooperative microbial network that may contribute to intestinal homeostasis.Discussion: The findings of this study highlight the beneficial effects of dietary fiber on calf intestinal microbiota, particularly in enhancing microbial diversity and enzymatic activity related to carbohydrate metabolism. The observed microbial symbiosis in PHP-fed calves suggests a potential role in maintaining intestinal homeostasis. These insights provide a theoretical foundation for optimizing dietary interventions to promote gut health in calves during the transition period. Further research is warranted to explore the mechanistic interactions between dietary fiber, gut microbiota, and host health outcomes.

Conjugated linoleic acid, a long-chain fatty acid, reduces milk fat content in dairy cows, which may alleviate the negative energy balance during the early lactation period by reducing lactation energy output and thereby decreasing the occurrence of ketosis and fatty liver due to a negative energy balance. Although research has revealed the milk fat inhibition mechanism, the mechanisms underlying the metabolic interactions among the liver, subcutaneous fat, and mammary gland tissues during this period remain unclear. In this study, the molecular mechanisms underlying CLA-induced milk fat depression in the liver, subcutaneous white adipose tissue, and mammary gland tissues of dairy cows were investigated. Chinese Holstein dairy cows at 30 d postpartum were fed a control diet (6 cows) or a CLA-supplemented diet (6 cows) for 7 d, and all cows were fed the control diet only for another 7 d. Tissue samples from the liver, subcutaneous white adipose tissue, and mammary gland were collected on d 0 (before CLA supplementation) and d 7 (after supplementation) for RNA sequencing analysis. In addition, blood samples were collected from control and CLA-treated cows on d 0, 4, 7, and 14 (post-treatment) to measure the concentrations of nonesterified fatty acids (NEFA) and β-hydroxybutyric acid (BHBA). Furthermore, from d 0 to 14, milk composition and milk yield were measured. The results of lactation performance indicated that supplementation with CLA led to a significant reduction in milk fat percentage, decreasing from 4.0% to 3.5%, and no other changes in the lactation performance of the CLA group were observed. With respect to the indicators of excessive fat mobilization and circulating ketone bodies, the CLA group presented significantly lower serum NEFA concentrations than did the control group, with levels decreasing from 0.70 to 0.55 mmol/L between one day before CLA supplementation (d 0) and d 7 of CLA supplementation (d 7), whereas the BHBA concentration tended to decrease over this period. Transcriptomic analysis of mammary tissue identified 674 differentially expressed genes (DEG). Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analyses revealed that the downregulated DEG in the CLA group were involved mainly in lipid biosynthetic processes, fatty acid biosynthesis, the p53 signaling pathway, and steroid biosynthesis. Transcriptomic analysis of subcutaneous white adipose tissue revealed 614 DEG between the CLA and control groups. These DEG were significantly enriched in pathways related to fatty acid metabolism, fatty acid transport, fatty acid β-oxidation, lipid metabolism, the peroxisome proliferator-activated receptor (PPAR) signaling pathway, and AMP-activated protein kinase (AMPK) signaling pathway. Transcriptomic analysis of liver tissue identified 479 DEG, which were enriched in pathways related to fatty acid metabolism, the PPAR signaling pathway, lipid transport activity, the AMPK signaling pathway, the phosphoinositide 3-kinase-protein kinase B (PI3K-AKT) signaling pathway, the insulin signaling pathway, and the insulin resistance pathway. Collectively, these findings indicate that CLA successfully induced milk fat depression in postpartum dairy cows. Based on the up- and downregulation of genes within fatty acid metabolism, fatty acid β-oxidation, and lipid metabolism pathways in adipose tissue, CLA may promote lipid synthesis while inhibiting fatty acid oxidation and lipolysis in subcutaneous white adipose tissue. Additionally, gene expression patterns related to lipid transport activity, fatty acid metabolism, PPAR, and AMPK pathways in liver tissue suggest that CLA may increase lipid transport capacity while reducing hepatic lipid accumulation. These findings provide theoretical evidence in support of the regulation of energy balance in early-lactating cows through the inhibition of milk fat synthesis by CLA, which may be used as a strategy for preventing type I ketosis.

This study aimed to (1) estimate the prevalence of cow-level high somatic cell count (SCC) in Chinese dairy herds and (2) identify potential factors associated with cow- and herd-level SCC variables. The monthly data on dairy herd improvement were collected from a total of 131 dairy herds in 11 provinces in China in 2019. Mixed models were constructed using the cow composite milk SCC and the variance of cow SCC as dependent variables separately and parity, seasons, days in milk (DIM), herd size, and farm types (family-owned vs. company-owned) as fixed effects, accounting for the nested random herd and cow effect. We used negative binomial regression using herd-level SCC-related variables, namely, monthly proportion of high SCC, monthly proportion of new high SCC, monthly proportion of chronic high SCC, and monthly proportion of new chronic high SCC as dependent variables separately against seasons, herd size, and farm types with the random herd effect. The overall average prevalence of high SCCs for each month per farm was 0.26 (2.5–97.5% quantile: 0–0.56). Company-owned farms performed better in herd SCC management. Seasons were significantly associated with all the aforementioned variables, and summer and autumn were the seasons associated with worse outcomes in herd SCCs. This study is the first to assess high SCC in a large number of Chinese dairy herds, which is useful for farms to tailor the on-farm mastitis control programs in China.

This study compared the regulatory effects of dietary supplementation with natural clinoptilolite (CLN) versus a dietary cation-anion difference (DCAD) regimen on calcium homeostasis in dairy cows during the last 21 days prepartum. Results showed that cows in the DCAD group exhibited significantly higher blood ionized calcium (iCa) and parathyroid hormone (PTH) concentrations than those in the CLN group (p < 0.05). Serum PTH concentrations displayed a declining trend in both groups prepartum, which deviates from the classical theory of compensatory PTH secretion, suggesting that alternative compensatory pathways may be involved in the regulation of calcium homeostasis during the periparturient period in dairy cows. Monitoring of calcium homeostasis and related metabolic parameters following postpartum oral calcium bolus administration revealed that the incidence of subclinical hypocalcemia in the DCAD group was 26%, lower than the 62% observed in the CLN group. However, blood iCa concentrations returned to normal levels more rapidly in the CLN group. Additionally, CLN supplementation was associated with more stable blood pH and lower prepartum serum potassium concentrations (p < 0.05) that remained within the physiological range, which may contribute to improved tissue sensitivity to PTH.