Heat stress (HS) decreases milk protein synthesis beyond what would be expected based on the concomitant reduction in feed intake. The aim of the present study was to evaluate the direct effects of HS on milk protein production. Four multiparous, lactating Holstein cows (101 ± 10 d in milk, 574 ± 36 kg of body weight, 38 ± 2 kg of milk/d) were individually housed in environmental chambers and randomly allocated to 1 of 2 groups in a crossover design. The study was divided into 2 periods with 2 identical experimental phases (control phase and trial phase) within each period. During phase 1 or control phase (9 d), all cows were housed in thermal neutral conditions (TN; 20°C, 55% humidity) and fed ad libitum. During phase 2 or treatment phase (9 d), group 1 was exposed to cyclical HS conditions (32 to 36°C, 40% humidity) and fed ad libitum, whereas group 2 remained in TN conditions but was pair-fed (PFTN) to their HS counterparts to eliminate the confounding effects of dissimilar feed intake. After a 30-d washout period in TN conditions, the study was repeated (period 2), inverting the environmental treatments of the groups relative to period 1: group 2 was exposed to HS and group 1 to PFTN conditions. Compared with PFTN conditions, HS decreased milk yield (17.0%), milk protein (4.1%), milk protein yield (19%), 4% fat-corrected milk (23%), and fat yield (19%). Apparent digestibility of dry matter, organic matter, neutral detergent fiber, acid detergent fiber, crude protein, and ether extract was increased (11.1-42.9%) in HS cows, as well as rumen liquor ammonia (before feeding 33.2%; after feeding 29.5%) and volatile fatty acid concentration (45.3%) before feeding. In addition, ruminal pH was reduced (9.5 and 6% before and after feeding, respectively) during HS. Heat stress decreased plasma free amino acids (AA; 17.1%) and tended to increase and increased blood, urine, and milk urea nitrogen (17.2, 243, and 24.5%, respectively). Further, HS cows had reduced plasma glucose (8%) and nonesterified fatty acid (39.8%) concentrations compared with PFTN controls. These data suggest that HS increases systemic AA utilization (e.g., decreased plasma AA and increased nitrogen excretion), a scenario that limits the AA supply to the mammary gland for milk protein synthesis. Furthermore, the increase in AA requirements during HS might represent the increased need for gluconeogenic precursors, as HS is thought to prioritize glucose utilization as a fuel at the expense of nonesterified fatty acids.

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The requirements of thiamine in adult ruminants are mainly met by ruminal bacterial synthesis, and thiamine deficiencies will occur when dairy cows overfed with high grain diet. However, there is limited knowledge with regard to the ruminal thiamine synthesis bacteria, and whether thiamine deficiency is related to the altered bacterial community by high grain diet is still unclear. To explore thiamine synthesis bacteria and the response of ruminal microbiota to high grain feeding and thiamine supplementation, six rumen-cannulated Holstein cows were randomly assigned into a replicated 3 × 3 Latin square design trial. Three treatments were control diet (CON, 20% dietary starch, DM basis), high grain diet (HG, 33.2% dietary starch, DM basis) and high grain diet supplemented with 180 mg thiamine/kg DMI (HG+T). On day 21 of each period, rumen content samples were collected at 3 h postfeeding. Ruminal thiamine concentration was detected by high performance liquid chromatography. The microbiota composition was determined using Illumina MiSeq sequencing of 16S rRNA gene. Cows receiving thiamine supplementation had greater ruminal pH value, acetate and thiamine content in the rumen. Principal coordinate analysis and similarity analysis indicated that HG feeding and thiamine supplementation caused a strong shift in bacterial composition and structure in the rumen. At the genus level, compared with CON group, the relative abundances of 19 genera were significantly changed by HG feeding. Thiamine supplementation increased the abundance of cellulolytic bacteria including Bacteroides, Ruminococcus 1, Pyramidobacter, Succinivibrio, and Ruminobacter, and their increases enhanced the fiber degradation and ruminal acetate production in HG+T group. Christensenellaceae R7, Lachnospira, Succiniclasticum, and Ruminococcaceae NK4A214 exhibited a negative response to thiamine supplementation. Moreover, correlation analysis revealed that ruminal thiamine concentration was positively correlated with Bacteroides, Ruminococcus 1, Ruminobacter, Pyramidobacter, and Fibrobacter. Taken together, we concluded that Bacteroides, Ruminococcus 1, Ruminobacter, Pyramidobacter, and Fibrobacter in rumen content may be associated with thiamine synthesis or thiamine is required for their growth and metabolism. In addition, thiamine supplementation can potentially improve rumen function, as indicated by greater numbers of cellulolytic bacteria within the rumen. These findings facilitate understanding of bacterial thiamine synthesis within rumen and thiamine's function in dairy cows.

of lysine to methionine alters expression of genes involved in milk protein transcription and translation and mTOR phosphorylation in bovine mammary cells.

The correlation between mastitis and the gastrointestinal microbiome in dairy cows has been demonstrated. Regulating the profile of rumen microorganisms may contribute to remission of subclinical mastitis (SCM).