RESUMEN
Feeding a higher-energy diet by increasing cereal grains at the expense of forage during the last 3 to 4 wk prepartum is a traditional approach to help the rumen "adapt" to the traditional diets fed at the onset of lactation. Increasing grain/concentrate in the diet changes ruminal fermentation and in sheep and goats elicits marked changes in mRNA expression of immune-related genes in ruminal epithelium. Whether such changes at the epithelial and systemic levels occur in dairy cows when the dietary energy content increases at a fixed level of concentrate is unknown. Fourteen nonpregnant, nonlactating Holstein cows were fed a control lower-energy (CON, 1.30 Mcal/kg of dry matter) diet to meet 100% of estimated nutrient requirements for 3 wk, after which half of the cows were assigned to a higher-energy diet (OVE, 1.60 Mcal/kg of dry matter) and half of the cows continued on CON for 6 wk. Levels of forage and concentrate for CON and OVE were 80 and 79% and 20 and 21%, respectively. Plasma samples were collected 1 d before slaughter to examine biomarkers of metabolism, liver function, inflammation, and oxidative stress. The reticulo-rumen mass was recorded at slaughter, and samples of epithelium were harvested from all cows. The expression of 29 genes associated with tight junctions, immune function, and nutrient transport (volatile fatty acids, urea, and trace minerals) was examined. Overfeeding energy led to consistently greater dry matter intake over time, and lowered plasma concentrations of haptoglobin, paraoxonase, bilirubin, fatty acids, and myeloperoxidase (secreted by neutrophils). In contrast, OVE resulted in greater hydroxybutyrate and cholesterol concentrations. A greater reticulo-rumen mass in cows fed OVE did not alter genes associated with tight junctions (CDLN1, CDNL4, OCLN, TJP1), immune function (IL1B, IL10, NFKB1, TLR2, TLR4, TNF), oxidative stress (SOD1, SOD2), or most nutrient transporters. However, feeding OVE upregulated the acute-phase protein SAA3 by 3.5-fold and downregulated a volatile fatty acid transporter (SLC16A1) and a Fe and Cu transporter (SLC11A2). The lack of effect on mRNA expression along with lower plasma concentrations of inflammation biomarkers indicates that long-term intake of a higher-energy diet ad libitum was not detrimental to ruminal epithelium integrity. In that context, a protective function of SAA3 could be envisioned with a role in opsonizing gram-negative bacteria that produce endotoxins. The long-term control of volatile fatty acid absorption and trace minerals from the rumen in cows overfed energy does not seem to be controlled at the gene transcription level. The relevance of these findings to the nutritional management of pregnant dry cows merits further research.