Abstract
Dairy cows are often fed high grain diets to meet the energy demand for high milk production or simply due to a lack of forages at times. As a result, ruminal acidosis, especially subacute ruminal acidosis (SARA), occurs frequently in practical dairy production. When SARA occurs, bacterial endotoxin (or lipopolysaccharide, LPS) is released in the rumen and the large intestine in a large amount. Many other bacterial immunogens may also be released in the digestive tract following feeding dairy cows diets containing high proportions of grain. LPS can be translocated into the bloodstream across the epithelium of the digestive tract, especially the lower tract, due to possible alterations of permeability and injuries of the epithelial tissue. As a result, the concentration of blood LPS increases. Immune responses are subsequently caused by circulating LPS, and the systemic effects include increases in concentrations of neutrophils and the acute phase proteins such as serum amyloid-A (SAA), haptoglobin (Hp), LPS binding protein (LBP), and C-reactive protein (CRP) in blood. Entry of LPS into blood can also result in metabolic alterations. Blood glucose and nonesterified fatty acid concentrations are enhanced accompanying an increase of blood LPS after increasing the amount of grain in the diet, which adversely affects feed intake of dairy cows. As the proportions of grain in the diet increase, patterns of plasma β-hydoxybutyric acid, cholesterol, and minerals (Ca, Fe, and Zn) are also perturbed. The bacterial immunogens can also lead to reduced supply of nutrients for synthesis of milk components and depressed functions of the epithelial cells in the mammary gland. The immune responses and metabolic alterations caused by circulating bacterial immunogens will exert an effect on milk production. It has been demonstrated that increases in concentrations of ruminal LPS and plasma acute phase proteins (CRP, SAA, and LBP) are associated with declines in milk fat content, milk fat yield, 3.5% fat-corrected milk yield, as well as milk energy efficiency.
Highlights
Dairy cows are often fed high grain diets to meet the energy demand for high milk production or due to a lack of forages at times
Lipopolysaccharide and Other Bacterial Immunogens Released in the Rumen and the Large Intestine It is widely accepted that free ruminal LPS concentrations increase after grain engorgement, especially during experimentally-induced subacute ruminal acidosis (SARA)
In the study of Khafipour et al [4], replacing 21% of the dry matter (DM) of the control diet with a forage to concentrate ratio (F:C) of 50:50 with pellets containing 50% ground wheat and 50% ground barley resulted in grain-based SARA, which exhibited a rise of free rumen LPS concentrations from 28,184 to 107,152 endotoxin units (EU)/mL
Summary
Dairy cows are often fed high grain diets to meet the energy demand for high milk production or due to a lack of forages at times. A recent study by Li et al [7] demonstrated a grain-pellet induced SARA challenge in dairy cows significantly increased LPS production in the cecum compared with control. High grain feeding can promote rapid growth of E. coli including the pathogenic E. coli in the digestive tract of dairy cattle as described previously, which could result in release of many of the immunogenic virulence factors. The absorption of propionate into blood circulation or its effects on rumen receptors may result in decreased feed intake in cows fed high grain diets [6]. Deceased feed intake with increasing the amount of grain in the diet may be due to enhanced release of endotoxin and other bacterial immunogens in the digestive tract and their translocation into blood. It was demonstrated in vitro that activated blood neutrophils had a cytotoxic effect on bovine mammary epithelial cells [52] potentially through the release of reactive oxygen species such as hydroxyl radicals [53]
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