Q. Zebeli ,S. M. Dunn , and B. N. Ametaj
Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, AB, Canada
Institute of Animal Nutrition, Department of Veterinary Public Health and Food Science, Vetmeduni Vienna, Vienna, Austria
A major challenge to the current feeding systems of dairy cattle is how to reconcile feeding of large amounts of cereal grains that support high milk production with the high incidence of metabolic disorders. Although an unbalanced nutrition and, in particular, the feeding of diets high in starch has been proposed as the major causal factor of various metabolic disorders in dairy cows (Ametaj et al., 2005; Goff, 2006), the exact mechanism(s) underlying this association is not yet completely understood.
Research conducted by our team and others demonstrated that feeding dairy cows large amounts of concentrate often results in alterations in the rumen environment, leading to major changes in the composition of rumen microbiota and accumulation of large amounts of potentially harmful compounds such as endotoxin, a bioactive cell-wall component of all gramnegative bacteria (Emmanuel et al., 2008; Khafipour et al., 2009; Ametaj et al., 2010). These studies have shown that the rise in the concentration of rumen endotoxin is associated with activation of an acute phase response (Emmanuel et al., 2008). New evidence suggests that cytokines such as tumor necrosis factor (TNF)-α, IL-1, and IL-6, released by liver macrophages when activated by binding of endotoxin, may alter various physiological functions in the host because most cell types express receptors for those cytokines (Elsasser et al., 2008). As an example, both i.v. (Steiger et al., 1999; Waldron et al., 2003) and intramammary (Waldron et al., 2006) infusions of single doses of endotoxin in cattle have been related to perturbations of different plasma metabolites such as enhanced plasma glucose, development of insulin resistance, greater concentration of lactate in the plasma, and lower circulating BHBA.
Nevertheless, a direct association between the rise of free endotoxin, released into rumen fluid during the feeding of diets high in starch, and the perturbations of plasma variables related to carbohydrate and lipid metabolism in dairy cows has not yet been documented. We hypothesized that alterations in the ruminal environment, including a rise in the concentration of endotoxin, in response to feeding graded amounts of barley grain, might play a role in oscillations of different plasma metabolites. Therefore, the main objectives were to investigate the effects of feeding graded amounts of barley grain on rumen pH patterns, concentration of endotoxin in the rumen fluid, and the profile of selected plasma metabolites in dairy cows. Furthermore, associations among grain-induced rise in the concentration of rumen endotoxin with different plasma metabolites allied to carbohydrate and lipid metabolism in lactating dairy cows were evaluated.