Milk metabolites: A novel noninvasive approach to assess energy balance in dairy cattle

International audienceIn dairy cattle, negative energy balance (NEB) can occur in early lactation or during feed restriction. It can negatively impact animal health and milk production. The aim of this study was to identify NEB biomarkers in milk. We hypothesized that mammary gland metabolism, which is impacted by feeding, can be evaluated through the measure of selected milk metabolite concentrations. Thirty Holstein dairy cows were studied both in early lactation (21 DIM), and in mid-lactation (173 DIM), during a 5-week feed restriction experiment. The feed restriction protocol aimed to reduce milk production by 20%, and to induce body reserves mobilization, i.e., a NEB. Thus, the diet was diluted with wheat hay to reduce the energy density and the diet digestibility by about 10%. Milk samples were collected at 21 DIM, before and during the first week of feed restriction. Data were analyzed by mixed models that included time, parity, and their interaction as fixed effects, and cow as random effect. NEB was observed in early lactation (−5.3 Mcal/d) and several milk metabolite concentrations varied: galactose (−52%, P < 0.001), glucose (−25%, P < 0.001) and glucose-6-phosphate (+291% P < 0.001) compared with during mid-lactation. The accumulation of glucose-6-phosphate may be due to a blockage of the pentose phosphate and isocitrate dehydrogenase pathways. Feed restriction at mid-lactation did not lead to a NEB. Milk production decreased by 10% (P < 0.001). Several milk metabolite concentrations also decreased as early as the first week of feed restriction: galactose (−24%, P < 0.001), glucose (−17%, P < 0.001), glucose-6-phosphate (−9%, P < 0.005), glutamate (−19%, P < 0.001) and isocitrate (−13%, P < 0.001). These variations suggest a global slowdown of mammary epithelial cell metabolism. The feed restriction model used in this study was not as severe as expected because feed intake remained high despite the reduced digestibility. Nonetheless, milk metabolite concentrations were still affected, showing their sensibility to energy intake variations. This study shows that milk metabolite concentration vary both with NEB at early lactation and feed level. Glucose-6-phosphate concentration varies differently between conditions suggesting that its increase may reflect a NEB state. These results need to be validated, and suggest new ways of using milk metabolites as markers of dairy cow energy stat