Milk metabolites as noninvasive indicators of nutritional status of mid-lactation Holstein and Montbéliarde cows

Abstract

The objective was to investigate the effects of feed restriction on concentrations of selected milk metabolites in midlactation Holstein and Montbéliarde cows, and explore their correlations with energy balance and classic plasma and milk indicators of nutritional status. Eight Holstein and 10 Montbéliarde cows (165 ± 21 DIM) underwent 6 d of feed restriction during which feed allowance was reduced to meet 50% of their net energy for lactation (NEL) requirements. The experiment was divided in four periods: Control (CONT; d -3 to -1), restriction (REST; d 1 to 6), WEEK1 (d 7 to 13) and WEEK2 (d 14 to 18) after refeeding at ad libitum intake. Intake, milk production, energy balance and plasma metabolites were used to validate the feed restriction model. Concentrations of seven milk metabolites, i.e. BHB, glucose, glucose-6-phosphate, isocitrate, glutamate, uric acid and free amino groups were measured in morning milk samples, and fatty acids in pooled PM and AM samples. Feed restriction induced a negative energy balance (-42.5 ± 4.4 MJ/d), increased plasma non-esterified fatty acids and BHB, and decreased plasma glucose concentrations. Feed restriction increased milk glucose-6-phosphate and isocitrate (+38% and +39%, respectively) and decreased milk BHB, glucose, glutamate, uric acid and free amino group concentrations (-20%, -57%, -65%, -42% and -14%, respectively), compared to pre- restriction. Milk concentrations of medium chain fatty acids (e.g. sum of C10 to C15) decreased and those of long chain (e.g. 18:0, cis-9 18:1) increased during restriction. Breed differences were not detected for the majority of variables. All studied milk metabolites were significantly correlated with energy balance (rs = 0.48, 0.63, -0.31, -0.45, 0.61 for BHB, glucose, glucose-6-phosphate, isocitrate and glutamate, respectively). Milk glucose and glutamate were the most correlated with plasma metabolites and milk FA associated with lipomobilization. These results suggest that milk metabolites may be used as noninvasive indicators of NEB and metabolic status of dairy cows