Metabolic and production profiles of dairy cows in response to decreased nutrient density to increase physiological imbalance at different stages of lactation.

Physiological imbalance (PI) is a situation in which physiological parameters deviate from the normal, and cows consequently have an increased risk of developing production diseases and reduced production or reproduction. Our objectives were to (1) determine the effect of stage of lactation and milk yield on metabolic and production responses of cows during a nutrient restriction period to experimentally increase PI; (2) identify major metabolites that relate to degree of PI; and (3) identify potential biomarkers in milk for on-farm detection of PI throughout lactation. Forty-seven Holstein cows in early [n=14; 49±22 d in milk (DIM); parity=1.6±0.5], mid (n=15; 159±39 DIM; parity=1.5±0.5), and late (n=18; 273±3 DIM; parity=1.3±0.5) lactation were used. Prior to restriction, all cows were fed the same total mixed ration ad libitum. All cows were then nutrient restricted for 4 d by supplementing the ration with 60% wheat straw to induce PI. After restriction, cows returned to full feed. Daily milk yield was recorded and composite milk samples were analyzed for fat, protein, lactose, citrate, somatic cells, uric acid, alkaline phosphatase, β-hydroxybutyrate (BHBA), and milk urea nitrogen. Blood was collected daily and analyzed for metabolites: nonesterified fatty acids (NEFA), BHBA, glucose, plasma urea nitrogen, and insulin. The revised quantitative insulin sensitivity check index (RQUICKI) was calculated for each cow. Liver biopsies collected before and during restriction were analyzed for triglycerides, glycogen, phospholipids, glucose, and total lipid content. A generalized linear mixed model was used to determine the effect of stage of lactation on responses during restriction. Regression analyses were used to examine the effect of pre-restriction levels on changes during restriction. Similar decreases in milk yield among groups indicate that the capacity of individual responses is dependent on milk yield but the coping strategies used are dependent on stage of lactation. Milk yield was a better predictor of feed intake than DIM. Plasma glucose decreased for all cows, and cows in early lactation had increased plasma BHBA, whereas cows in later lactation had increased NEFA during restriction. Milk citrate had the greatest increase (58%) during restriction for all cows. Results reported here identified metabolites (i.e., glucose, NEFA, BHBA, cholesterol) as predictors of PI and identified milk citrate as a promising biomarker for PI on farm.