Oxidized protein biomarkers in the blood and milk of cows supplemented with flaxseed during the dry period

Oxidative stress (OS) is the result of an imbalance between reactive oxygen species (ROS) production and neutralizing capacity of antioxidant mechanisms. A state of OS is observed in the periparturient cow, when a great amount of ROS is formed in response to copious milk yield and mammary gland involution. Flaxseed is an excellent source of alpha-linolenic acid, and many studies have considered the supplementation of flaxseed in diets for lactating cows. Conversely, few studies have dealt with flaxseed administration during the dry period. Here, we investigated whether flaxseed administration during the dry period can affect the cows’ OS response around calving. Because of OS biomarkers in colostrum and milk may reflect the OS status of the mammary gland, we also investigated if biomarkers of oxidized protein can be measured in colostrum and milk. At the beginning of the dry period, high yielding Holstein cows in the same parity and production level and with a close date of drying off were randomly assigned to a control (CTRL; n=35), or to an experimental flaxseed supplement dry-off diet (FLAX; n=38; 200 g/head/day). Blood samples were taken at d 55, 30, 8, and 3 before the expected calving. Blood and colostrum/milk samples were collected at d 4, 15 and 30 after calving. The OS was monitored by measuring reactive oxygen metabolites (ROM), advanced oxidation protein products (AOPP) and carbonyl groups (CG). Somatic cell count (SCC) was also recorded. Variables were analysed using a hierarchical mixed model for repeated measures (PROC MIXED, SAS). Flaxseed administration did not affect any of the biomarkers measured in plasma and colostrum/milk. The day of sample significantly affected all measured parameters (P<0.001). Weak but significant correlations were found in milk between AOPP and SCC (r=0.305; P<0.01), AOPP and ROM (r=0.333; P<0.01), AOPP and CG (r=-0.309; P<0.01). Plasma and milk AOPP were negatively correlated (r=-0.220; P<0.01). Around calving, AOPP and CG showed an opposite trend both in plasma and in milk, which deserves further investigations. It is possible that different radicals can produce different alteration in the protein structure and, thus, a panel of parameters may better characterize the OS status.