Editor's note: The following article was written by Peng Ji and published in the Miner Institute Farm Report, available here. Peng was a post-doctoral researcher at Miner Institute from fall 2011 through late summer 2013. Peng now has a post-doctoral research position at the University of Illinois.
The first time that I heard of oxidative stress was from cosmetics advertisements. The products report that they help prevent aging of skin (a.k.a. wrinkles) through the elimination of over-accumulated free radicals or reactive oxygen species (ROS) that resulted from oxidative stress. The skin care products are essentially antioxidants. Currently, there is increasing evidence that oxidative stress is causatively linked to several health problems in transition dairy cows.
Reactive oxygen species are the by-product primarily produced during oxidative phosphorylation inside of mitochondria, which is a process that converts nutrients (fatty acids and sugar) into cellular energy substrate (ATP) in most of mammalian cells. Thus, ROS is generated as with normal metabolic process of cells in the human body as well as in dairy cows. Interestingly, ROS is a two-edged sword. Under normal conditions, the existence of several endogenous antioxidant mechanisms can efficiently neutralize ROS to prevent its accumulation; in addition, steady-state ROS helps immune cells to kill pathogens, which is beneficial to health. However, excessive accumulation of ROS that occurs when energy requirements are high and/or deficient in endogenous antioxidants will result in oxidative stress. This is destructive to the protein and lipid components of cell membrane and DNA (especially immune cells) that cause tissue damage and suppression of the immune response.
For high-producing dairy cows, the energy requirement is generally doubled shortly after calving compared with late pregnancy, which greatly increases the metabolic rate of tissues and organs (e.g. mammary gland, liver, and muscles), which are more susceptible to oxidative stress. A recent study that used a new and more comprehensive parameter, called oxidative stress index which is the ratio of ROS to total serum antioxidant capacity, confirmed that both the ROS and oxidative stress index are higher in the post-fresh period than that in mid (4 to 5 months) or late gestation in dairy cows. The antioxidant capacity is lower during peak lactation than other times, which is probably due to depletion of fat-soluble antioxidants by milk and consumption by ROS.