Milk yield differences between 1× and 4× milking are associated with changes in mammary mitochondrial number and milk protein gene expression, but not mammary cell apoptosis or SOCS gene expression

Milking frequency is known to affect milk production and lactation persistence in dairy cows. Despite this, the mechanisms underlying this effect are only partially understood. Previous work in dairy cows examining increases in milk yield due to increased milking frequency have identified changes in apoptosis and expression of genes regulating cytokine signaling. In addition, changes in mitochondrial biogenesis and function have been suggested to play a role during the lactation cycle in regulating milk production. The goal of this study was to test the hypothesis that, when maintained over an entire lactation, extreme differences in milking frequency would be reflected in differences in apoptosis, mammary mitochondrial number, and the mammary expression of genes known to inhibit cytokine signaling. Primiparous Holstein cows (n = 6) were assigned to the study 40 d before parturition after which 1 udder half was milked once daily (1×) and the other 4 times daily (4×) Mammary biopsies were collected at 15, 60, 120, and 230 d of lactation. Average milk yield from the 4× side was 3 times higher than from the 1× side. Analysis of milk composition revealed that protein, lactose, and solids-not-fat percentages were lower in 1× than 4× udder halves. Mammary cell apoptosis was not affected by milking frequency. Mammary cell mitochondrial number, as estimated by succinate dehydrogenase staining, was higher in early lactation, decreasing as days in milk increased, and with increased milking frequency. Although mammary expression of α-lactalbumin (LALBA) and β-casein (CSN2) was significantly increased in 4× glands, the expression of suppressors of cytokine signaling were similar between 1×- and 4×-milked halves. These results support the conclusion that changes in milk production in response to extreme differences in milking frequency may be related to alterations in mitochondrial number and lactose synthesis, but not apoptosis.