Effect of milk yield and energy balance on neutrophil function in dairy cows

Neutrophils perform a series of sequential functions to kill pathogens entering the body. The objective of this study was to investigate whether variation across cows in neutrophil function tests is related to milk production and energy balance. Blood samples from 37 Holstein-Friesian heifers were taken early morning after milking and prior to feeding. Each cow was sampled about 30 days before calving and at 30 and 100 days after calving. Three groups of neutrophil function tests were used: (i) measurement of phagocytosis of bacteria (with heat inactivated serum (no complement) or complete serum (complement and antibodies)); (ii) measuring production of reactive oxygen species (ROS) with 30, 100 or 300 nmol/l phorbol myristate acetate (PMA); and (iii) after ingestion of bacteria (Staphylococcus or Streptococcus). Milk production (including fat, protein and lactose percentage), live weight and food intake were recorded for the first 100 days of lactation and blood metabolic concentrations (non-esterified fatty acid, glucose, beta-hydroxybutyrate) were measured fortnightly (six samples per cow). Restricted maximum likelihood was used to estimate the regression coefficients of neutrophil function on these traits and cow identity was fitted as random effect to account for the repeated measurements. There was no effect of milk yield on the neutrophil function but animals with a higher food intake and a more positive energy balance had a lower ROS production across the lactation (PMA). Of the metabolic characteristics, (beta hydroxybutyrate was positively associated with the capacity of reactive oxygen species production (significantly for 300 nmol/l PMA and close to significant for 100 nmol/l PMA; P < 0.05) but negatively with reactive oxygen species after ingestion of bacteria. Hence, results from these two reactive oxygen species tests contradict each other. The results suggest that immune function is not affected by selection for milk yield, as long as food intake capacity is increased sufficiently.