Milk somatic cell count affects feed efficiency through increased heat production of lactating dairy cows

The objective was to evaluate the relationships between somatic cell count (SCC) and feed efficiency using the data from two respiration chamber studies using Nordic Red cows (n = 136). The cows were fed ad libitum a total mixed ration (forage: concentrate 45:55 on dry matter (DM) basis) containing 162 and 379 g crude protein and neutral detergent fibre in kg DM, respectively. Average (± SD) DM intake and energy corrected milk (ECM) yield were 23.6 ± 3.4 and 37.7 ± 6.1 kg/d, respectively. Blood samples were collected in one of the studies (n = 86). Feed efficiency was expressed as ECM/DM intake, residual feed intake (RFI) and residual ECM yield (RECM). Feed efficiency decreased significantly with increased SCC irrespective of how the efficiency was expressed. Data from energy metabolism indicated that conversion of dietary gross energy to metabolizable energy (ME) was not influenced by SCC, but the efficiency of ME utilization decreased as a result of increased heat production. Increased heat production might be associated with the greater energy demand for immune function. Blood haptoglobin increased with SCC also indicating the role of immune function in increased heat production. Methane intensity increased by 1.0 g/kg ECM above cut-off value of 47 000 cells/mL. It is concluded that in addition to economic losses and animal welfare issues, increased milk SCC has negative environmental impact.