Reduction in milk yield associated with somatic cell counts up to 600 000 cells/ml in French Holstein cows without clinical mastitis

The reduction in milk yield (kg) on test day associated with individual som atic cell count (SCC) up to 600 000 cells/ml was assessed using monthly cow -records collected for a 1-year period in 105 Holstein herds located in wes tern France. Monthly records with SCC over 600 000 cells/ml, and/or occurri ng after a clinical case of mastitis were excluded. The resulting dataset c omprised 32 148 records from 4968 cows. A mixed autoregressive linear model was used to assess, on test day, the ef fects on the milk yield (dependent variable) of herd-season (random), SCC ( after natural logarithmic transformation), days in milk on which SCC were r ecorded and an interaction term between SCC and days in milk. Regression co efficients associated with SCC were expressed as a polynomial of days in mi lk. The reduction in milk yield in kg associated with an increase in SCC wa s calculated in deviation from a reference SCC value set at 50 000 cells/ml as a function of SCC and days in milk on which SCC were recorded. In primiparous cows, the reduction in milk yield was 0.30 kg for an SCC of 100 000 cells/ml, 0.61 kg for an SCC of 200 000 cells/ml and 1.09 kg for an SCC of 600 000 cells/ml. In multiparous cows, the reduction in milk yield in kg increased with parity and with days in milk. For cows in parity 2 on day 50 pp, the reduction in milk yield was 0.32 kg for an SCC of 100 000 ce lls/ml, 0.63 kg for an SCC of 200 000 cells/ml and 1.13 kg for an SCC of 60 0 000 cells/ml. For cows in parity 2 with SCC of 200 000 cells/ml, the redu ction in milk yield was 0.63 kg on day 50, 0.92 kg on day 150 and 1.77 kg o n day 250 pp. For cows in parity greater than or equal to 3 on day 50 pp, t he reduction in milk yield was 0.30 kg for an SCC of 100 000 cells/ml, 0.60 kg for an SCC of 200 000 cells/ml and 1.07 kg for SCC of 600 000 cells/ml. For cows in parity greater than or equal to 3 with SCC of 200 000 cells/ml , the reduction in milk yield was 0.60 kg on day 50, 0.92 kg on day 150 and 1.85 kg on day 250 pp. (C) 1999 Elsevier Science B.V. All rights reserved.