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

Abstract

The reduction in milk yield (kg) on test day associated with individual somatic 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 western France. Monthly records with SCC over 600 000 cells/ml, and/or occurring after a clinical case of mastitis were excluded. The resulting dataset comprised 32 148 records from 4968 cows. A mixed autoregressive linear model was used to assess, on test day, the effects on the milk yield (dependent variable) of herd-season (random), SCC (after natural logarithmic transformation), days in milk on which SCC were recorded and an interaction term between SCC and days in milk. Regression coefficients associated with SCC were expressed as a polynomial of days in milk. The reduction in milk yield in kg associated with an increase in SCC was 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 cells/ml, 0.63 kg for an SCC of 200 000 cells/ml and 1.13 kg for an SCC of 600 000 cells/ml. For cows in parity 2 with SCC of 200 000 cells/ml, the reduction in milk yield was 0.63 kg on day 50, 0.92 kg on day 150 and 1.77 kg on day 250 pp. For cows in parity ≥3 on day 50 pp, the 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 an SCC of 600 000 cells/ml. For cows in parity ≥3 with SCC of 200 000 cells/ml, the reduction in milk yield was 0.60 kg on day 50, 1.09 kg on day 150 and 1.85 kg on day 250 pp.