Non-genetic factors affecting milk differential somatic cell count in specialized and dual purpose dairy cows

In addition to somatic cell count (SCC), milk differential somatic cell count (DSCC) has been proposed in recent years to monitor cow’s udder health status. Milk DSCC (%) represents the proportion of polymorphonuclear neutrophils out of the total SCC and, according to the literature, values > 65 % indicate inflammation of the mammary gland. The present study aimed at investigating the DSCC variability using test-day data (TD) col lected in multibreed herds and from cows belonging to i) a specialized dairy (Holstein) and ii) a dual purpose breed (Simmental) and sampled in North of Italy. Only herds with both breeds present were kept and restrictions on SCC allowed outliers elimination. For each TD, DSCC_n (cells*1,000/mL) was calculated by multiplying SCC (cells*1,000/mL) and DSCC. Finally, 112,748 TD from 12,009 Holstein and 3,830 Simmental cows in 266 herds were available to estimate the least square means of DSCC and DSCC_n through a linear mixed model. The fixed effect of breed, parity (1, 2, 3 and ≥4), lactation stage (12 classes), and sampling season were considered together with the first-order interac tion between breed and lactation stage, breed and parity and parity and lactation stage. The random effects were: cow, herdβtestβdate, and residual. Estimates of the two breeds were significantly different (P<0.001) being DSCC equal to 64.19±0.06 and 62.54±0.10% and DSCC_n equal to 296±2.52 and 210±4.40 cells*1,000/ml in Holstein and Simmental, respectively. Overall, both traits were greater in Holstein than in Simmental cows, regard less of parity and lactation stage. However, in late lactation estimates of the two breeds were similar (P>0.05). Preliminary results suggest that the conventional DSCC thresh old might be partially adjusted to better fit the udder health status in Simmetal breed.