Effects of a decrease in milk yield, caused by once-daily milking or by restricted feeding, on the somatic cell count in milk from cows with or without subclinical mastitis

1. Two experiments were carried out with grazing Holstein Friesian cows which had either low somatic cell counts (SCC) or high SCC before the treatment period began. They were designed to measure the effects of two factors, other than bacterial infection, on SCC. In experiment 1, 32 cows in mid lactation were subjected to a 3 week experimental period of either generous or severely restricted feeding. In experiment 2, 36 cows in late lactation were subjected to a 4 week experimental period of either twice daily or once daily milking. 2. The yield, composition and SCC in milk from each cow were measured twice weekly, and the bacteriological status of each quarter was examined on two days before and after the experimental period. 3. In both experiments the cows with high initial SCC had a higher proportion of quarters which were infected, some with major pathogens, while none of the cows with low initial SCC had quarters which shed major pathogens. 4. In experiment 1, the large decrease in milk yield (-30%) caused by the restricted level of feeding had no consistent effect on the yield of somatic cells or the SCC. However, the decrease in milk yield was associated with decreases in the yield of somatic cells and the SCC in the cows with low initial SCC but relative increases in the cows with high initial SCC; and the interactions were significant (P < 0.05; 0.12). 5. In experiment 2, once daily milking caused a larger decrease in daily milk yield in cows with the high initial SCC than in those with the low initial SCC (interaction P < 0.01). Once daily milking did not affect yield of somatic cells, but it did cause increases in SCC, with the effect being larger in the cows with high initial SCC (interaction P = 0.10). 6. The results of both experiments showed that SCC can be used to distinguish between uninfected and infected cows in mid and late lactation despite wide variations in feeding and milking. In fact the difference in SCC between infected and uninfected cows may be increased by the effects of reduced feeding or once daily milking. 7. In practical terms, the results of Experiment 1 indicate that maintenance of a high daily yield may help to reduce the adverse effects of subclinical mastitis, and the results of Experiment 2 indicate that cows with low SCC are more resilient to the adverse effects of once daily milking in late lactation.