A common observation on individual farms is that SCC increases with advancing age and stage of lactation. However, research has shown that if cows were separated into groups by infection status, it becomes obvious that there is little change in SCC in uninfected cows, either as a cow ages or in late lactation.

Additional work has confirmed the finding that milk from uninfected quarters displayed little change in SCC with increasing number of lactations or with advancing stage of lactation. Inspection of stage of lactation data showed that the SCC of milk from uninfected quarters rose from 83,000 at 35 days postpartum to 160,000 by 285 days. However, Staphylococcus aureus infected quarters rose from 234,000 to 1,000,000 over the same period. All quarters, regardless of infection status had elevated SCC immediately postpartum, but those quarters with no infections or with minor pathogen infections showed a rapid decline in SCC to 35 days postpartum. Further, a 1997 study found no significant effect of parity or stage of lactation on SCC in bacteriologically negative (uninfected) cows. One study showed a rise in SCC at the end of lactation only after production had dropped below 4 kg (8.8 lb) per day, but the infection status in these animals was not determined. Feed or water deprivation results in decreased milk yield and increased SCC which can be interpreted as a dilution phenomenon. It has been suggested that a modest rise in SCC in uninfected quarters at the end of lactation could be a dilution effect. Thus the major influence of parity and stage of lactation on SCC is related to intramammary infection status.

Source: National Mastitis Council 1998 Regional Meeting Proceedings, pg. 40