The association between clinical mastitis and milk yield in dairy cows

Abstract

A study of 1514 lactations that occurred between 1977 and 1982 in 790 Friesian, Ayrshire and Holstein crossbred cows was undertaken. One thousand and eighty-four of these lactations were used to consider the association between the occurrence of clinical mastitis and changes in milk yield, and to illustrate a technique that enables the previous lactation history of a cow to be used for estimating changes associated with the disease.The lactation curves were described using the model y(n) = an^be^−cn from which values for peak yield, persistency, rate of decline at week 25, and recorded and extrapolated 305-day yield were calculated. A significant correlation between successive lactations of the same animal was observed for each of these variables. Pairs of successive lactations free from clinical mastitis were then compared by regression analysis with lactations in which the preceding lactation was free from the disease whilst the second lactation of the pair was infected.Significant reductions in peak yield, recorded and extrapolated 305-day yield were found when mastitis occurred before the week of peak yield. A reduction in recorded 305-day yield was also observed when the disease occurred between peak and 10 weeks after peak. Mastitis later in the lactation had no significant effect.The mean reduction in recorded 305-day yield of 540 kg when mastitis occurred before peak for a cow with a mean yield of 4830 kg, was higher than previous estimates. No evidence of recovery was observed in subsequent lactations that were free of clinical infection, indicating that once a cow has contracted mastitis it may not achieve its full milk-yield potential in the next lactation.