Over three seasons (1998, 1999 and 2000) groups of Holstein-Friesian cows of New Zealand (NZ) or overseas (OS) origin were monitored for mastitis when managed in two contrasting systems. Cows were fed either on pasture or total mixed ration (TMR) with cows on TMR confined to small loafing paddocks or a concrete feedpad. All quarters were sampled for bacteriology at calving, at dry off, in mid and late lactation and when clinical mastitis was detected. Individual cow somatic cell counts (SCC) were determined weekly throughout lactation. In 1998, there was no difference in incidence of clinical mastitis but in later seasons, cows on TMR experienced significantly more clinical, and sub-clinical mastitis, than cows on pasture, probably reflecting differences in bacterial challenge within the environment. Genotype differences were less evident compared to dietary effects. Compared to NZ cows, OS cows experienced more sub-clinical infections in 1998 and more clinical infections in 1999 whilst in 2000, genotype effects were confounded by diet. Seasonal SCC was higher for OS cows in 1999 and 2000, reflecting differences in incidence of sub-clinical mastitis among treatment groups. Despite the strong influence of environment on mastitis incidence, genotype differences were detectable between NZ and OS cows.
Proceedings of the New Zealand Society of Animal Production, Volume 62, Palmerston North, 24-29, 2002
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