The study was conducted in the years 2006–2008. The experimental materials comprised 368 Holstein-Friesian cows purchased from Germany as in-calf heifers. The objective of this study was to determine the health status of cows in a commercial herd kept in the Olsztyn region, based on the type and incidence of diseases that occurred during three consecutive lactations, and to analyze the effect of these diseases on milk yield and composition in the first, second and third lactation cycle. The cows were divided into five groups: HEA – clinically healthy cows (showing no disease symptoms), MAS – cows with mastitis, LAM – cows with foot/leg defects and lameness, REP – cows with reproductive problems (retention of the placenta, endometritis, ovarian cysts), MET – cows with metabolic diseases (ketosis, abomasal displacement). It was found that the most common diseases during three consecutive lactations in the investigated herd were endometritis (37.63%) which occurred soon after calving (on day 18 post-partum), mastitis (35.48%), formation of ovarian cysts (10.10%), ketosis (8.39%) and leg/foot defects (6.44%). Retention of the placenta and abomasal displacement were diagnosed much less frequently (1.62% and 0.34% respectively). The percentage of healthy cows decreased in successive lactations (19.81% in the first lactation, 12.28% in the second lactation, 6.22% in the third lactation). During each lactation, more than one third of cows suffered from mastitis. The proportion of cows showing the symptoms of ketosis increased with age, from 5.12% in the first lactation to 12.23% in the third lactation. The highest yields of milk and milk components over a 305-day lactation cycle were noted in cows with reproductive diseases (ROZ). Foot/led defects and lameness (LAM), mastitis (MAS) and metabolic diseases (MET) had the most significant effect on a decrease in milk production.
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