THE EFFECTS OF LAMENESS ON MILK PRODUCTION IN DAIRY COWS(19.SAYFA ) KOÇAK, Ö. , EKİZ, B., YILMAZ, A. Istanbul University, Faculty of Veterinary Medicine, Department of Animal Husbandry, 34320 Avcılar, Istanbul, TURKEY Abstract This study was carried out to investigate the effects of lameness on milk yield of Holstein cows. Data includes 204045 daily milk yields from 477 Holstein cows collected in a private farm. The milk productions per day were recorded based on automated milk weight measurements at each milking. The GLM procedure was used to analyze data. The model included fixed effects of parity, week of lactation, months in milk and whether a cow ever became lame. All these effects were found to be significant (P<0.001). The decrease in milk production in cows having lameness began two weeks before the diagnosis of lameness and continued until four weeks after the diagnosis. After the 4 th week of the illness the milk production returned back to its normal level. The milk production was 31.0 kg/day for cows that had not yet been diagnosed lame, 27.3 kg/day in the first week of lameness, 27.2 kg/day in the second week of lameness and 30.1 kg/day in the 4 th week of lameness. The results of this study showed that lameness has a significant impact on milk production. Introduction Lameness is an important disease of dairy cows as it has a high incidence and causes financial and welfare losses. In the studies carried out a lameness incidence the results differed between 2.1 – 50% (Mclennan, 1998; Warninck et al., 2001; Barkema et al., 1994; Green et al., 2002; Rojala-Schultz et al, 1999). These differences between the studies were explained by the difficulties in the diagnosis of lameness (Green et al., 2002) and the effects of several diseases like trauma, infectious diseases and nutrition (Harris et al., 1988). Some of the financial losses caused by lameness are the decreases in milk production, the need for extra work and expenses of treatment. In the studies carried out, different results were reported for the effects of lameness on milk production. Some authors reported that lameness caused decreases in milk production (Warninck et al., 2001; Green et al., 2002, Rojala-