Aim of the present study was to determine the effect of the lactation number on milk production and to detect the most economical lactation period in Holstein dairy cows. Materials and The animal materials of the study were 392 Holstein dairy cows with a similar dry period and lactation length. The cows were divided into 6 groups according to the number of lactations. The groups were formed as 1st lactation (Group 1; n=67), 2nd lactation (Group 2; n=124), 3rd lactation (Group 3; n=51), 4th lactation (Group 4; n=62), 5th lactation (Group 5; n=41) and 6th lactation (Group 6; n=47).Average lactation milk yields for the 1st, 2nd, 3rd, 4th, 5th, and 6th lactations of the cows used in the study were 7860.03 kg, 9010.02 kg, 10207.15 kg, 10165.89 kg, 8430.57 kg and 8069.78 kg, respectively. It was determined that the highest milk production and highest daily income were obtained during the 3rd and 4th lactation periods (p<0.05).In conclusion, milk production increased in the first three lactations, 3 and 4 lactation remained stable and decreased in 5th and 6th lactations. But, regardless of the number of lactations during the first 6 lactation periods, it is considered economical to use healthy cows for milk production in dairy farms.

This study was planned to assess the possible relationships with pregnancy-associated glycoproteins (PAG) concentrations by the determination of the biomarkers of oxidative stress in the plasma of dairy cattle during the early period of gestation. Materials and Methods: Blood samples were collected from coccygeal vessels in pregnant (n = 54) and nonpregnant (n = 45) cows. Measurement of biomarkers of oxidative stress (LPO, GSH and SOD) was carried out in females using spectrophotometric method.Plasma PAG concentrations increased and continuously over the both periods investigated. There were significant differences between pregnant and non-pregnant groups (P < 0.001). The concentration of SOD were significantly lower (P< 0.05) in pregnant females from day 25 to 35 (7.08 ±0.31 U.ml-1) and day 36 to 50 after AI (6.6 ± 0.29 U.ml-1) compared with non-pregnant cows (7.59 ± 0.35 U.ml-1). Concerning the concentrations of LPO and GSH, the values obtained were also significant lower (P < 0.05) in pregnant females in the period 25-35 days post AI (122.7 ±10.27 μM and 6.46 ±1.24 μmol/min.ml-1, respectively) and 36-50 days post AI (108.05±6.17 μM and 6.2±0.77 μmol/min.ml-1, respectively) than in the non-pregnant females (124.8 ± 12.16 μM and 6.96 ± 0.92 μmol/min.ml-1, respectively).It was observed that the markers of oxidative stress tended to be higher in non-pregnant females compared with pregnant females during the early period of gestation in dairy cattle. Our results suggest the existence of a relationship among the concentration of oxidative stress markers and PAG during early pregnancy.