The relationship between the level of inbreeding and milk production of cows in the first lactation period

The objective of the present study was to explore the effect of the level of inbreeding on milk efficiency of cows and on the content of milk components (fat and protein) in the 1st standard lactation period (305 days). The level of inbreeding (Fx) of inbred cows ranged between 1.25 and 12.5%. Efficiency parame- ters of inbred cows were compared with their contemporaries – half-sisters after the father (n=2567), raised on the same farm whose first lactation ended in same year. Out of the total database (567,036 cows) created in 1980–2002 we incorporated 885 inbred cows. To allow detailed analyses the inbred cows were divided into 3 groups according to the coefficient Fx (1.5–2.3%, 3.0–5.0% and 8.0–12.5%). The database was evaluated using the analysis of variance with 8 fixed effects including the commercial type, own breeding value and breeding value of mother, average efficiency of mother, reproduction parameters (year and age at calving and interval between the 1st and 2nd lactation) and total number of lactations. The statistical programme SAS 8.2 – GLM procedure (Copyright c 1999–2001 by SAS Institute Inc., Cary, NC, USA) was used for statistical analyses. Compared to their contemporaries the 323 kg reduction of milk production of inbred cows with the lowest value of Fx (1.5–2.3%) was insignificant; the decrease in the relative content of fat and protein was insignificant by 0.01 and 0.02%, respectively. Comparisons between contemporaries and inbred cows with Fx at the level of 3.0–5.0% showed a highly significant 407 kg reduction in milk efficiency. The fat content in milk of inbred cows increased by 0.11% (p<0.01) and the protein content by 0.03% (p<0.05) compared to their contemporaries. The reduction in milk production of inbred cows with the highest level of Fx (8.0–12.5%) was significant, i.e. –572 kg; the fat content increased insignificantly by 0.02%, while the protein content was the same in inbred cows and contemporaries. Comparisons of the milk production of inbred cows at various levels of Fx, taking into consideration the fixed effects, showed that the differences were most marked between groups Fx=1.5–2.3% and Fx=8.0–12.5%; the reduction in milk efficiency was –731 kg. A Fx coefficient higher by 1% reduced the milk efficiency of cows by 90.07 kg, while the fat and protein content increased by 0.001% and 0.002%, respectively.