Research was based on data from a herd of Black-and-White breed cows which
had an average milk yield ca. 7000 kg per cow per year. Upgrading crossing
with Holtein-Friesian breed (HF) was employed in the herd and over 15 years
of the breeding scheme the percentage of HF blood increased from 18 to 87%
. Evaluation and comparative analysis of performance indices (length of per
formance, life-time milk production and per 1 year of performance) for 16 g
roups (2448 animals in total) of cows which started production between 1979
-1994 were carried out.
A tendency to shorten the length of performance was observed in proportion
to the increase of HF genes which greatly influenced and lowered life-time
milk production. A speedy improvement in milk yield per one year of perform
ance due to an increase in HF blood share of up to 75% was noted. Stagnatio
n in milk yield was observed in the next generations of hybrids with more t
han 75% share of HF genes.