EXPECTED IMPACT OF SELECTION FOR MILK YIELD ON REPRODUCTIVE PERFORMANCE TRAITS IN HOLSTEIN FRIESIAN COWS UNDER EGYPTIAN CONDITIONS

Estimation of genetic and phenotypic parameters of productive (305-day yields of milk, MY305; fat, FY305 and protein, PY305) and reproductive performance traits (days open, DO; calving interval, CI and number of inseminations per conception, NSC) were calculated on 3398 lactation records of 1054 Holstein Friesian cows, daughters of 94 bulls and 691 dams using multi-trait animal model with repeated records. Expected impact of direct selection for MY305, FY305 and PY305 on reproductive performance was calculated. Heritability estimates for productive traits varied from 0.08 to 0.26 and from 0.04 to 0.19 for reproductive traits. The productive traits were highly inter-correlated genetically (0.995 to 0.998) than the reproductive traits (0.241 to 0.786). Cows which producing abundant MY305, FY305 and PY305 tended to show less reproductive efficiency in terms of longer DO (rG = 0.942 to 0.947), longer CI (rG =0.587 to 0.673) and more NSC (rG =0.769 to 0.829). Direct selection, after one round of selection, for MY305, FY305 and PY305 is expected to result in cows with more advantageous productive performance in terms of higher MY305 (+180.22 to 323.27 kg), FY305 (+13.79 to 24.95 kg) and PY305 (+12.41 to 22.48 kg). This enhancement in productive performance is expected to be associated with deterioration in reproductive performance in terms of longer days open (+16.27 to 29.48 days), longer calving interval (+15.26 to 25.22 days) and higher number of inseminations per conception (+.08 to 0.16 services). It could be concluded that the undesirable relationship between productive and reproductive performance traits in Holstein Frisian cows must be taken into consideration when planning breeding programs in dairy cattle (e.g. improving productive traits by genetic selection and reproductive traits by enhancement management or by using restricted selection indices).