GENETIC FACTORS AFFECTING TOTAL MILK YIELD, LACTATION PERIOD AND CALVING INTERVAL OF CROSSBRED FRIESIAN COWS RAISED ON NILE DELTA

The main objective of the study is to estimate the various genetic parameters to determine the status of the productive herd, as well as the effect of non-genetic factors such as (parity, season and year of calving also interactions between these factors) for total milk yield (TMY kg), lactation period(LP, days) as milk production traits and calving interval (CI, days) as reproductive traits of Crossbred Friesian (local & Friesian) at generation F₈, cows raised on Nile Delta in Gamaza herd, Egypt. A total number of 1308 lactation records of 402 cows (daughters of 279 dams and 73 sires). The analysis was performed using SAS (2003).  The model included the random effects of genetic factors (sire effect) and fixed effects (parity, season and year of calving). In addition the animal model MTDFREML was used to estimate heritability (h²) and breeding value (BV).

Actual means of TMY, LP and CI were 1918 kg; 231 day and 440 day respectively.
(h²) estimated of productive trait (TMY, LP) were high h² 0.44±0.002 and 0.46±0.01 while CI was low h² (0.01±0.005).
The correlation coefficients between TMY and LP were positive with 0.28 for the genetic correlation between the two traits while the phenotypic correlation was 0.79, which means that the genetic improvement of one of the two traits could be improved by the genetic improvement of other trait.
The range of (BV) for cows was high for most of studied traits. The ranges of breeding values of cows for TMY, LP and CI were 2075.1kg, 307.2 day and 232.8 day, respectively.
The genetic trend of all studied traits was the lowest for the genetic trend in 2003 and 2015 years while the highest values of the genetic trend of all traits in 2017.

In conclusion, the difficulty of genetic improvement of CI by selection due to the low heritability of this trait can be improved by increasing improving environmental conditions and good care together. Higher ranges of BV for cows for most of studied traits indicate higher genetic variation and higher opportunity for selection of top cows in breeding value, which would result in rapid genetic progress in the future generations. The study recommends that this can be confirmed by molecular genetics to detect the location of quantitative traits that affect milk production and selection of the cows carrying these genes.