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The purpose of this study was to estimate the genetic and phenotypic trends of 305- day milk yield of Holstein cows in a commercial farm. Data consisted of 2654 lactation records for 850 Holstein cows sired by 316 sires, collected from 1991 to 2007. The data were analyzed using Least Squares technique to examine the effect of year, season of calving, parity and interaction between season of calving and parity factors on 305- day milk yield. Heritability of 305 day milk yield trait and breeding values were estimated using VCE 6.0.2 software. Highly significant effects for year of calving, parity and the interaction between season of calving and parity on the 305- day milk yield were observed. However, season of calving had a non significant effect on the 305- day milk yield. The average of 305- day milk yield was 7042 kg. The heritability estimate of 305- day milk yield was 0.06. The regression of 305-day milk yield on year of calving showed a significant negative phenotypic trend being -91.6 kg per year. However, the genetic trend depicted a significant positive trend (+ 2.19 kg per year). In conclusion, although the trend in the estimated breeding values found for cows was positive, it still did not lead to change in the average milk production due to the probable reduction in the nutritional and /or management conditions of the herd.

GENETIC AND PHENOTYPIC TRENDS OF 305-DAY MILK YIELD OF HOLSTEIN COWS RAISED AT COMMERCIAL FARM IN EGYPT