Selection and genetic gain in dairy cattle: relationship between longevity and production

#1, The present investigation was based upon an analysis of 11,250 lactation records obtained from 4,538 cows belonging to 24 pedigree dairy herds in England. #2. The coefficient of variation of each of the 1, 2, 3 and 4th lactations were 23.47, 24.64, 25.23 and 26. 59 respectively for milk yield, and 9.75, 10.33, 10.78 and 10.50 respectively for the butterfat percentage. #3. The coefficient of variation of each of the dams' 1, 2, 3 and 4th lactations was 22.99, 24.68, 25.98 and 26.01 respectively for milk yield, and 9. 71, 10.36, 10.94 and 10.42 respectively for butterfat percentage. #4. The average repeatability estimate for 305 day milk yield was found to be 0.556, and the corresponding value for butterfat percentage was 0.532. It appeared as if there was a higher correlation between successive records than between non-successive records. #5. The heritability estimates for 305 day milk yield for the 1, 2, 3 and 4th lactation records were 0.35, 0.27, 0.26 and 0.34 respectively, The corresponding values for the butterfat percentage were 0.53, 0.40, 0.54 and 0.34 respectively. #6. The heritability estimates of the dams' average of the first 2, 3, and 4 lactation records were 0.48, 0.47 and 0.53 respectively for milk yield, and 0.57, 0.71 and 0.68 respectively for butterfat percentage. #7. The genetic 'superiority of dams of cows was found to be 7.54 gallons, and the corresponding value for the dams of bulls was 27.38 gallons, with the annual genetic gain of 1.82 gallons. #8. The genetic superiority of dams of bulls used in the herds and those sold to other pedigree dairy herds were 45.28 gallons and 22.18 gallons respectively. #9. In the case of butterfat percentage, the genetic superiority of dams of cows and dams of bulls were -0.04 and -0.06 respectively. #10. The average length of productive herd life of dairy cows was 3.4 lactations. #11. There appeared to be a slight relationship between high production and longevity