Relationships among First-Lactation and Lifetime Measurements in a Dairy Population

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Abstract
Relationships between first-lactation and lifetime traits were investigated for 6,468 lactation records of 1,861 Holsteins in seven herds of the North Carolina Institu- tional Breeding Association. The pheno- typic coi~relation between first-lactation milk yield and lifetime production on a within herd-year of first calving basis was 0.48 and the correlation between first-lacta- tion milk yield and productive life was 0.43. For these data the optimum age at first calving for maximum lifetime produc- tion was 27 months. Cows calving first at 24 months of age produced 579 kg less milk in their lifetime than those calving first at 27 months; whereas, those calving first at 30 months produced 458 kg less milk during their productive life. Genetic parameters were estimated from 1,017 daughter-dam pairs. The heritability esti- mates of lifetime milk yield and length of productive life were 0.18 and 0.15, respec- tively. The genetic correlation between first-lactation milk yield and lifetime pro- duction was 0.85 and between first-lactation milk yield and length of productive life, 0.76. The genetic correlations between the measures of lifetime production; namely, lifetime milk yield, length of productive life, and number of lactations were 0.95 or above. Even though selection is directed toward high first-laetation production, this should not produce a decline in lifetime performance in these herds. Concern has been expressed about the pos- sible deterioration of lifetime performance as a result of continued selection for high first- lactation performance. Most selection for eeo-