Date of Award

1985

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Abstract

Direct and correlated responses from two different sire selection strategies were compared for twelve characteristics of dairy cattle. Daughters of sires selected on genetic merit for milk production (ST) were compared to daughters of sires selected on genetic merit for an index of milk production, fat production and type scores (MT). Data consisted of repeated measures of twelve dairy traits on 275 foundation through third generation Holstein cows in a single herd. First through third generation animals represented daughters of 42 AI sires. Phenotypic response to selection was change in first lactation performance and producing ability per generation and per year. Expected and actual genetic response to selection was change in sire Predicted Difference and daughter breeding value per generation and per year. Breeding value and producing ability was estimated using best linear unbiased prediction techniques. Mean changes in milk production from first through third generation ranged from 292 kg for breeding value to 650 kg for first lactation phenotype in the ST line and from -525 kg for first lactation phenotype to 150 kg for sire Predicted Difference in the MT line. The ST line increased genetically across generations for milk and fat production, udder dimensions and milking speed. The MT line decreased in genetic merit for improved reproductive performance. Phenotypic and genetic changes across generations for the remainder of the traits were similar across selection lines. Annual changes in phenotypic performance and breeding value for milk production in the ST line were -15 kg and 11 kg as compared to -56 kg and -5 kg in the MT line. Correlations between breeding values and first lactation performance for milk and fat production ranged from .70 to .84 in the ST and MT lines. Results indicated that selection for yield increased phenotypic performance and genetic merit for milk yield, fat yield, udder dimensions and milking speed. Selection for an index of milk yield, fat yield and type decreased genetic merit for improved reproductive performance.

Pages

151

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