Studies on Lactation Yield and Individual Test-day Yields of Swedish Dairy Cows

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Abstract
The variation in persistency for 1st lactation yield and the correlation between persistency and lactations of various length were investigated. The materials analyzed comprised 8154 first lactation records of the Swedish Red and White breed (SRB) and 1913 records of Swedish Friesian (SLB). Two variants of the P2:1 measure from Johansson and Hansson (1940) were used as a measure of persistency. Month of calving and level of herd yield were the environmental factors producing most of the variation in persistency. The phenotypic standard deviation for persistency varied between 9.6 and 11.8 and the genetic SD ranged between 2.3 and 4.2. The heritability ranged from 0.07-0.15, which is about half the level of heritability for milk yield. the phenotypic correlations between cumulative milk yield and persistency were negative when short lactations were considered, but zero or positive correlations were estimated with 305-day yield. The genetic correlations were also negative with the short lactations, but highly positive genetic correlations were found between persistency and 305-day yield (0.32-0.65).