Genetic and Phenotypic Parameters for Production and Days Open in the First Three Lactations of Swedish Dairy Cattle

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Abstract
The material consisted of completed 305-day records of daughters of Al bulls. The analyses were carried out for each of the two predominant breeds where one of the breeds was divided into two separate geographical areas. The material was further divided into five years of first calving (1977–81) giving a total of 15 data sets. The traits were 305-day lactation milk (M) or fat (F) yield and days open (DO) in the first three lactations (1, 2, 3). All traits were preadjusted for month and age of calving, production was also adjusted for days open. The multiple trait mixed model used had the same effects for all traits analyzed simultaneously. These were effects of herd (fixed), group (fixed) and sire within group. The latter was random if the sire was a young bull and fixed for old progeny-tested bulls. The fixed sires were not used for estimating variance components. A triangular transformation was applied which made the residual variance-covariance matrix an identity matrix. The EM-algorithm was used to get approximate REML estimates of the variance components. Forty rounds of iteration were carried out. Overall average h 2 for milk yield were 0.268, 0.246 and 0.248 for the first three lactations respectively. The genetic correlations between the lactations were high, 0.855, 0.836 and 0.966 for r g12, r g13 and r g23 respectively. Heritabilities for fat yield were somewhat lower: 0.250, 0.213 and 0.228 respectively. Genetic correlations were 0.920, 0.846 and 0.938 respectively. For days open low heritabilities were found: 0.040, 0.050 and 0.066 respectively. Fairly high genetic correlations were found between these traits, however: 0.582, 0.493 and 0.220. Genetic correlations between milk yield in first lactation (Ml) and days open in first and second lactations (DO1, DO2) were moderately high and unfavourable: 0.414 and 0.335 respectively. There were no systematic trends in the estimated parameters over the different production years. The unfavourable genetic correlations obtained in this study between milk production and days open was not reflected in the estimated genetic trends for days open in a previous study.