Genetic evaluation of somatic cell score in dairy cattle considering first and later lactations as two different but correlated traits

Abstract

A test-day (TD) random regression model (RRM) was described for the genetic evaluation of somatic cell score (SCS) where first and later lactations were considered as two different but correlated traits. A two-step covariance function procedure was used to estimate variance-covariances and associated genetic parameters. Analysis of estimated breeding values (EBV), ranking of top bulls and cows and some computational aspects were used to compare RRM with TD repeatability model (RPM) and lactation average model (LAM). Residuals were analysed to assess the relative fit of TD models. Comparison between RRM and RPM showed that RRM has lower mean squared error and gave better fit to the data. For young bulls and cows, the standard deviation (SD) of EBVs was highest for RRM and lowest for LAM implying efficient utilization of information on SCS, in terms of revealing more genetic variation. A much lower correlation of EBVs ranging from 0.80 to 0.92 and significant re-ranking of top bulls and cows were observed between RRM and LAM. The lower across-lactation correlation between RRM and LAM indicated that LAM is directed to give more weight to first lactation breeding values. The RRM, where SCS in the first and later lactations was considered as two different but correlated traits was able to make effective use of available information on young bulls and cows, and could offer an opportunity to breeders to utilize EBVs for first and later lactations.

Genetic parameters of test-day somatic cell scores for the first three lactations of Polish Holstein-Friesian cattle

“…The lowest correlations within lactation were between the 5 th and the 305 th day of lactation, and they were equal to 0.21, 0.29, 0.29 for the first, second and third lactation, respectively. This was in agreement with other studies, in which the lowest genetic correlations were between SCS at the most distant TD (Haile-Mariam et al, 2001;Koivula et al, 2002;Odegard et al, 2003;Koivula et al, 2004;Negussie et al, 2006). The estimates ranged from 0.33 to 0.71 for the first lactation (Haile-Mariam et al, 2001;Odegard et al, 2003;Koivula et al, 2004) and were higher than in the present study.…”

“…For the third parity, Haile-Mariam et al (2001) reported very low genetic correlations between the beginning and the end of lactation (0.14). Koivula et al (2002) and Negussie et al (2006) reported that within lactation genetic correlations were higher in the first than in the second lactation, which was not confirmed in our study.…”

“…Similar differences were observed for genetic correlation between SCS at 30 and 305 DIM. Our estimate was equal to 0.47 and in the other reports this correlation ranged from 0.65 to 0.71 (Liu et al, 2000;Negussie et al, 2006). Few studies have taken into account later lactations.…”

“…Genetic correlations across lactation are high at the same stage of lactation, especially in the midlactation (Haile-Mariam et al, 2001;Mrode and Swanson, 2003;Negussie et al, 2006), being the highest between the second and third lactation (Pösö and Mäntysaari, 1996;Liu et al, 2000;HaileMariam et al, 2001;Mrode and Swanson, 2003;Carlen et al, 2004). Because in many studies this genetic correlation was close to unity, it has been suggested that SCC in the second and third lactation can be considered as the same trait.…”

Genetic parameters and single versus multi-trait evaluation of udder health traits

“…Thus, for genetic evaluation of animals, a MT model treating first and later lactations as different but correlated traits would be a practical strategy. Negussie et al (2006) described a method of treating first and later lactations as two different traits in genetic evaluation of SCS in dairy cows.…”