Genetic correlations between milk production traits and somatic cell scores on test day within and across first and second lactations in Holstein cows
Introduction
Mastitis is the most common health disorder in dairy cows (Bigras-Poulin et al., 1990, Rajala-Schultz and Gröhn, 1999). Somatic cell counts (SCCs), generally log-transformed to somatic cell scores (SCSs), are used widely as indicators of clinical and subclinical mastitis (Detilleux et al., 1997, Schutz, 1994). SCSs increase with intramammary infection (Schepers et al., 1997, Schukken et al., 2003) and are highly correlated with the occurrence of clinical mastitis (reviewed by Mrode and Swanson, 1996). There have been many estimates of the genetic parameters of SCSs and their relationship to production traits (Haile-Mariam et al., 2001, Jamrozik et al., 2010, Jamrozik and Schaeffer, 2011, Jamrozik and Schaeffer, 2012, Koivula et al., 2005, Miura and Suzuki, 2005, Miura and Suzuki, 2006, Negussie et al., 2008, Samoré et al., 2008). In addition, several reports have addressed the relationships between average SCS and lactation persistency (Haile-Mariam et al., 2003, Weller et al., 2006). It has been suggested that lactation persistency is related to the lactation curve (Togashi and Lin, 2003) and that modifying the curve by improving persistency could increase total milk yield without increasing metabolic stress and disease susceptibility in early lactation (Dekkers et al., 1998).
Haile-Mariam et al. (2003) reported a negative genetic correlation between lactation persistency and average SCS in first lactation. Weller et al. (2006) reported that this correlation was quite low in the first lactation and negative in the second lactation. These results were estimated only by using a lactation model; however, a random regression test day (TD) model has not yet been used to assess the genetic correlations between daily SCS and lactation persistency. Applying the TD model in this context can be important, because the genetic correlations between daily milk yield and daily SCS may differ with lactation stage and parity (Haile-Mariam et al., 2001, Jamrozik et al., 2010). The results of such a study could reveal the genetic relationship between these traits at various lactation stages and might suggest the idea of modifying the lactation curve to increase milk yield without increasing SCS during the first and second lactations.
Therefore, the aims of this study were to investigate the genetic correlations between daily SCS and milk production traits (especially persistency) during first and second lactations by using a multiple trait random regression TD model.
Section snippets
Data
Monthly TD milk and SCS records for 305 days in milk (DIM) in first- and second-lactation Holstein cows in the Hokkaido region that had calved between 2004 and 2009 were obtained from the Hokkaido Dairy Milk Recording and Testing Association (Hokkaido, Japan). SCSs were calculated from SCCs as follows: SCS=log2 (SCC/100,000)+3 (Ali and Shook, 1980). The data set consisted of 200,095 TD records from 21,238 cows in their first lactations and 143,051 records from 15,281 cows in their second
Results and discussion
Average lactation curves for TD milk and SCS within 305 DIM in the data set (Fig. 1) revealed that daily milk yield peaked at 36–95 DIM in first lactation and at 36 to 65 DIM in second lactation and gradually declined thereafter. The changes in SCS during the first and second lactations were inverted relative to those of milk yield within lactation. After reaching a nadir (first lactation, 2.00 at about 70 DIM; second lactation, 1.80 at about 40 DIM), mean SCS started to increase in both
Conclusions
The genetic correlations between daily milk yield in first lactation and SCS on the same DIM in first and second lactations were positive and peaked in early lactation. In contrast, the genetic correlations between persistency and daily SCS were negative for most of both the first and the second lactation, after an initial brief positive period. These correlations suggested that focusing selection on increasing milk yield early in the first lactation would likely increase SCS, whereas selecting
Conflictions of interest
There are no conflictions of interest for publishing this manuscript.
Acknowledgments
We thank Mrs. K. Yukawa for her support with the experiments.
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