Evaluation of models for somatic cell score lactation patterns in Holsteins
Introduction
Mastitis is an udder health disorder that causes substantial economic losses to the dairy industry (Shook, 1989). This inflammation of the mammary gland, usually in response to invasive agents, can be characterized by an increase in the somatic cell count (SCC) in milk. This trait or a logarithmic transform, called somatic cell score (SCS), is used as an indicator of udder health status for management and selection purposes.
After the beginning of lactation, SCS decreases to a minimum at around 60 days post-calving and increases thereafter (Wiggans and Shook, 1987). Variation in the shape and level of the SCS pattern is related to lactation number (Wiggans and Shook, 1987), to udder infection status (Sheldrake et al., 1983) and to individual cows. Heritability estimates of test-day SCS range between 0.07 and 0.44 (Ali and Shook, 1980, Kennedy et al., 1982, Emanuelson and Philipsson, 1984 Gadini et al., 1996). Monardes and Hayes (1985) reported repeatability estimates of test-day SCS of 0.36–0.42 across parities, and repeatability within lactations ranged from 0.47 to 0.59 (Emanuelson and Persson, 1984). These results suggest the need to consider both the variation within and between cows for an adequate modeling of SCS trends. Further, an important fraction of the variation seems to be related to genetic factors.
Lactation average SCS measures are used currently in some national genetic evaluation schemes, the objective being to lower the prevalence of mastitis by indirect selection for SCC. This approach does not use all the information and masks short-term variation in SCS (Shook and Schutz, 1994, Emanuelson, 1997). Test-day somatic cell records are taken on the same cow at various times, providing longitudinal information that can be related more meaningfully to episodes of infection. A statistical model for longitudinal data may provide more accurate estimates of the influence of risk factors than lactation average models. This additional information should be beneficial in the study of a trait with seemingly low heritability, such as SCS.
A reasonable model for describing SCS variation during lactation must consider the ‘typical’ lactation pattern and allow for differences associated with explanatory variables (e.g. age of cow) and with the individual cows themselves. Nonlinear mathematical functions are natural candidates for describing SCS lactation patterns, and their parameters can be modeled to account for known sources of variation. Estimates of these parameters can be used for management and breeding purposes to the extent that at least some of the variation detected is heritable.
The main objective of this study was to explore the feasibility of nonlinear mixed effects models to describe the SCS lactation curves. Another objective was to compare the fit of four nonlinear models and two linear models when applied to SCS lactation records in Holstein cows. An approximate maximum likelihood analysis was used for these purposes.
Section snippets
Data
The Ohio Agricultural Research and Development Center (Wooster) provided the SCC data used. After edits, the data consisted of 2387 test-day observations from 217 first to third lactation Holstein cows recorded between July 1982 and June 1989. The herd was maintained free of Streptococcus agalactiae, and less than 1% of the quarters were infected with Staphylococcus aureus at any one time (Todhunter et al., 1991). The bacteriological status was assessed by 0.01 ml of milk streaked onto the
Results
Mean 305-day, mature equivalent milk yield was about 7600 kg. Average SCC and standard deviation was 671,000 and 897,000 cells/ml, respectively. A few cows, with clinical mastitis symptoms, have very high levels of SCS, causing the average to be higher than the values usually associated to minor pathogens. As shown in Table 1, SCS decreased to a nadir at about 60 DIM and then increased, although not in a monotonic fashion, without regaining the initial level. The standard deviation followed an
Discussion
The overall trend of SCS during lactation is consistent with reports by Emanuelson and Philipsson (1984) and Wiggans and Shook (1987). The nonlinear (in time) SCS pattern observed could be explained by effects of stress at the onset of lactation and of dilution thereafter (Wiggans and Shook, 1987). Some of the models studied here have been used, with variable success, to describe milk yield. However, it is a challenge to find an appropriate model for SCC or SCS. Immunological responses may
Conclusion
Somatic cell score lactation patterns in dairy cows were described with nonlinear mixed effects models. Parameter estimates may be useful for selection schemes or management. In this study, four nonlinear and two linear mixed effects model were used. Within these, Morant and Gnanasakthy’s model provided the best fit. There was a significant association between intramammary infection and SCS trajectory. Cows calving between 1983 and 1985 had higher SCS levels and flatter curves, based on the
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