Test-day fat to protein ratio as an indicator trait for sub-clinical ...

White cattle. Estimates of Meinert et al. (1989) for this trait were 0.31 for the part of
lactation from 40 to 100 DIM and 0.69 for the entire 305 d lactation for the same
population. Lower heritability of F:P (0.14) was estimated for Austrian Simmentals
(Gredler et al. 2006) and for Spanish Holsteins (0.19) (Peña, 2006). Genetic correlations
between milk yield and protein to fat ratio was 0.19 for the entire 305 d lactation and -
0.15 for DIM from 40 to 100 (Meinert et al. 1989), and 0.18 (Vos and Groen, 1998).
Estimates of phenotypic correlation between these traits ranged from 0.19 (Vos and
Groen, 1998) to 0.88 (Meinert et al. 1989). To our knowledge, estimates of correlation
between F:P and SCS have not been up till now a subject of any published study.
Producer-recorded data collection of clinical mastitis cases started in 2007 in
Canadian dairy herds, with the objective to develop genetic selection tools for resistance
to clinical mastitis. Selection index based on genetic evaluations of SCS and mammary
system conformation will likely continue to complement direct selection against clinical
mastitis. Test-day milk yield and F:P, especially at the beginning of lactation, seem to be
potential traits that could contribute to the accuracy of such an index, with the overall
goal of reducing mastitis incidence in Canadian dairy cattle.
CONCLUSIONS
A mixture model for TD milk, F:P and SCS allowed classification of the data into
two components: healthy and infected with putative, sub-clinical mastitis. Distributions
of data differed in means and variances between mixture components and across different
parts of lactation. Frequency of mastitic records was high immediately after calving and
decreased as lactation progressed. Records from infected cows had lower TD milk yield,
and higher F:P and SCS, as compared with healthy cows. Fat to protein ratio, easily
available, highly heritable and relatively independent from milk and SCS, could serve as
an additional indicator for indirect selection against mastitis in dairy cattle.
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