Abstract
The object of this study was to investigate the genetics of lactation curve parameters derived from a biological model of lactation and the relationships among them. This biological model fitted 2 logistic curves to mimic the initial increase in milk secretory cell numbers in early lactation and the progression of apoptosis in late lactation. Records from 82,255 Holstein-Friesian heifers from commercial dairy herds in the United Kingdom, recorded from 1994 to 2003, were analyzed. The heritabilities of 2 lactation curve parameters, maximum secretion potential and relative cell death rate, were 0.27 and 0.08 respectively. Maximum secretion potential was highly genetically correlated with peak yield (0.99), and relative cell death rate was highly correlated with persistency of lactation (0.84). Heritability values for the traits analyzed showed a characteristic pattern. Total milk yield traits, maximum secretion potential, and peak yield had similar and moderate heritabilities (∼0.3). Traits associated with late lactation had lower heritability values (∼0.1), whereas day of peak yield and early lactation traits had little genetic variation. The permanent environmental variance of the various traits ranged from 0.08 to 0.26 of the phenotypic variance. Parameters from the 2 logistic curves were not highly correlated, suggesting that selection programs could be devised to exploit genetic variation in both aspects of lactation independently.
Highlights
Milk production during lactation from dairy animals has been widely studied using a range of mathematical models (Masselin et al, 1987)
The results showed that the biological models fitted dairy sheep lactations with a smaller or similar residual mean square than the other models with the same number of parameters
The objective of this study was to determine the genetics of, and the relationship among, parameters of the lactation curve of dairy cows using a biological model of lactation on records from commercial dairy cow herds
Summary
Milk production during lactation from dairy animals has been widely studied using a range of mathematical models (Masselin et al, 1987). The incomplete gamma function proposed by Wood (1967) has been the most widely used model to fit the lactation curve (Olori et al, 1999) This model accounts for the main parameters of the lactation curve such as milk yield at the start of lactation, rate of increase to peak yield, and persistency of lactation. A biological approach to lactation curve models was proposed by Dijkstra et al (1997) and Pollott (2000) The latter outlined a biological model that described the curve parameters in terms of secretory cell differentiation in early lactation, cell death rate after parturition, and a change in secretion rate per cell during lactation. This approach was based on the dynamics of the mammary cell population as described by Knight and Wilde (1993), Wilde et al (1997), and Knight et al (1998)
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