Abstract
A recently developed biological model of lactation described changes in daily milk yield throughout lactation as the result of 3 processes, secretory cell differentiation, cell death, and secretion rate per cell. This paper extends the model to describe the production of milk components (fat, protein, lactose, and water) throughout lactation by replacing milk secretion rate of the original model with the secretion rates of the four components. The milk component model approach was used to examine the relationship between milk yield and the major determinants of its production, using the secretion of milk components throughout lactation. Newly derived models were tested on 461 lactations from a single Holstein herd and used to estimate variability of secretion rates throughout lactation. Because the pattern of cell numbers throughout lactation is not precisely known, an alternative pattern of cell numbers was modeled and the concomitant change in secretion rates outlined. Fat secretion rate was the most variable, as measured by its weekly coefficient of variation throughout lactation. Secretion rates of lactose and water were nearly constant throughout lactation and highly correlated (0.94). Fat and protein secretion rates also were well correlated (0.53). The known biochemistry of milk component production related well to the secretion rate observations derived from the model. Lactose secretion rate and numbers of active secretory cells primarily determined daily milk yield.
Highlights
Milk is a mixture of fat, protein, lactose, vitamins, and minerals, either dissolved or suspended in water
In their review of mammary development and function, Knight et al (1998) concluded that the decline in milk yield in late lactation paralleled the decline in secretory cell numbers and that secretory cell activity remained high throughout declining lactation
Proportions of fat, protein, lactose, water, and total solids can be described in terms of the number of active secretory cells and the secretion rate per cell
Summary
Milk is a mixture of fat, protein, lactose, vitamins, and minerals, either dissolved or suspended in water. In their review of mammary development and function, Knight et al (1998) concluded that the decline in milk yield in late lactation paralleled the decline in secretory cell numbers and that secretory cell activity remained high throughout declining lactation. They stated that secretory cells continued to be produced until around the peak of lactation. Patterns of secretion rate of milk components per cell are largely unknown due to the difficulty of measuring the number of active cells on any given day of lactation, or individual cell metabolism throughout lactation
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