Modelling Extended Lactations in Polish Holstein-Friesian Cows

Area of Science:

• Animal Science
• Dairy Science
• Quantitative Genetics

Background:

• Accurate modeling of lactation curves is crucial for dairy herd management and genetic evaluations.
• The Wilmink function is a common nonlinear model for describing milk production dynamics.
• Understanding lactation curve shapes beyond 305 days in milk (DIM) is important for extended lactation analysis.

Purpose of the Study:

• To evaluate different lactation curve shapes using the Wilmink function for milk, fat, protein, lactose, and milk urea content.
• To determine the best-fitting function for test-day records beyond 305 DIM in Polish Holstein-Friesian cows.
• To compare the modeling accuracy of different functions for various milk components.

Main Methods:

• Utilized 6,955,768 test-day records from 284,193 Polish Holstein-Friesian cows across six lactations (2001-2018).
• Fitted three models: Wilmink for entire lactation, Wilmink (5-305 DIM) + linear (306-400 DIM), and Wilmink (5-305 DIM) + squared (306-400 DIM).
• Assessed model fit for milk yield, fat, protein, lactose, and milk urea content.

Main Results:

• The Wilmink function effectively modeled lactation curves up to 305 DIM for all tested milk components.
• Milk urea content showed slightly poorer model fit compared to milk yield, fat, protein, and lactose.
• Linear and squared functions provided suitable descriptions for lactation records beyond 305 DIM.

Conclusions:

• Nonlinear models like the Wilmink function are effective for modeling lactation up to 305 DIM.
• Post-305 DIM lactation can be adequately described by linear or squared functions.
• Model selection should consider the specific milk trait and lactation stage for optimal accuracy.