Abstract
The inclusion of reproductive performance in dairy cow breeding schemes has resulted in a cumulative improvement in genetic merit for reproductive performance; this improvement should manifest in longer productive lives through a reduced requirement for involuntary culling. Nonetheless, the average length of dairy cow productive life has not changed in most populations, suggesting that risk factors for culling, especially in older cows, are possibly more associated with lower yield or high somatic cell score (SCS) than compromised reproductive performance. The objective of the present study was to understand the dynamics of lactation yields and SCS in dairy cows across parities and, in doing so, quantify the potential to alter this trajectory through breeding. After edits, 3,470,520 305-d milk, fat, and protein yields, as well as milk fat and protein percentage and somatic cell count records from 1,162,473 dairy cows were available for analysis. Random regression animal models were used to identify the parity in which individual cows reached their maximum lactation yields, and highest average milk composition and SCS; also estimated from these models were the (co)variance components for yield, composition, and SCS per parity across parities. Estimated breeding values for all traits per parity were calculated for cows reaching ≥fifth parity. Of the cows included in the analyses, 91.0%, 92.2%, and 83.4% reached maximum milk, fat, and protein yield in fifth parity, respectively. Conversely, 95.9% of cows reached their highest average fat percentage in first parity and 62.9% of cows reached their highest average protein percentage in third parity. In contrast to both milk yield and composition traits, 98.4% of cows reached their highest average SCS in eighth parity. Individual parity estimates of heritability for milk yield traits, milk composition, and SCS ranged from 0.28 to 0.44, 0.47 to 0.69, and 0.13 to 0.23, respectively. The strength of the genetic correlations per trait among parities was inversely related to the interval between the parities compared; the weakest genetic correlation was 0.67 (standard error = 0.02) between milk yield in parities 1 and 8. Eigenvalues and eigenfunctions of the additive genetic covariance matrices for all investigated traits revealed potential to alter the trajectory of parity profiles for milk yield, milk composition, and SCS. This was further demonstrated when evaluating the trajectories of animal estimated breeding values per parity.
Highlights
The inclusion of functional traits, like reproductive performance, in dairy cow breeding programs has contributed to a cumulative improvement in the genetic merit for reproductive performance in most dairy cow populations (Berry et al, 2014; Cole and VanRaden, 2018; De Vries, 2020)
The improvement achieved in genetic merit for reproductive performance should have manifested itself in a longer dairy cow productive life via reduced involuntary culling for reproductive failure
The Akaike information criterion improved with each increasing polynomial order for the additive genetic term in the model; the fourth eigenvalue of cubic animal random regression explained less than 1.4% of the genetic variance for all traits
Summary
The inclusion of functional traits, like reproductive performance, in dairy cow breeding programs has contributed to a cumulative improvement in the genetic merit for reproductive performance in most dairy cow populations (Berry et al, 2014; Cole and VanRaden, 2018; De Vries, 2020). The improvement achieved in genetic merit for reproductive performance should have manifested itself in a longer dairy cow productive life via reduced involuntary culling for reproductive failure. Poor lactation yield and high SCC are already 2 of the primary reasons reported for voluntary culling in dairy cows (Berry et al, 2005; Kerslake et al, 2018; De Vries and Marcondes, 2020) and the rate of culling due to high SCC will likely increase as European dairy producers are required to implement selective dry cow therapy. Age-linked reductions in lactation yield or increases in SCC could potentially explain why productive life has not improved in line with the genetic gain in reproductive performance
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