Genetic parameters for efficiency of crude protein utilisation and its relationship with production traits across lactations in grazing dairy cows

Abstract

ABSTRACT The objective of the study was to estimate genetic parameters of efficiency of crude protein utilisation (ECPU) and its genetic correlation (r G) with other production traits across lactation in grazing dairy cows. Data were originated from 467 (2016–17 season) and 460 cows (2017–18 season) representing Holstein-Friesian (F), Jersey (J) and their crosses (F×J), from two New Zealand farms. The ratio between crude protein yield in milk and crude protein intake is defined as ECPU. (Co)variance components between ECPU and other traits were estimated using bivariate random regression models (RRM) that included fixed effects of herd-test-date, parity, calving date, proportion of F, and F×J heterosis, with random effects of animal additive genetic, and within-lactation permanent environment. Estimates of heritability (h2 ) of daily ECPU ranged from 0.02 to 0.41. Estimates of r G between daily ECPU and milk yield traits were strong and positive throughout lactation suggesting that selection for milk production traits improves ECPU of dairy cows. Daily r G between ECPU and mid-infrared predictions of milk urea (MU) concentration were mostly negative during lactation confirming that cows with high ECPU had genetically lower MU. Estimates of genetic relationships between ECPU and fertility traits are required to assess implications of including ECPU in breeding objective.