Performance of grazing dairy cows genetically divergent in genetic merit for feed intake

Genetic evaluations are the cornerstone of modern dairy breeding programs, facilitating selection for traits of economic and environmental importance. The utility of any genetic evaluation is dependent on its ability to predict phenotypic performance in an independent population. This is of particular importance for traits that are not routinely measured on commercial farms, such as feed intake and efficiency. The objective of the present study was to characterize the performance of grazing dairy cows stratified on their EBV for energy intake. After edits, 14,451 net energy intake (NEI) records each with a corresponding live weight, BCS, and milk production record from 5,060 lactations on 2,521 cows were available; the cows were a mixture of Holstein-Friesian, Jersey, Montbéliarde, Norwegian Red, and crossbreds across 5 separate Irish research farms. These NEI records were divided into a calibration and a corresponding validation dataset using 2 validation scenarios: (1) validation by year of recording or (2) validation by farm. Cows in the validation populations were stratified, within experimental treatment by year, into 4 strata based on their within-breed EBV for NEI. Predicted marginal means were estimated using linear mixed models for a series of performance traits in each EBV stratum. Predicted marginal means for NEI increased progressively across strata of increasing EBV for NEI in both the validation-by-year of recording and validation-by-farm scenarios. In fact, for the validation-by-year scenario, cows ranked in the bottom 25% on genetic merit for NEI (i.e., genetic merit for lower NEI) consumed, on average, 1.0 unité fourragère du lait (UFL)/d less than cows in the top 25%; average NEI in the dataset was 17.2 UFL. Similarly, when based on the validation-by-farm scenario, cows ranked in the bottom 25% on genetic merit for NEI (i.e., genetic merit for lower NEI) consumed, on average, 0.5 UFL/d less than cows in the top 25%. Additionally, cows genetically predisposed to higher NEI produced more milk, were heavier, and produced more milk relative to their metabolic BW than their herd contemporaries genetically predisposed to consume less. When validated by farm, cows ranked in the top 25% on genetic merit for NEI produced 3% more milk (NEL) and weighed 9.2 kg more than cows in the bottom 25%; the mean NEL and mean live weight in the entire dataset was 9.8 UFL/d and 513 kg, respectively. The findings of the present study will help inform the potential of direct selection for feed intake to deliver phenotypic change in feed intake among grazing dairy cows.