Feed intake and efficiency of grazing Holstein-Friesian dairy cows divergent in genetic merit for live-weight or milk production.

Accurately measuring individual animal DMI in grazing dairy cows remains expensive, invasive, and time-consuming making the inclusion of direct measure of feed intake or efficiency in genetic evaluations for pasture-based systems challenging. Consequently, live-weight is commonly used as an indicator of feed requirements in dairy cow total merit indexes, with the economic value for feed intake often applied directly to an animal's genetic merit for live-weight. The biological appropriateness of this strategy, however, at a population level, has not been evaluated using large-scale intake data. The objective of the present study was to quantify the mean phenotypic net energy intake (NEI) and energy efficiency across lactation of grazing Holstein-Friesian cows stratified into deciles based on genetic merit for live-weight, testing the hypothesis that energy intake, on average, increases with increasing genetic merit for live-weight; stratification on genetic merit for fat and protein corrected milk was also explored. After edits, 3,163 DMI records from 2,201 lactations on 1,220 cows were available in early lactation, with corresponding mid- and late-lactation data sets comprising 3,703 records from 2,500 lactations and 2,758 records from 2,234 lactations, respectively. Within each stage of lactation, cows were stratified into 10 strata within their experimental treatment by year contemporary group based on their national paternal average EBV for live-weight and separately on their EBV for fat and protein corrected milk; the mean phenotypic difference between extreme deciles was 60 kg (equivalent to 1.4 genetic SD) for live-weight and 274 kg (equivalent to 0.88 genetic SD) for fat and protein corrected milk. Predicted marginal means for NEI, and NEI adjusted for energy sinks, in each EBV strata were estimated using linear mixed models. Across all stages of lactation, average NEI increased approximately linearly with increasing genetic merit for live-weight, with mean differences of 0.9, 1.0, and 1.1 unité fourragère du lait (UFL) between cows in the extreme deciles in early, mid, and late lactation, respectively; regression of NEI means on stratum mean live-weight EBV confirmed a strong linear association (R 2 = 0.89, 0.90, and 0.80). When live-weight EBV was fitted as a continuous covariate, individual cow NEI increased, on average, by 0.11, 0.08, and 0.10 (SE ranged from 0.02 to 0.03) UFL per 10-kg increase in live-weight EBV in early, mid, and late lactation, respectively. In contrast, mean NEI adjusted for energy sinks declined with increasing genetic merit for live-weight, with genetically heavier cows consuming, on average, 0.8, 0.9, and 0.6 UFL less net energy than genetically lighter cows in early, mid, and late lactation, respectively. When stratified on their genetic merit for fat and protein corrected milk, no stratum differences in mean NEI were observed in early lactation, whereas in mid and late lactation, cows of higher genetic merit for fat and protein corrected milk consumed, on average, more net energy than contemporaries that were genetically inferior for fat and protein corrected milk. Collectively, these results demonstrate a strong and largely linear relationship between genetic merit for live-weight and phenotypic energy intake at the sub-population level in grazing dairy cows, supporting the continued use of live-weight EBV as tool to shift mean (sub-)population feed intake in pasture-based breeding objectives.