Sustainability & efficiency

The association between genetic merit for live-weight and energy intake and efficiency in grazing dairy cows

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Abstract

Accurately measuring dry matter intake (DMI) in grazing dairy cows remains expensive, invasive, and time-consuming. In contrast to countries with predominantly indoor dairy systems, where DMI phenotypes can be more easily measured and incorporated into total merit indexes, pasture-based systems lack cost-effective feed intake measurement strategies for grazing animals. Consequently, direct measures of feed intake have largely been restricted to research herds with live-weight measures used as genetic predictors of feed requirements in total merit indexes. Questions are now being asked as to whether a truly linear relationship exists between genetic merit for live-weight and actual feed intake in grazing animals. The objective, therefore, of the present study was to characterize energy intake and energy efficiency in grazing Holstein-Friesian cows stratified by their genetic merit for live-weight. Following data editing, 3,812 mid-lactation (i.e., 91 to 180 days in milk) net energy intake (NEI) records from 1,418 grazing dairy cows were available. Energy efficiency (NEIadj) was defined as the residual of NEI after adjusting for differences in energy sinks. Parental average estimated breeding value (EBV) for live-weight were available from the Irish national genetic evaluation. Cows were stratified into 10 groups on their parental average EBV for live-weight. EBV for live-weight was standardized and a variable derived representing 10 equally spaced points to impose equal distance between strata to facilitate modelling of linear differences in performance across the live-weight EBV strata. Predicted marginal means for phenotypic NEI and NEIadj were estimated using linear mixed models. Days in milk (classified into 30-day intervals), parity (1, 2, 3, 4, ≥5), and EBV stratum for live-weight as well as the EBV covariate were included as fixed effects; experimental treatment by year group nested within farm and a permanent environmental effect were included as random effects. Predicted marginal means for NEI increased progressively across strata of increasing parental average EBV for live-weight, though the increase was not linear. Conversely, predicted marginal means for NEIadj decreased relatively linearly as genetic merit for live-weight increased. On average, the difference in genetic merit for live-weight between cows ranked in the extreme deciles was 77 kg, while cows ranked in the lightest 10% on genetic merit in live-weight consumed, on average, 0.8 unité fourragère du lait (UFL) per day less than cows in the heaviest 10%; the mean NEI of the dataset was 17.8 UFL. The positive, though non-linear association between parental average EBV for live-weight and phenotypic NEI confirms that live-weight is an effective proxy for estimating energy intake in grazing dairy cows. Nevertheless, cows with higher genetic merit for live-weight had lower NEIadj, suggesting that while genetically heavier cows consumed more energy overall, they utilized that energy more efficiently relative to their expected requirements.

Keywords: 2026

How to Cite:

Berry, D. & Williams, M., (2026) “The association between genetic merit for live-weight and energy intake and efficiency in grazing dairy cows”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2286418. doi: https://doi.org/10.31274/wcgalp.23995

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