When is too small, too small? Evaluating the trade-offs of smaller dairy cows in pasture-based systems

Interest in dairy cow size as a determinant of overall production efficiency is intensifying globally across different production systems, particularly in pasture-based systems where energy intake is constrained. Smaller cows are theoretically more efficient, as a lower proportion of their energy intake is required for maintenance. However, concerns persist that reducing genetic merit for cow live weight may compromise productivity, both in terms of milk yield and progeny carcass weight. A key concern is that output could decrease rapidly if live weight drops below a critical threshold. The objective of the present study was to quantify the association between genetic merit for live weight and phenotypic performance for milk production and progeny carcass weight in Irish pasture-based Holstein-Friesian cows. Following all edits, phenotypic 305-d milk yield, fat yield, and protein yield were available on 831,307 lactations from 583,361 cows calving in 2023 and 2024, whereas live weight records were available for 27,600 cows, with carcass weight data available for 49,906 of their progeny. Cows were stratified into 20 groups based on live weight PTA, and associations with milk production traits and progeny carcass weight were quantified using linear mixed models. Phenotypic cow live weight increased almost linearly with genetic merit for live weight across all parities. Milk yield and milk solids yield also increased with live weight PTA, at rates of ~1.1%, 1.3%, and 1.3% per 10-kg increase in live weight PTA for parities 1, 2, and 3, respectively. Milk protein concentration did not change with cow live weight PTA but milk fat concentration reduced as cows got genetically heavier. Progeny carcass weight increased linearly with dam live weight PTA. In all, within the range of cow sizes typical of pasture-based systems, genetically lighter Holstein-Friesian cows produced marginally less milk and had lighter progeny, but no evidence of a threshold below which productivity declined rapidly was detected. These results suggest that moderate genetic reductions in cow live weight may improve herd efficiency without materially compromising milk solids or progeny carcass output. Therefore, given that the increase in output associated with genetically heavier cows was modest compared with the potential increased maintenance costs of heavier cows, the economic advantage of selecting for heavier cows in pasture-based systems may be limited.