Influence of herd and environmental factors and phenotypic trends in productive and reproductive traits in dairy farms in Argentina

Abstract

Argentina hosts a wide range of dairy production systems, from pasture-based to confined models. Despite this diversity, genetic improvement has relied almost exclusively on imported semen from high-merit Holstein bulls. Unregistered Holstein populations managed under pasture-based systems account for a substantial share of national milk production, yet their productive and reproductive performance remains poorly documented. To address this gap, this study evaluated the effects of herd and environmental factors on production and fertility traits and assessed recent phenotypic trends in pasture-based dairy herds. The dataset comprised 46,588 lactations from 19,936 cows across 16 commercial herds recorded between 2010 and 2023. Traits analyzed included 305-day milk, fat and protein yields, calving-to-first-service interval, days open and calving-to-calving interval. Herd, lactation number, calving month and calving year significantly affected all traits (p≤0.0001). Peak milk yield occurred between the third and fifth lactations, while reproductive performance declined with increasing parity. Cows calving in autumn–winter (May–August) showed superior production and fertility, coinciding with months exhibiting fewer number of days with temperature–humidity index values above the heat-stress threshold. Variance component analysis indicated that cow effects explained 16–19% of the variance in production traits and 8–11% in fertility traits. Phenotypic overall trends suggested stabilization in milk yield and limited progress in fertility, although more favourable trends were observed during 2020–2023. While these recent improvements may reflect changes in herd-level conditions over time, potential genetic contributions cannot be ruled out. In pasture-based systems, the expression of imported high-merit genetics may differ across environmental conditions, underscoring the potential value of breeding programs adapted to local production conditions