Dairy cattle

The effect of heat stress on daily milk yield in Holstein Nordic cattle

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Abstract

Climate change is increasing the frequency and intensity of heat events, exposing even temperate regions to prolonged warm periods that can induce heat stress in dairy cattle, affecting milk production. This study sought to investigate how warm weather affects milk yield in Nordic Holstein cattle in Sweden. Deviations from the target lactation curve for each lactation were analyzed to identify the onset of heat stress and predictors of sensibility to heat stress with daily milk records from milking robots. Data included 560,877 daily milk yield (DMY) records for 1,056 multiparous cows. Expected lactation curves were fitted using fourth-order polynomial quantile regression, and deviations from the target curve (RES) were estimated as the difference between the expected and actual DMY. Heat stress was quantified by evaluating the effect of a 5-day average of maximum temperature (T5d) on RES and DMY using generalized additive model (GAM). The model includes the fixed effects for herd, lactation number and calving group (combining calving year and calving season), age at calving as linear and quadratic terms, a random effect of the cow, and smoother terms for days in milk (DIM) and T5d were modeled using cubic regression spline functions to capture the potential nonlinear effects. The heat stress threshold (±SE) was determined as the point where the fitted value of the smoother for T5d began to decline, using segmented regression analyses. This threshold occurred at 23.6 ± 0.4°C for DMY and 21.1± 0.2°C for RES. Based on highest coefficients of determination (0.97) and lower SE, the RES threshold was used calculate the heat load (HL) function, defined as HL = max(0,T5d-21), which was then included as a linear covariate in an additional GAM. The HL was intended to measure the amount of heat exceeding a given threshold. The results showed that cows were most sensitive to heat stress between DIM 60 and 140. Otherwise, cows in their first lactation were less affected by high temperatures. This study suggests that RES is a reliable approach to estimate the effect of temperature on milk yield, allowing the identification of heat stress onset. The findings suggest that even in regions with cold climates, such Nordic countries, summer temperatures can reach levels that negatively impact milk yield.

Keywords: 2026

How to Cite:

Ask-Gullstrand, P., Dominguez-Castaà±o, P., Johnsson, M., Klingström, T., Ohlsson, I., Olmos Antillón, G., Svensson, J. & Tamminen, L., (2026) “The effect of heat stress on daily milk yield in Holstein Nordic cattle”, World Congress on Genetics Applied to Livestock Production Digital Archive 2026(1): 2280038. doi: https://doi.org/10.31274/wcgalp.23437

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