Evaluation of heat stress effects on production traits and somatic cell score of Holsteins in a temperate environment

Abstract

This study was aimed to evaluate the degree of thermal stress exhibited by Holsteins under a continental temperate climate. Milk, fat, protein, and somatic cell count test-day records collected between 2000 and 2011 from 23,963 cows in 604 herds were combined with meteorological data from 14 public weather stations in Luxembourg. Daily values of 6 different thermal indices (TI) weighted in term of temperature, relative humidity, solar radiation, and wind speed were calculated by averaging hourly TI over 24h. Heat stress thresholds were first identified by a broken-line regression model. Regression models were thereafter applied to quantify milk production losses due to heat stress. The tipping points at which milk and protein yields declined were effectively identified. For fat yield, no valid threshold was identified for any of the studied TI. Daily fat yields tended to decrease steadily with increasing values of TI. Daily somatic cell score patterns were marked by increased values at both lowest and highest TI ranges, with a more pronounced reaction to cold stress for apparent temperature indices. Thresholds differed between TI and traits. For production traits, they ranged from 62 (TI(1)) to 80 (TI(3)) for temperature-humidity indices (THI) and from 16 (TI(5)) to 20 (TI(6)) for apparent temperature indices. Corresponding somatic cell score thresholds were higher and ranged from 66 (TI(1)) to 82 (TI(3)) and from 20 (TI(5)) to 23 (TI(6)), respectively. The largest milk decline per unit of mild, moderate, and extreme heat stress levels of 0.164, 0.356, and 0.955 kg, respectively, was observed when using the conventional THI (TI(1)). The highest yearly milk, fat, and protein losses of 54, 5.7, and 4.2 kg, respectively, were detected by TI(2), the THI index that is adjusted for wind speed and solar radiation. The latter index could be considered as the best indicator of heat stress to be used for forecast and herd management in a first step in temperate regions under anticipated climate changes.

Dairy cattle in a temperate climate: the effects of weather on milk yield and composition depend on management

“…The importance of direct temperature metrics in explaining productivity is consistent with a wealth of studies on the impact of heat stress in dairy cattle (Renaudeau et al, 2012). Relatively few studies have assessed the impact of other weather variables on milk traits, but thermal indices that account for WS and solar radiation perform better than those that do not (Hammami et al, 2013). In our study, weather metrics summarised across a week's timescale from the TD usually explained milk traits (particularly yield and fat content) better than shorter-scale summaries.…”

“…AIC has been used previously to compare temperature indices in explaining milk traits (Bruegemann et al, 2012;Hammami et al, 2013). As the metrics for summarising a given element were closely correlated, and high proportions of shared variance can lead to unreliable estimates, we fitted each metric in a separate linear mixed effects model (LMM) (equation (3)) using maximum likelihood to produce a series of non-nested models.…”

“…High temperatures are associated with a greater incidence of heat stress in livestock, which can have negative effects on milk yield (Bohmanova et al, 2007;Hammami et al, 2013), fertility (Hansen, 2009) and health , and increase the risk of mortality (Vitali et al, 2009). Heat stress occurs when animals experience conditions above their thermal comfort zone and are unable to dissipate enough heat to maintain † E-mail: davina.hill@sruc.ac.uk Animal (2015), 9:1, pp 138-149 © The Animal Consortium 2014 doi:10.1017/S1751731114002456 animal 138 thermal balance (Kadzere et al, 2002).…”

“…Cattle generate metabolic heat as a by-product of milk synthesis and so higher yielding animals experience heat stress at lower THIs than lower yielders (Kadzere et al, 2002). An animal's thermal tolerance is also affected by solar radiation and the velocity of ambient air (Dikmen and Hansen, 2009;Graunke et al, 2011;Hammami et al, 2013), while increasing precipitation (ppt) is associated with declining milk production (Stull et al, 2008). Weather-related stressors could potentially affect performance immediately or have a delayed impact, and yet few studies have explored the time interval between weather events occurring and impacting milk traits (St-Pierre et al, 2003).…”

“…While the effects of heat stress on dairy cows has been well-documented in tropical and subtropical regions (e.g. West et al, 2003;Dikmen and Hansen, 2009), a growing number of studies has also reported associations between THI and milk traits in temperate regions where tolerance to heat stress is lower (Bruegemann et al, 2011;Hammami et al, 2013;Dunn et al, 2014). Moreover, temperatures are predicted to increase over the 21st century in southern Scotland, especially in summer, with an expected mean daily maximum temperature increase of 4.3°C by the 2080s with a very slight reduction (0% to 5%) in humidity (Jenkins et al, 2009).…”