# Long-lasting effects of in utero heat stress on subsequent performances of heifers and primiparous cows

A. Vinet, C. Fouéré, B.C.D. Cuyabano, S. Mattalia, R. Vallée, A. Barbat, C.

Bertrand, C. Hoze, D. Boichard

► To cite this version:

A. Vinet, C. Fouéré, B.C.D. Cuyabano, S. Mattalia, R. Vallée, et al., Long-lasting effects of in utero heat stress on subsequent performances of heifers and primiparous cows. Journal of Dairy Science, 2024, 107 (12), pp.11065-11077. 10.3168/jds.2024-25168. hal-04885784

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# Long-lasting effects of in utero heat stress on subsequent performances of heifers and primiparous cows

A. Vinet,<fnref n="1" />* C. Fouéré,<fnref n="1" />,<fnref n="2" /> B. C. D. Cuyabano,<fnref n="1" /> S. Mattalia,<fnref n="3" /> R. Vallée,<fnref n="3" /> A. Barbat,<fnref n="1" />

C. Bertrand,<fnref n="4" /> C. Hoze,<fnref n="2" /> and D. Boichard<fnref n="1" />

## ABSTRACT

The performance of an adult dairy cow may be influenced by heat stress that occurs during her gestation. The present study investigated potential effects of temperature-humidity index (THI) experienced by a cow during pregnancy on the gestated daughter's performance on her first lactation, for the French Holstein and Montbéliarde dairy cattle populations. We analyzed 14 traits, all measured on genotyped cows: 305-d milk, fat, and protein yields; 305-d SCS; clinical mastitis (both occurrence and number of events); body conformation traits; and heifer and cow conception rate. To study the effect of heat stress, we considered the THI experienced by the gestating cow, averaged for each month of her pregnancy and then categorized in 7 classes (≤40, [40,45], [45,50], [50,55], [55,60], [60,65], and >65). These average THI classes were then fitted as categorical covariates in the regression models used for this study, which included other fixed effects, and the GEBV as a covariate, both specific to each trait, the latter being previously obtained from the official French evaluations. The THI effect was therefore estimated as the deviation between the observed and predicted performances. In general, the estimated heat stress effects were small, presenting limited practical impact on the studied traits, and particularly for fertility and udder health, the estimated heat stress effects were not statistically significant. For the production traits (i.e., milk, fat, and protein yields), the estimated effect associated with high THI experienced at the beginning of the gestation was negative, and slightly positive when associated with high THI experienced by the dam at the end of her pregnancy. Finally, our results suggest that under the current French climate conditions, heat stress experienced by cows during any stage of their

pregnancy has limited impact on the future performance of their gestated daughters; however, we cannot exclude that a significant in utero heat stress effect may be present in climate conditions warmer than the French.

**Key words:** dairy cattle, prenatal effects, environmental effects, temperature-humidity index, fetal programming

## INTRODUCTION

Climate change is expected to have major negative consequences for livestock production. One of the main risks will come from heatwaves, which will become more frequent and intense. Exposure to very hot environments can challenge cattle homeothermy, thereby inducing heat stress (Shephard and Maloney, 2023) and the onset of behavioral, physical, biochemical, and physiological processes as a response to counteract the negative effects of heat stress and maintain thermal equilibrium (Silanikove, 2000; Collier et al., 2017). Among these processes responding to heat stress, some, such as panting, sweating, or greater chemical reaction rates, can increase maintenance requirements in cattle (Baumgard and Rhoads, 2012). On the other hand, a decrease in feed intake due to heat exposure is an adaptive mechanism to decrease diet-induced thermogenesis (Kadzere et al., 2002; Collier et al., 2017). In addition to these direct responses to heat stress, and as a consequence of many of them, animals exposed to heat stress present a decrease in productivity, with negative short-term effects of heat stress reported for many commercial cattle traits, such as production, reproduction, and health or welfare traits (Polsky and Von Keyserlingk, 2017; Lees et al., 2019; Vinet et al., 2023, 2024).

For pregnant cows, an increase of their core body temperature may affect not only the cow's own performance, but also the gestated offspring (De Rensis and Scaramuzzi, 2003; Wolfenson and Roth, 2019). Heat stress compromises the intrauterine environment, with a decrease in blood flow to the uterus and an increase

<footnote n="1">¹Université Paris Saclay, INRAE, AgroParisTech, GABI, 78350 Jouy-en-Josas, France</footnote>

<footnote n="2">²Eliance, 75012 Paris, France</footnote>

<footnote n="3">³Idele, 75012 Paris, France</footnote>

<footnote n="4">⁴INRAE, US210 CTIG, 78350 Jouy-en-Josas, France</footnote>
in uterine temperature (De Rensis and Scaramuzzi, 2003). Several authors reported that heat stress inhibits embryonic development, increases early embryonic loss, and reduces the proportion of successful inseminations (Roth, 2008; De Rensis et al., 2015; Hansen, 2019). High ambient temperature also affects the pre-attachment stage of embryos. Nevertheless, as pregnancy progresses, embryos become more resistant to the adverse effects of maternal heat stress (Wolfenson and Roth, 2019). It is generally believed that fetal temperature is completely dependent on the maternal temperature (Asakura, 2004) because the body temperature of the fetus is dependent on its metabolic heat production and on the heat transfer with the dam. The fetus exchanges heat with the dam mainly through the fetal-placental circulation, a process that accounts for approximately 85% of total fetal heat loss; the remaining heat is exchanged conductively through fetal membranes, amniotic fluid, and the uterine wall (Asakura, 2004). Consequently, changes in maternal core temperature affect fetal temperature.

When pregnancy is carried to term, heat stress may have long-term effects on the calf's performance, including postnatal survival and its adult performance up to the third lactation (Pinedo and De Vries, 2017; Ouellet et al., 2021). With a previous study, we showed that in mainland France, a temperate climate zone, dairy cows experience a decline in their overall performance immediately after a temperature-humidity index (THI) increase (Vinet et al., 2023). We therefore wondered whether in utero heat stress could happen for the French dairy cattle breeds, even in our temperate climate.

The present study aimed to assess if heat stress during the dam's pregnancy has a time-lagged effect on the subsequent daughter's performances (for both heifers and primiparous cows), to determine the potential magnitude of the effect and to identify the most critical period and level of heat stress for the embryo/fetus.

# MATERIALS AND METHODS

## Ethics Statement

This study was carried out with existing data collected on commercial farm and no experiment was performed on animals. Thus, no ethical approval was required.

## Animal Data

The animal data used in this study, comprising the insemination and calving dates of the dams, the performances of their offspring, and the pedigree information, were extracted from the French national database (INRAE-CTIG, Jouy-en-Josas). The start and end dates of gestation were deducted from the date of successful

insemination of the dam and the date of birth of the offspring. Gestations shorter than 270 d or longer than 294 d for Holstein or 300 d for Montbéliarde were excluded from the analysis. The offspring performance data used were from animals born between 2015 and 2020. The dams and their offspring were purebreed cows from either the French Holstein (HOL) or Montbéliarde (MON) populations, and each breed was studied separately.

The progeny traits analyzed for both breeds were conception rate after the first insemination for heifers (CRh) and cows (CRc); 305-d milk (MY), fat (FY), and protein yields (PY); 305-d SCS; clinical mastitis occurrence (Mast) and number of clinical mastitis events (MastNb); height at sacrum; body depth; and chest width. For the French HOL breed, BCS was also analyzed, and for the MON breed, muscularity at thighs (MuscT) and at withers (MuscW) was also analyzed. Except for CRh, all traits were measured during the first lactation. For each studied trait, the contemporary groups (either herd-year or herd-date-of-visit, depending on the trait) with fewer than 3 cows were removed. Depending on the trait, the final data set contained between 97,815 and 419,624 HOL offspring and between 23,826 and 149,987 MON offspring. Descriptive statistics of the studied traits are presented in Table 1.

Values for CRh and CRc were determined as in Barbat et al. (2010): the outcome was set to 100 (i.e., successful) if the cow calved following the expected gestation length of the breed (281 ± 15 d and 285 ± 15 d for Holstein and Montbéliarde respectively), otherwise it was set to 0. Only purebred inseminations were considered. For heifers, only first inseminations performed between 12 and 28 mo of age were kept. For primiparous cows, only those that calved between 22 and 40 mo of age and that were first inseminated between 15 and 200 d after calving were retained.

All 305-d production traits and SCS records were discarded for lactations lasting less than 250 d. The 305-d SCS was obtained by averaging test-day SCS records, with test-day SCS being a log-transformation of the test-day records of SCC: $SCS = \log2(SCC/100,000) + 3$ (Ali and Shook, 1980).

Clinical mastitis occurrence was a binary trait with value 1 when at least one clinical mastitis event was detected during the first 150 d of the lactation, and 0 otherwise; MastNb is the number of occurrences of clinical mastitis during the first 150 DIM. To ensure that the health information was sufficiently complete, only herd-year combinations with a mastitis incidence of at least 10% in primiparous cows were included.

The morphological traits were recorded by classifiers accredited by the breeding organizations. For Holstein, morphological traits were scored from 1 to 9. For Montbéliarde, the muscularity traits were scored, whereas the
<table><caption>**Table 1.** Number of females (with means and SD) by trait in Holstein and Montbéliarde</caption><thead><tr><th rowspan="2">Item</th><th colspan="3">Holstein</th><th colspan="3">Montbéliarde</th></tr><tr><th>Number</th><th>Mean</th><th>SD</th><th>Number</th><th>Mean</th><th>SD</th></tr></thead><tbody><tr><td>Milk yield (kg)</td><td>214,151</td><td>7,876</td><td>1,467</td><td>69,848</td><td>6,825</td><td>1,268</td></tr><tr><td>Fat yield (kg)</td><td>214,151</td><td>315</td><td>58</td><td>69,848</td><td>265</td><td>52</td></tr><tr><td>Protein yield (kg)</td><td>214,151</td><td>254</td><td>48</td><td>69,848</td><td>228</td><td>44</td></tr><tr><td>SCS</td><td>213,405</td><td>2.033</td><td>1.215</td><td>69,796</td><td>2.203</td><td>1.278</td></tr><tr><td>Mastitis occurrence</td><td>97,815</td><td>0.14</td><td>0.34</td><td>23,826</td><td>0.12</td><td>0.33</td></tr><tr><td>Mastitis number</td><td>97,815</td><td>0.18</td><td>0.52</td><td>23,826</td><td>0.15</td><td>0.46</td></tr><tr><td>Conception rate for heifers (%)</td><td>419,624</td><td>57.3</td><td>49.5</td><td>149,987</td><td>58.3</td><td>49.3</td></tr><tr><td>Conception rate for cows (%)</td><td>284,337</td><td>49.0</td><td>50.0</td><td>76,678</td><td>56.1</td><td>49.6</td></tr><tr><td>Height at sacrum<sup>1</sup></td><td>105,883</td><td>6.33</td><td>1.53</td><td>49,795</td><td>145.89</td><td>3.82</td></tr><tr><td>Body depth<sup>1</sup></td><td>105,883</td><td>5.74</td><td>1.20</td><td>49,795</td><td>74.74</td><td>2.56</td></tr><tr><td>Chest width<sup>1</sup></td><td>105,883</td><td>5.25</td><td>1.05</td><td>49,795</td><td>44.17</td><td>3.34</td></tr><tr><td>BCS<sup>2</sup></td><td>105,883</td><td>4.62</td><td>1.08</td><td></td><td></td><td></td></tr><tr><td>Muscularity at thighs<sup>3</sup></td><td></td><td></td><td></td><td>49,795</td><td>5.38</td><td>1.74</td></tr><tr><td>Muscularity at withers<sup>3</sup></td><td></td><td></td><td></td><td>49,795</td><td>5.33</td><td>2.05</td></tr></tbody></table>

other traits (i.e., height at sacrum, body depth and chest width) were measured.

## Meteorological Data

The meteorological data used for this study were the Système d'Analyse Fournissant des Renseignements Atmosphériques à la Neige (SAFRAN) data, provided by the French national meteorological agency (Météo-France). These data contain daily interpolated estimates of various climate variables for each one (9,892 total) of the 8 × 8-km squares that cover the entire French territory. These daily interpolated estimates of climate variables are the result of a model developed for meteorological forecasting based on continuous local recording from a wide network of meteorological stations. As in Vinet et al. (2023), the daily meteorological data assigned to each herd (geographically identified by its postal code) were weighted averages of the values at each of the 8 × 8-km squares that covered the location, with the weights equal to the proportion of the surface of the village in each square.

The meteorological variables used in this study were the average temperature and relative humidity, combined into a single THI. The daily THI (NRC, 1971) was calculated as THI = (1.8 × T + 32) - (0.55 - 0.0055 × RH) × (1.8 × T - 26), where T is the 24-h mean temperature (°C), and RH is the 24-h mean relative humidity (%). The climate heat stress variable, whose effects were to be assessed, was the mean THI for each month of the dam's pregnancy. In a preliminary analysis, different heat stress indicators were tested for each month or trimester of the dam's pregnancy: the mean THI, the mean maximum

temperatures (both for each month or for each trimester), and the sum of daily THI >65 or THI >70 for each month only. Because all these heat stress indicators presented similar estimated effects during the same periods of in utero development, only the results based on the average THI for each month of the dam's pregnancy are presented and discussed in this article (Supplemental Figure S1, see Notes). We kept only the data from gestations of at least 270 d with all THI information available for gestational mo 1 to 9 (M1–M9, with M1 spanning from d 1 to 30, M2 spanning from d 31 to 60, and so on up to M9 spanning from d 241 to 270). To assess a possible effect of heat stress during the very last period of the gestation, an additional period (M9.5) was considered. This period spanned from d 250 to 279. For each month of gestation, mean THI values ranged from 26 or 27 to 73 or 74, with at least 6.3% of the mean THI values above 65 (Table 2).

## Statistical Analyses

This study was based on a comparison between observed and predicted performances. The performances were predicted by regression on the GEBV in addition to the usual fixed effects. Specifically, the following model was used:

$$y = X\beta + Wh + \alpha g + e, \quad [1]$$

where y is the vector of the observed performances of the female offspring for a given trait; β is the vector of the fixed effects similar to those used in the French routine genetic evaluation (International Bull Evaluation Service, https://interbull.org/ib/geforms; Supplemental

<footnote n="1">¹The unit of measurement of these traits depends on the breed: a score from 1 to 9 (the greater the height, depth, or width, the higher the score) for Holstein, and a distance in cm for Montbéliarde.</footnote>

<footnote n="2">²Measured with a score from 1 (thinnest) to 9 (fattest).</footnote>

<footnote n="3">³Measured with a score from 1 (narrow withers/hollow thighs) to 9 (broad withers/rounded thighs).</footnote>
<table><caption>**Table 2.** Number of observations in the THI >65 class and percentage of observations (shown in parentheses) in this THI >65 class out of the total number of performances, for each month of gestation and each trait, for the Holstein (HOL) and the Montbéliarde (MON) breeds</caption><thead><tr><th rowspan="2">Item, n (%)</th><th rowspan="2">M1</th><th rowspan="2">M2</th><th rowspan="2">M3</th><th rowspan="2">M4</th><th colspan="6">Month of gestation of the dam</th></tr><tr><th>M5</th><th>M6</th><th>M7</th><th>M8</th><th>M9</th><th>M9.5</th></tr></thead><tbody><tr><td>Milk, protein, and fat yields, and SCS</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>HOL</td><td>15,118<br>(7.1)</td><td>16,671<br>(7.8)</td><td>18,532<br>(8.7)</td><td>20,494<br>(9.6)</td><td>23,566<br>(11)</td><td>26,867<br>(12.5)</td><td>29,277<br>(13.7)</td><td>28,938<br>(13.5)</td><td>24,488<br>(11.4)</td><td>23,339<br>(10.9)</td></tr><tr><td>MON</td><td>4,828<br>(6.9)</td><td>5,098<br>(7.3)</td><td>5,659<br>(8.1)</td><td>6,897<br>(9.9)</td><td>8,893<br>(12.7)</td><td>10,796<br>(15.5)</td><td>12,488<br>(17.9)</td><td>13,363<br>(19.1)</td><td>11,106<br>(15.9)</td><td>9,995<br>(14.3)</td></tr><tr><td>Type traits</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>HOL</td><td>7,256<br>(6.9)</td><td>8,013<br>(7.6)</td><td>9,100<br>(8.6)</td><td>10,249<br>(9.7)</td><td>11,637<br>(11)</td><td>13,129<br>(12.4)</td><td>14,535<br>(13.7)</td><td>14,778<br>(14)</td><td>12,783<br>(12.1)</td><td>12,098<br>(11.4)</td></tr><tr><td>MON</td><td>3,160<br>(6.3)</td><td>3,379<br>(6.8)</td><td>3,719<br>(7.5)</td><td>4,479<br>(9)</td><td>5,638<br>(11.3)</td><td>6,764<br>(13.6)</td><td>7,671<br>(15.4)</td><td>8,310<br>(16.7)</td><td>6,938<br>(13.9)</td><td>6,236<br>(12.5)</td></tr><tr><td>Mastitis traits</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>HOL</td><td>6,393<br>(6.5%)</td><td>7,019<br>(7.2%)</td><td>7,819<br>(8%)</td><td>8,745<br>(8.9%)</td><td>10,077<br>(10.3%)</td><td>11,509<br>(11.8%)</td><td>12,787<br>(13.1%)</td><td>12,549<br>(12.8%)</td><td>10,625<br>(10.9%)</td><td>10,076<br>(10.3%)</td></tr><tr><td>MON</td><td>1,843<br>(7.7%)</td><td>1970<br>(8.2%)</td><td>2,246<br>(9.4%)</td><td>2,759<br>(11.5%)</td><td>3,526<br>(14.7%)</td><td>4,255<br>(17.8%)</td><td>4,920<br>(20.6%)</td><td>5,118<br>(21.4%)</td><td>4,161<br>(17.4%)</td><td>3,739<br>(15.6%)</td></tr><tr><td>Conception rate for heifers</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>HOL</td><td>27,733<br>(6.6)</td><td>29,647<br>(7.1)</td><td>31,870<br>(7.6)</td><td>34,758<br>(8.3)</td><td>43,418<br>(10.3)</td><td>52,464<br>(12.5)</td><td>56,525<br>(13.5)</td><td>56,399<br>(13.4)</td><td>49,359<br>(11.8)</td><td>46,852<br>(11.2)</td></tr><tr><td>MON</td><td>10,331<br>(6.9)</td><td>10,983<br>(7.3)</td><td>11,979<br>(8)</td><td>14,315<br>(9.5)</td><td>18,246<br>(12.2)</td><td>22,051<br>(14.7)</td><td>24,913<br>(16.6)</td><td>26,781<br>(17.9)</td><td>22,801<br>(15.2)</td><td>20,652<br>(13.8)</td></tr><tr><td>Conception rate for cows</td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>HOL</td><td>20,365<br>(7.2)</td><td>22,269<br>(7.8)</td><td>24,360<br>(8.6)</td><td>26,883<br>(9.5)</td><td>30,666<br>(10.8)</td><td>34,883<br>(12.3)</td><td>37,793<br>(13.3)</td><td>37,618<br>(13.2)</td><td>32,215<br>(11.3)</td><td>30,773<br>(10.8)</td></tr><tr><td>MON</td><td>5,420<br>(7.1)</td><td>5,757<br>(7.5)</td><td>6,406<br>(8.4)</td><td>7,696<br>(10)</td><td>9,770<br>(12.7)</td><td>11,693<br>(15.2)</td><td>13,342<br>(17.4)</td><td>14,267<br>(18.6)</td><td>11,798<br>(15.4)</td><td>10,642<br>(13.9)</td></tr></tbody></table>
Table S1, see Notes), and **X** is their incidence matrix; **h** is the vector of monthly THI effects during the dam's gestation, in 7 classes (≤40, ]40,45], ]45,50], ]50,55], ]55,60], ]60,65], and >65), and **W** is their incidence matrix; **g** is the vector of GEBV of the female offspring with performance in **y**, fitted as a covariate, and **α** is their regression coefficient; and **e** is the vector of random residuals. The GEBV are from the French official genomic evaluations. To avoid any direct dependencies between the GEBV and the performances, the March 2018 evaluation was used, as it did not account for the own performances of the females whose performances were used for our analyses. Finally, for each trait, separate models were fit to estimate the THI effects during each gestational month, totaling 10 individual models per trait (M1 to M9 and M9.5), with each estimating the effects of all 7 THI classes. The regression models were implemented using the GLM procedure of SAS software version 9.4 (SAS Institute Inc., 2013, Cary, NC). To account for multiple testing (420 independent tests classes of THI-month of gestation-breed-trait by group of traits: production, type, fertility, or health), the effects were deemed significant if the nominal *P*-value was smaller than 0.0001, corresponding to an overall *P*-value of 4% at the scale of the group of traits.

## RESULTS

Figure 1 provides the *P*-values associated with the estimated effects of the mean THI by the dam's gestational month, on the progeny performances. In this figure, we observed that for both breeds, in general, the mean THI per gestational month presented a statistically significant effect on the type traits (BCS, MuscW, MuscT, height at sacrum, body depth, and Weight), MY, FY, PY, and SCS. Conversely, the mean THI per gestational month did not present a statistically significant effect on Mast, MastNb, CRc, and CRh, and results for these traits using other climate indicators did not differ in their statistical significance (Supplemental Figure S1, see Notes). Hence, further results related to these traits are mentioned only briefly in the subsequent sections.

Figures 2 to 6 present the estimated effects of mean THI during each month of gestation on the progeny's MY, FY, PY, SCS, height at sacrum, body depth, and chest width, respectively, for both breeds. Figure 7 presents the estimated effects of mean THI during each month of pregnancy on the progeny's BCS for the HOL breed only, and on MuscT and MuscW for the MON breed only. The corresponding *P*-values associated with the estimated effect of each monthly THI class presented in Figures 2, 3, 4, 5, 6, and 7 are available in Supplemental Table S2 (see Notes). For every trait studied, the effect of THI during

the in utero development on the subsequent daughter's performances was low. The effect of the different THI classes during the dam's gestation on the daughter's MY was at maximum 96 kg for HOL (i.e., 1.2% of the average HOL whole lactation milk yield). This effect was obtained for the THI effect during the seventh month of the dam's gestation. It was observed between the effect of THI ≤40 and the effect of THI >65 on the daughters' performance. For MON, the greatest difference in MY was 72 kg, (i.e., 1.1% of the average MON whole lactation milk yield). It was observed between the second coldest and the hottest THI classes (i.e., ]40,45] and >65) during the sixth month. The direction of the THI effect varied according to the gestational stage and breed. Overall, for HOL, a high THI level had a negative effect on MY, FY, and PY only when experienced during the dam's initial months of gestation, whereas this high THI effect was positive when experienced during the dam's mid to final months of gestation (Figures 2, 3, and 4). For the MON breed, the trend was the same as observed for HOL for M1 to M8, although with weaker effects (Figures 2, 3, and 4) and some statistically nonsignificant effects (Supplemental Table S2), and we observed negative effects

<figure><img src="image_3.png" /><figcaption>**Figure 1.** *P*-values of the effect of the average temperature-humidity index by month of the gestation of the dam on the progeny performances for both breed, Holstein (HOL, top) and Montbéliarde (MON, bottom). Results are given by colors according to nominal *p*-values. When accounting for the multiplicity of tests, only dark green cells indicate significant results at a 5% risk. MuscW = muscularity at withers; MuscT = muscularity at thighs; Height = height at sacrum; Depth = body depth; Width = chest width; MY = 305-d milk yield; FY = 305-d fat yield; PY = 305-d protein yield; Mast = occurrence of mastitis during the first 150 d days of lactation; MastNb = number of mastitis events; CRh = conception rate for heifers; CRc = conception rate for cows; M1-M9 = months 1 to 9 of the gestation of the dam (d 1 to d 270, in 30-d increments); M9.5 = d 250 to d 279 of the gestation of the dam.</figcaption></figure>
<figure><img src="image_4.png" /></figure>

**Figure 2.** Effect of the mean temperature-humidity index (THI) during each month of fetal development on the first lactation's 305-d milk yield for Holstein (HOL, top) and Montbéliarde (MON, bottom). For each month of gestation, the estimated effect is the difference in kg of milk compared with the effect of the THI class [40,45]. M1–M9 = months 1 to 9 of the gestation of the dam (d 1 to d 270, in 30-d increments); M9.5 = d 250 to d 279 of the gestation of the dam.

<figure><img src="image_5.png" /></figure>

**Figure 4.** Effect of the mean temperature-humidity index (THI) during each month of fetal development on the first lactation's 305-d protein yield for Holstein (HOL, top) and Montbéliarde (MON, bottom). For each month of gestation, the estimated effect is the difference in kg of protein compared with the effect of the THI class [40,45]. M1–M9 = months 1 to 9 of the gestation of the dam (d 1 to d 270, in 30-d increments); M9.5 = d 250 to d 279 of the gestation of the dam.

<figure><img src="image_6.png" /></figure>

**Figure 3.** Effect of the mean temperature-humidity index (THI) during each month of fetal development on the first lactation's 305-d fat yield for Holstein (HOL, top) and Montbéliarde (MON, bottom). For each month of gestation, the estimated effect is the difference in kg of fat compared with the effect of the THI class [40,45]. M1–M9 = months 1 to 9 of the gestation of the dam (d 1 to d 270, in 30-d increments); M9.5 = d 250 to d 279 of the gestation of the dam.

<figure><img src="image_7.png" /></figure>

**Figure 5.** Effect of the mean temperature-humidity index (THI) during each month of fetal development on the first lactation's 305-d SCS for Holstein (HOL, top) and Montbéliarde (MON, bottom). For each month of gestation, the estimated effect is the difference in SCS compared with the effect of the THI class [40,45]. M1–M9 = months 1 to 9 of the gestation of the dam (d 1 to d 270, in 30-d increments); M9.5 = d 250 to d 279 of the gestation of the dam.
<figure><img src="image_8.png" /><figcaption>**Figure 6.** Effect of the mean temperature-humidity index (THI) during each month of fetal development on body conformation traits height at sacrum, body depth, and chest width for Holstein (left) and for Montbéliarde (right). For each month of gestation, the estimated effect is the difference in score (from 1 to 9, for Holstein) or measurement (in cm, for Montbéliarde) compared with the effect of the THI class [40,45]. M1–M9 = months 1 to 9 of the gestation of the dam (d 1 to d 270, in 30-d increments); M9.5 = d 250 to d 279 of the gestation of the dam.</figcaption></figure>

associated with high THI levels experienced by the dam during M9 and M9.5; however, these effects were not statistically significant.

For both breeds, an unfavorable effect of a high THI was observed for SCS when experienced by the dam at

the beginning of the gestation, with a favorable effect observed when high THI was experienced by the dam at the end of the pregnancy (Figure 5). These described effects on the daughter's SCS of high THI experienced by the dam presented relatively high significance levels (Supplemental Table S2). Note that in the case of SCS, the adverse effect is an increase in SCS (reflecting an inflammatory response).

For both breeds, the morphological traits in general presented a negative effect of THI >65, when experienced by them dam during the middle of the gestation, and positive when experienced by the dam during the end of the gestation. For MON, all these traits tended to be positively influenced by a high THI during the dam's first month of pregnancy (Figure 6 and Supplemental Table S2).

To summarize, the effects of different prenatal THI on the subsequent daughter's milk performance was minor. The effects were higher for SCS and type traits. The adverse effects of high THI were mainly observed at the beginning or in the middle of the gestation of the dam. No significant effect was observed on fertility or clinical mastitis of the daughters, probably due to insufficient power.

<figure><img src="image_9.png" /><figcaption>**Figure 7.** Effect of the mean temperature-humidity index (THI) during each month of fetal development on BCS in Holstein (BCS - HOL) and muscularity at thighs (MuscT - MON) and at withers (MuscW - MON) for Montbéliarde. For each month of gestation, the estimated effect is the difference in BCS score (from 1 = thinnest to 9 = fattest) compared with the effect of the THI class [40,45]. M1–M9 = months 1 to 9 of the gestation of the dam (d 1 to d 270, in 30-d increments); M9.5 = d 250 to d 279 of the gestation of the dam.</figcaption></figure>

## DISCUSSION

This study estimated THI effects as deviations of the observed performance from the expected performance. The expected performance was fitted in the model through the linear regression of the observed performance on the GEBV from a prior evaluation that did not contain
the performances analyzed in this present study. Due to
the large size of the reference populations in both the
French Holstein and Montbéliarde breeds, these GEBV
are considered accurate. These regression coefficients
were close to their expectations, and a further detailed
discussion about these regression coefficients can be
found in Fouéré et al. (2024), a study of potential factors
of maternal programming.

This present study highlighted very low or statistically insignificant effects of THI during the fetal stage on the later onset of clinical mastitis and on fertility traits. These traits also presented a low adjustment of the regression models used for the analyses, with R² values between 0.1 and 0.15, contrary to production and morphological traits, whose regression models presented R² values between 0.25 and 0.7 and yielded several significant THI effects during embryonic and fetal life, even when these effects were small.

In contrast to our results, several studies have shown substantial and significant effects of heat stress during the dam's gestation on the reproduction of the daughter. Akbarinejad et al. (2017) found significant detrimental effects of maternal heat stress for most of the primiparous fertility traits in Iranian cattle, with higher estimated effects when heat stress occurred during the last trimester of the gestation of the dam. The authors estimated a decrease of the first service conception rate for primiparous cows from 37.5% when there was no heat stress during the gestation, to 17.8% for gestations in which at least 2.5 mo of the last trimester occurred between June and August. Adverse effects of heat stress during the end of the gestation of the dam on progeny fertility were also reported by both Dahl et al. (2016) and Kipp et al. (2021). According to Dahl et al. (2016), heifers affected by late gestational heat stress required more services to conceive when compared with their herdmates under in utero thermoneutral conditions. Kipp et al. (2021) reported that heifers from dams exposed to high THI during the last 8 weeks of pregnancy presented a lower conception rate than those from dams exposed to high THI only during the 3 last weeks of gestation. Moreover, the authors reported that the interval from calving to first insemination for primiparous cows was prolonged when their dams were exposed to high THI during the last 2 mo of gestation. Other authors highlighted detrimental effects of heat stress at the beginning of the dam's gestation on the progeny's reproductive performances. For Holsteins from Florida, Pinedo and De Vries (2017) reported that cows conceived in winter exhibited the best reproductive performance (i.e., lower age at first calving and shorter calving-first breeding and calving-conception intervals). For Holsteins raised in Argentina, Recce et al. (2021) observed the most adverse effects of prenatal heat stress

when experienced by the dam during the first trimester
of the gestation. The authors reported that cows exposed
to a high THI (i.e., THI ≥72) in the first trimester of their
intrauterine development had longer intervals of calving
to conception and calving to first service.

Our results have shown that for the French Holstein
and Montbéliarde, the effects of in utero heat stress are
weaker on subsequent performance than what is gener-
ally found in similar studies with other breeds or with
equivalent breeds in different countries. In utero heat
stress effects previously reported in the literature for
the first lactation 305-d MY range from 200 to 900 kg
(Barash et al., 1996 for Israeli cattle; Brown et al., 2015
and 2016, Laporta et al., 2020, and Pinedo and De Vries,
2017 for US cattle; and Van Eetvelde et al., 2017 for
Belgian cattle). One could hypothesize that the small ef-
fect of maternal heat stress observed in our present study
could be due to the moderate French climate (Vinet et
al., 2023), as suggested in Kipp et al. (2021) in a similar
study performed in Germany. However, in a study with
74 Holstein heifers reared in Belgium, a country with
equivalently moderate climate (3% of the days in Bel-
gium present THI above 68 according to Van Laer et al.,
2014), Van Eetvelde et al. (2017) reported high effects
of gestational heat stress on offspring performance, re-
porting a difference of 900 kg on 305-d energy-corrected
milk yield. Finally, this difference was observed with a
limited dataset and was not confirmed in a subsequent
study by the same authors with a large dataset of Dutch
and Belgian Holsteins (Van Eetvelde et al., 2020).

According to the results obtained with our data, high
THI is unfavorable for progeny production traits (MY,
FY, PY) when heat stress is experience by the dam at the
beginning of the gestation, during embryonic and early
fetal life, but it is favorable when experienced by the
dam at the end of the gestation. These negative effects
of heat stress at the beginning of the gestation (and posi-
tive effects at the end of the gestation) on the progeny's
production performance have also been reported by most
studies analyzing the seasons of conception or the season
of birth in warm regions. Barash et al. (1996) reported
that Israeli Holstein cows born in summer and fall (i.e.,
cows potentially exposed to maternal heat stress at the
end of the gestation) presented higher production than
cows born in early spring (i.e., cows potentially exposed
to maternal heat stress during the conception). Although
it is more pronounced in first lactation, when the effect
is around 240 kg greater milk yield for summer-fall
born cows, this effect is observed for the first 3 parities
(Barash et al., 1996).

In studies that compared US Holstein cows con-
ceived in summer (June–August, i.e., potentially heat
stress–conceived cows) with cows conceived in winter
(December–February, i.e., thermoneutral-conceived cows), Brown et al. (2015, 2016) had similar conclusions as Barash et al. (1996). In most instances of the studies from Brown et al. (2015, 2016), the thermoneutral-conceived cows produced significantly more milk than the heat stress conceived-cows. However, they observed a significant interaction between heat stress at the time of conception and season of calving, whereas a higher milk production of thermoneutral conceived cows was not observed for cows that calved in spring (Brown et al., 2015, 2016). Finally, the authors assumed that cows experiencing maternal heat stress around the time of conception may present better thermotolerance, giving them an advantage during subsequent periods of heat stress. Pinedo and De Vries (2017) have also observed that milk and fat production were generally greater for winter-conceived Holstein cows from Florida, compared with cows that were conceived in summer. As in Brown et al. (2015), this effect of the season of conception decreased with the parity of the cow (Pinedo and De Vries, 2017).

On a study that focused their analyses on the effect of heat stress at the end of the dam's gestation, Kipp et al. (2021) reported that daughters from dams who experienced the highest THI from wk 1 to 3 antepartum presented a lower first test-day MY than those not exposed to maternal antepartum heat stress; however, this negative effect was small (i.e., less than 0.1 kg/d) and not significant. Moreover, no negative effect was observed for the whole end of the gestation (8 wk antepartum), and events of THI ≥50 during weeks in greater distance from calving (i.e. wk 5 to 8) were associated with a minor increase of MY (Kipp et al., 2021). However, a THI of 50 can be considered too low to be associated with heat stress, and in many situations can in fact be considered an optimal climate condition for dairy cattle (Vinet et al., 2023).

Successive experiments were conducted in Florida comparing the effect of heat stress vs cooling during the last 46-d period of dams' gestations (Dado-Senn et al., 2020). These studies have shown that heifers that experienced late gestational heat stress produced less milk in their first lactation, compared with their counterparts that had their in utero development under thermoneutral conditions, i.e., dams that were actively cooled when the ambient temperature exceeded 21°C (Laporta et al., 2020). In these experiments, the decrease observed in milk production of in utero heat-stressed heifers was remarkable, with losses ranging from 2.2 kg to 5.1 kg per diem for wk 1 to 35 of the first lactation, depending on the datasets analyzed by each study (Dahl et al., 2016; Monteiro et al., 2016a; Laporta et al., 2020). Moreover, lower body weights from birth to one year of age have been reported for heifers from late-gestational

heat-stressed dams, but no association has been found between this lower weight in the heifer's first year and its later milk production through the first lactation (Monteiro et al., 2016a; Dado-Senn et al., 2020). Furthermore, mammary biopsies collected from first-lactation heifers showed that in utero heat-stressed heifers had smaller mammary alveoli comprising fewer mammary epithelial cells, as well as a lower rate of mammary epithelial cell proliferation, relative to heifers that did not experience heat stress in utero heat (Skibiel et al., 2018a; Dado-Senn et al., 2020). Finally, in these last studies, the negative effect of late in utero heat stress was observed across lactations for milk, fat, protein, and lactose yields.

Although Monteiro et al. (2016a) observed no heat stress effect at the end of maternal gestation (the sole period investigated in their study) on the progeny's SCS during their first lactation, Kipp et al. (2021) observed a trend similar to ours for this trait, with a decrease in SCS when THI is higher at the end of gestation (for SCS, a decrease is a favorable effect). In the latter study, the positive effect of higher THI at the end of the dam's gestation was small, but the estimated difference between the extreme THI classes (i.e., THI ≤39 vs THI ≥60) was significant (*P* < 0.05) between 29 and 56 d prepartum. Although studies are available on the effect of prenatal heat stress on the progeny's SCS, such studies on the adult progeny's pathologies are still rare in cattle. We studied mastitis indicators, but our data did not show any adverse effect of a high THI during in utero development on the subsequent occurrence of clinical mastitis, and to our knowledge, no other study has done so.

In their experiments that compared the effect of heat stress versus cooling during the last 46-d period of gestation of dams, beyond birth and one-year-old weight, Monteiro et al. (2016a) also studied BCS at calving, yet they did not observe significant differences between the 2 groups. These results are somewhat at odds with our own, considering that we have observed differences in primiparous BCS, probably due to differences in THI during in utero development. However, the differences we observed are extremely small, and we do not know whether they are consistent with differences that might exist on birth weight. For our study, we chose not to study birth weight due to a lack of reliable information about birth weight for the French dairy cattle records.

Several hypotheses have been proposed to explain the long-term effects of heat stress during the dam's pregnancy, but the biological processes involved in the long-term effects of in utero heat stress are not fully defined. These hypotheses mainly concern changes in hormonal profiles or epigenetic modifications during embryonic or fetal development, with lasting consequences throughout the animal's life. Despite conflicting results and
hypotheses concerning the effect of heat stress during maternal gestation on blood insulin concentration in neonatal calves, several authors suggested that this hormone could explain the differences in performance between stressed and unstressed calves during in utero life. Van Eetvelde et al. (2017) reported a negative correlation between the neonatal basal insulin concentrations and the environmental temperature at birth. Moreover, the same study reported that winter-born heifers presented the highest basal insulin concentrations 3 d after birth, whereas glucose concentrations were similar irrespective of the season of birth. According to the authors, this last result suggested a lower insulin sensitivity of peripheral tissues. They therefore concluded that, conversely, heifers born in the warmer months, with higher peripheral insulin sensitivity at birth, might later develop higher insulin resistance and better tissue accretion, including mammary tissue, ultimately leading to a higher first lactation milk yields. Conversely, Monteiro et al. (2016b) observed no differences in plasma insulin or in glucose concentration from d 1 to d 56 after birth between heat-stressed and cooled calves during the end of their dam's gestation. Nevertheless, after a glucose tolerance test and after an insulin challenge, the 2 groups diverged in their responses in glucose, insulin, and nonesterified fatty acid concentrations. Monteiro et al. (2016b) suggested that compared with cooled calves, heat-stressed calves exhibited a relatively high level of insulin resistance in peripheral tissues, such as muscle and adipose tissue, and thus limited insulin-mediated glucose entry into the peripheral tissues. However, the authors did not hypothesize that these alterations persist into maturity and whether they can account for any reduced milk production during first and second lactations (Ouellet et al., 2020).

Using different methodologies, such as analyzing the
performance of offspring from ancestors that did or did
not experience heat stress during pregnancy over several
generations, or studying the methylome and transcrip-
tome of these offspring, several authors have suggested
that heat stress during the dam's pregnancy can modify
the epigenetic marks of the fetus. Weller et al. (2021)
found a significant association of the month of birth, the
season of pregnancy, and the heat stress index of F₀ fe-
males, with the performance of their F₂ and F₃ progenies.
To explain the inheritance of the environmental effect ex-
perienced by the F₀ ancestors, these authors hypothesized
that an epigenetic modification that occurred during the
formation of the F₁ gametes could be preserved, at least
partially, during the formation of the F₂ gametes. In other
words, they suspected an inefficient restoration or era-
sure of the epigenetic marks (Weller et al., 2021). Similar
transgenerational effects due to heat stress during the F₀

gestation on their F₂ or F₃ progeny were confirmed by
both Laporta et al. (2020) and Macciotta et al. (2023).

Such modifications of epigenetic marks due to heat
stress were highlighted by Skibiel et al. (2018b) who
identified a substantial number of differentially expressed
genes and differentially methylated cytosines from a
comparison of gene expression profiles and methyla-
tion patterns of in utero heat-stressed or cooled calves.
The differences in pattern of methylation between heat-
stressed or cooled calves were observed on 2 different
tissues: liver tissue of the male calves collected just after
the birth, and mammary gland tissue of the 2-yr lactating
females. Interestingly, among the differently methylated
genes (i.e., genes with at least one differently methylated
cytosine) that were identified between the heat-stressed
and the cooled groups, Skibiel et al. (2018b) reported
50 genes or ribosomal RNA that were common to both
tissues examined. This result led the authors to suggest
that there are common patterns of DNA methylation that
were induced by heat stress, regardless of the animal's
sex, age, or tissue type. Finally, these methylation pat-
terns may affect fundamental processes, including cel-
lular repair, oxidative defense, energy metabolism, and
development (Skibiel et al., 2018b). Lastly, although the
general biological functions of the differentially methyl-
ated genes were similar to the biological functions of the
differentially expressed genes, the authors found weak
correlations between changes in methylation patterns and
changes in gene expression. Our results did not address
epigenetic effects, but such phenomena could explain
the persistence of differences between those animals that
were exposed to in utero stress, and those that were not,
throughout their lives.

Most studies available in the literature of in utero heat
stress have investigated the effects of heat stress either
at the early stages of the dam's gestation (warm or ther-
moneutral months of conception, sometimes excluding
other months of birth, as in Brown et al., 2015, 2016),
or at the late stages of the dam's gestation (month of calf
birth, effect of THI on late-gestation stages). As a result,
the beginning and the end of the gestation are the periods
most often reported to be sensitive to heat stress. Never-
theless, of the few studies that considered all gestation
stages, Akbarinejad et al. (2017) and Recce et al. (2021)
reported that the second trimester of the dam's gestation
was also potentially sensitive to heat stress. In our study,
the progeny's production traits and SCS were shown to
be negatively affected by high THI during the dam's early
gestation, whereas morphological traits would be more
affected by high THI during the dam's mid-gestation.

We would like to make a remark that, although the
THI for different gestational periods were tested in-
dependently, this independence is not true. Due to the bovine gestation length and the temperate climate in mainland France, cows that experience heat stress (THI >65) during the last trimester of their pregnancy are likely to consistently experience an average THI below 65 during the first trimester of their pregnancy (Supplemental Figure S2, see Notes). In other words, cows that experienced a high THI at the end of the gestation are cows whose pregnancies began during the colder seasons. Therefore, we cannot eliminate the hypothesis that the positive effects of high THI during the last months of gestation may be a consequence of the positive effects of an absence of high THI at the beginning of the dam's pregnancy. We did test some models that included not one, but 2 average monthly THI, one for the beginning and the other for the end of the dam's pregnancy. These monthly THI were considered either as 2 separated effects, or as their combination. Due to the partial dependence of the THI at the beginning and end of the gestation period, some combinations were not observed. However, these models did not suggest different results (not shown) from those reported as the main results from our study: a negative effect of high THI on production only when experienced by the dams at the beginning of their pregnancy, and a positive effect of high THI on production when experienced by the dams in the last part of their pregnancy.

When compiling the results from all previous studies regarding the effects of prenatal heat stress, along with ours, it is in fact difficult to define one single period of a dam's pregnancy that is more sensitive to heat stress than others, with respect to their effects on the later performances of the gestated progeny. It is likely that in utero heat stress has multiple effects on embryos and fetuses. These effects include a decrease in nutrient intake due to reduced maternal intake, fetal hypoxia and malnutrition due to reduced blood flow to the uterus, the modification of epigenetic reprogramming (Ghaffari, 2022), and changes in the balance between reactive oxygen generation and antioxidant protection (Hansen, 2013). Finally, it is possible that the sensitivity to each of these phenomena differs according to the stage of development of the embryo or fetus, and that long-term effects on the progeny are due to multiple processes.

## CONCLUSIONS

Our study estimated the effect of heat stress during the dam's pregnancy on different performance metrics of the future progeny during their first lactation for the French Holstein and Montbéliarde breeds. This effect was estimated as the deviation of the observed performance from the expected one. In both breeds and for all traits, THI

effects during gestation appeared to have little impact on adult performance under French climatic conditions. Although results on fertility and udder health were not significant, milk, fat, and protein yields were more sensitive to heat stress at the beginning than at the end of the gestation. This study was carried out with data from animals exposed to a rather temperate climate, and we cannot exclude that the impact of stress may be stronger for animals exposed to warmer weather.

## NOTES

This project has received funding from APIS-GENE through the CAICALOR project and from the European Union's Horizon 2020 Programme for Research and Innovation under grant agreement no. 101000226 (Rumigen). Rumigen is part of EuroFAANG (https://eurofaang.eu). The SAFRAN climatic data were provided by Météo-France and were downloaded via the SICLIMA platform developed by AgroClim-INRAE. Animal recording data were provided by Eliance's (Paris, France) network. We are grateful to the breeders who participate in on-farm performance testing. Supplemental material for this article is available at https://doi.org/10.6084/m9.fishshare.26433787. This study was carried out with existing data collected on commercial farm and no experiment was performed on animals. Thus, no ethical approval was required. The authors have not stated any conflicts of interest.

**Nonstandard abbreviations used:** CRc = conception rate at first insemination for cows; CRh = conception rate at first insemination for heifers; Depth = body depth; FY = 305-d fat yield; Height = height at sacrum; HOL = Holstein; M1–M9 = gestational months 1 to 9, in 30-d increments; M9.5 = last period of gestation, from d 250 to 279; Mast = clinical mastitis occurrence; MastNb = number of clinical mastitis events; MON = Montbéliarde; MuscT = muscularity at thighs; MuscW = muscularity at withers; MY = 305-d milk yield; PY = 305-d protein yield; SAFRAN = Système d'Analyse Fournissant des Renseignements Atmosphériques à la Neige; THI = temperature-humidity index; Width = chest width.

## REFERENCES

* Akbarinejad, V., F. Gharagozlou, and M. Vojgani. 2017. Temporal effect of maternal heat stress during gestation on the fertility and anti-Mullerian hormone concentration of offspring in bovine. *Theriogenology* 99:69–78. https://doi.org/10.1016/j.theriogenology.2017.05.018.

* Ali, A. K. A., and G. E. Shook. 1980. An optimum transformation for somatic cell concentration in milk. J. Dairy Sci. 63:487–490. https://doi.org/10.3168/jds.S0022-0302(80)82959-6.

* Asakura, H. 2004. Fetal and neonatal thermoregulation. J. Nippon Med. Sch. 71:360–370. https://doi.org/10.1272/jnms.71.360.
Barash, H., N. Silanikove, and J. I. Weller. 1996. Effect of season of birth on milk, fat, and protein production of Israeli Holsteins. *J. Dairy Sci.* 79:1016–1020. https://doi.org/10.3168/jds.S0022-0302(96)76453-6

Barbat, A., P. Le Mézec, V. Ducrocq, S. Mattalia, S. Fritz, D. Boichard, C. Ponsart, and P. Humblot. 2010. Female fertility in French dairy breeds: Current situation and strategies for improvement. *J. Reprod. Dev.* 56(Suppl.):S15–S21. https://doi.org/10.1262/jrd.1056S15

Baumgard, L. H., and R. P. Rhoads. 2012. Ruminant nutrition symposium: Ruminant production and metabolic responses to heat stress. *J. Anim. Sci.* 90:1855–1865. https://doi.org/10.2527/jas.2011-4675.

Brown, B. M., J. W. Stallings, J. S. Clay, and M. L. Rhoads. 2015. Peri-conceptional heat stress of Holstein dams is associated with differences in daughter milk production and composition during multiple lactations. *PLoS One* 10:e0133574. https://doi.org/10.1371/journal.pone.0133574.

Brown, B. M., J. W. Stallings, J. S. Clay, and M. L. Rhoads. 2016. Peri-conceptional heat stress of Holstein dams is associated with differences in daughter milk production during their first lactation. *PLoS One* 11:e0148234. https://doi.org/10.1371/journal.pone.0148234.

Brown, B. M., J. W. Stallings, J. S. Clay, and M. L. Rhoads. 2016. Peri-conceptional heat stress of Holstein dams is associated with differences in daughter milk production during their first lactation. *PLoS One* 11:e0148234. https://doi.org/10.1371/journal.pone.0148234.

Collier, R. J., B. J. Renquist, and Y. Xiao. 2017. A 100-year review: Stress physiology including heat stress. *J. Dairy Sci.* 100:10367–10380. https://doi.org/10.3168/jds.2017-13676.

Dado-Senn, B., J. Laporta, and G. E. Dahl. 2020. Carry over effects of late-gestational heat stress on dairy cattle progeny. *Theriogenology* 154:17–23. https://doi.org/10.1016/j.theriogenology.2020.05.012.

Dahl, G. E., S. Tao, and A. P. A. Monteiro. 2016. Effects of late-gestation heat stress on immunity and performance of calves. *J. Dairy Sci.* 99:3193–3198. https://doi.org/10.3168/jds.2015-9990.

De Rensis, F., I. Garcia-Ispierto, and F. López-Gatius. 2015. Seasonal heat stress: Clinical implications and hormone treatments for the fertility of dairy cows. *Theriogenology* 84:659–666. https://doi.org/10.1016/j.theriogenology.2015.04.021.

De Rensis, F., and R. J. Scaramuzzi. 2003. Heat stress and seasonal effects on reproduction in the dairy cow—A review. *Theriogenology* 60:1139–1151. https://doi.org/10.1016/S0093-691X(03)00126-2.

Fouéré, C., M. P. Sanchez, M. Boussaha, S. Fritz, A. Vinet, H. Kiefer, D. Boichard, and C. Hozé. 2024. A large population study to assess the magnitude of prenatal programming in dairy cattle. *J. Dairy Sci.* 107:5913–5923. https://doi.org/10.3168/jds.2023-24051.

Ghaffari, M. H. 2022. Developmental programming: Prenatal and postnatal consequences of hyperthermia in dairy cows and calves. *Domest. Anim. Endocrinol.* 80:106723. https://doi.org/10.1016/j.domaniend.2022.106723.

Hansen, P. J. 2013. Cellular and molecular basis of therapies to ameliorate effects of heat stress on embryonic development in cattle. *Anim. Reprod.* 10:322–333.

Hansen, P. J. 2019. Reproductive physiology of the heat-stressed dairy cow: Implications for fertility and assisted reproduction. *Anim. Reprod.* 16:497–507. https://doi.org/10.21451/1984-3143-AR2019-0053.

Kadzere, C. T., M. R. Murphy, N. Silanikove, and E. Maltz. 2002. Heat stress in lactating dairy cows: A review. *Livest. Prod. Sci.* 77:59–91. https://doi.org/10.1016/S0301-6226(01)00330-X.

Kipp, C., K. Brügemann, P. Zieger, K. Mütze, S. Möcklinghoff-Wicke, S. König, and K. Halli. 2021. Across-generation effects of maternal heat stress during late gestation on production, female fertility and longevity traits in dairy cows. *J. Dairy Res.* 88:147–153. https://doi.org/10.1017/S0022029921000327.

Laporta, J., F. C. Ferreira, V. Ouellet, B. Dado-Senn, A. K. Almeida, A. De Vries, and G. E. Dahl. 2020. Late-gestation heat stress impairs daughter and granddaughter lifetime performance. *J. Dairy Sci.* 103:7555–7568. https://doi.org/10.3168/jds.2020-18154.

Lees, A. M., V. Sejian, A. L. Wallace, C. C. Steel, T. L. Mader, J. C. Lees, and J. B. Gaughan. 2019. The impact of heat load on cattle. *Animals* (Basel) 9:322. https://doi.org/10.3390/ani9060322.

Macciotta, N. P. P., C. Dimauro, L. Degano, D. Vicario, and A. Cesarani. 2023. A transgenerational study on the effect of great-granddam birth month on granddaughter EBV for production traits in Italian Simmental cattle. *J. Dairy Sci.* 106:2588–2597. https://doi.org/10.3168/jds.2022-22455.

Monteiro, A. P. A., J. R. Guo, X. S. Weng, B. M. Ahmed, M. J. Hayen, G. E. Dahl, J. K. Bernard, and S. Tao. 2016b. Effect of maternal heat stress during the dry period on growth and metabolism of calves. *J. Dairy Sci.* 99:3896–3907. https://doi.org/10.3168/jds.2015-10699.

Monteiro, A. P. A., S. Tao, I. M. T. Thompson, and G. E. Dahl. 2016a. In utero heat stress decreases calf survival and performance through the first lactation. *J. Dairy Sci.* 99:8443–8450. https://doi.org/10.3168/jds.2016-11072.

National Research Council. 1971. A Guide to Environmental Research on Animals. National Academy of Sciences, Washington, DC.

Ouellet, V., A. Boucher, G. E. Dahl, and J. Laporta. 2021. Consequences of maternal heat stress at different stages of embryonic and fetal development on dairy cows' progeny. *Anim. Front.* 11:48–56. https://doi.org/10.1093/af/vfab059.

Ouellet, V., J. Laporta, and G. E. Dahl. 2020. Late gestation heat stress in dairy cows: Effects on dam and daughter. *Theriogenology* 150:471–479. https://doi.org/10.1016/j.theriogenology.2020.03.011.

Pinedo, P. J., and A. De Vries. 2017. Season of conception is associated with future survival, fertility, and milk yield of Holstein cows. *J. Dairy Sci.* 100:6631–6639. https://doi.org/10.3168/jds.2017-12662.

Polsky, L., and M. A. G. von Keyserlingk. 2017. Invited review: Effects of heat stress on dairy cattle welfare. *J. Dairy Sci.* 100:8645–8657. https://doi.org/10.3168/jds.2017-12651.

Recce, S., E. Huber, U. S. Notaro, F. M. Rodríguez, H. H. Ortega, F. Rey, M. L. Signorini, and N. R. Salvetti. 2021. Association between heat stress during intrauterine development and the calving-to-conception and calving-to-first-service intervals in Holstein cows. *Theriogenology* 162:95–104. https://doi.org/10.1016/j.theriogenology.2021.01.002.

Roth, Z. 2008. Heat stress, the follicle, and its enclosed oocyte: Mechanisms and potential strategies to improve fertility in dairy cows. *Reprod. Domest. Anim.* 43:238–244. https://doi.org/10.1111/j.1439-0531.2008.01168.x.

SAS Institute Inc. 2013. Base SAS 9.4 Procedures Guide: Statistical Procedures, Second Edition. SAS Institute Inc., Cary, NC.

Shephard, R. W., and S. K. Maloney. 2023. A review of thermal stress in cattle. Aust. Vet. J. 101:417–429. https://doi.org/10.1111/avj.13275.

Silanikove, N. 2000. Effects of heat stress on the welfare of extensively managed domestic ruminants. *Livest. Prod. Sci.* 67:1–18. https://doi.org/10.1016/S0301-6226(00)00162-7.

Skibiel, A. L., B. Dado-Senn, T. F. Fabris, G. E. Dahl, and J. Laporta. 2018a. In utero exposure to thermal stress has long-term effects on mammary gland microstructure and function in dairy cattle. *PLoS One* 13:e0206046. https://doi.org/10.1371/journal.pone.0206046.

Skibiel, A. L., F. Peñagaricano, R. Amorín, B. M. Ahmed, G. E. Dahl, and J. Laporta. 2018b. In utero heat stress alters the offspring epigenome. *Sci. Rep.* 8:14609. https://doi.org/10.1038/s41598-018-32975-1.

Van Eetvelde, M., G. de Jong, K. Verdru, M. L. van Pelt, M. Meesters, and G. Opsomer. 2020. A large-scale study on the effect of age at first calving, dam parity, and birth and calving month on first-lactation milk yield in Holstein Friesian dairy cattle. *J. Dairy Sci.* 103:11515–11523. https://doi.org/10.3168/jds.2020-18431.

Van Eetvelde, M., M. M. Kamal, L. Vandaele, and G. Opsomer. 2017. Season of birth is associated with first-lactation milk yield in Holstein Friesian cattle. *Animal* 11:2252–2259. https://doi.org/10.1017/S1751731117001021.

Van Laer, E., C. P. H. Moons, B. Sonck, and F. A. M. Tuyttens. 2014. Importance of outdoor shelter for cattle in temperate climates. *Livest. Sci.* 159:87–101. https://doi.org/10.1016/j.livsci.2013.11.003.

Vinet, A., S. Mattalia, R. Vallee, C. Bertrand, A. Barbat, J. Promp, B. C. D. Cuyabano, and D. Boichard. 2024. Effect of temperature-humidity index on the evolution of trade-offs between fertility and production in dairy cattle. *Genet. Sel. Evol.* 56:23. https://doi.org/10.1186/s12711-024-00889-4.
Vinet, A., S. Mattalia, R. Vallee, C. Bertrand, B. C. D. Cuyabano, and D. Boichard. 2023. Estimation of genotype by temperature-humidity index interactions on milk production and udder health traits in Montbéliarde cows. Genet. Sel. Evol. 55:4. https://doi.org/10.1186/s12711-023-00779-1.

Weller, J. I., E. Ezra, and M. Gershoni. 2021. Broad phenotypic impact of the effects of transgenerational heat stress in dairy cattle: A study of four consecutive generations. Genet. Sel. Evol. 53:69. https://doi.org/10.1186/s12711-021-00666-7.

Wolfenson, D., and Z. Roth. 2019. Impact of heat stress on cow reproduction and fertility. Anim. Front. 9:32–38. https://doi.org/10.1093/af/vfy027.

ORCIDS

A. Vinet, https://orcid.org/0000-0001-7327-6049

C. Fouéré, https://orcid.org/0009-0005-8735-2918

B. C. D. Cuyabano, https://orcid.org/0000-0003-0060-8879

S. Mattalia, https://orcid.org/0000-0002-7672-4414

A. Barbat, https://orcid.org/0009-0001-9264-6225

C. Bertrand, https://orcid.org/0009-0009-3373-432X

C. Hoze, https://orcid.org/0000-0002-5900-5506

D. Boichard, https://orcid.org/0000-0003-0361-2961