Effect of supplementary feeding and shade in grazing beef steers during summer in a subtropical humid climate

An experiment was conducted to evaluate the effects of supplementary feeding and shade on animal performance and physiological response of growing beef cattle grazing native grasslands during summer. Thirty-two Angus steers were randomly allocated to 1 of 8 paddocks (2 ha and 4 steers per paddock). Treatments were randomly assigned to paddocks in a 2 x 2 factorial arrangement: control unsupplemented and unshaded (Control-Sun), control with shade (Control-Shade), supplemented unshaded (Suppl-Sun) and supplemented with shade (Suppl-Shade). Supplemented steers were group-fed early in the morning 5 days per week with a totally mixed energy–protein ration (16.8% CP; 25.1% NDF; 5.1% ADF) at 1.1% body weight (BW) on days of feeding (DM basis). Shade was provided by a black cloth (4 m 2 /animal) intercepting 80% of solar radiation mounted 4 m above the ground in grazing paddocks. The temperature-humidity index (THI) was recorded hourly and assigned to heat stress categories. Animal average daily gain (ADG), respiration rate (RR), and body temperature (BT) were analyzed as a 2 x 2 factorial design with the group of animals in each paddock considered the experimental unit. Supplemented steers had higher (P < 0.05) ADG (0.74 ± 0.09 kg/a/d) compared with Controls (0.21 ± 0.02 kg/a/d), while Shade and Sun steers had similar ADG (P > 0.05). RR was 9% lower (P < 0.05) for steers in the Shade compared with steers in the Sun treatment at 11:00 and 15:00 h. Differences in BT between Sun and Shade steers were significant (P < 0.05) and increased in magnitude from alert (Sun: 39.3 °C; Shade: 38.7 °C) to danger (Sun: 40.6 °C; Shade: 39.3 °C) and emergency levels (Sun: 41.1 °C; Shade: 39.5 °C). Environmental THI thresholds ranged from 66.0 to 71.6, beyond which RR and BT increased significantly. Control-Sun steers had the sharpest RR and BT increase (2.65 breaths/min and 0.22 °C for each unit increase in the THI above the threshold, respectively). Supplementary feeding and the provision of shade are complementary strategies that can be used in combination to increase animal performance and improve thermal comfort of grazing steers during summer in subtropical humid climates.

Under heat stress conditions, beef cattle experience increased physiological thermoregulatory responses, reduced feed intake, impaired growth, and altered behavior (Brown-Brandl, 2018; Kim et al., 2023; Silva Neto et al., 2025). Most research assessing cattle heat stress has been conducted in grazing cattle (Nielsen and Wredle, 2023; Eckhardt et al., 2025; Hendriks et al., 2025) or feedlot systems (Azevedo et al., 2024; Mejia Turcios et al., 2024; Slayi and Jaja, 2025). Assessment of cattle heat stress in grazing systems with supplementary feeding is relevant because of the current intensification of pasture-based systems. Global beef meat consumption is projected to grow 13% by 2034 due to population and income growth (OECD/FAO, 2024). In this context, intensification of grass-based production systems is an opportunity to satisfy the increasing demand.

As the risk of heat-stress events continue to occur, mitigation strategies may need to be implemented to improve livestock efficiency and animal comfort. Previous research has demonstrated that providing shade to beef cattle during periods of heat stress can improve growth performance and animal welfare (Blackshaw and Blackshaw, 1994; Rovira, 2014; Edwards-Callaway et al., 2021; Schuck-Paim et al., 2026). For cattle on pasture with supplementary feeding, however, it is largely unknown whether the benefits of shade would be altered by feeding supplements.

Supplementation is a practical method for improving performance of grazing cattle during summer, but little is known about its impact on the risk of heat stress of cattle. Quantity and quality of feed consumed, in relation to environmental conditions, may influence the animal’s ability to cope with heat load (Mader et al., 1999). For example, cattle supplementation with energy-rich supplements can decrease internal heat production as they have a lower heat increment than forage (Sudarman and Ito, 2000). However, total heat production may increase as total dry matter intake (DMI) of cattle consuming pasture plus supplements is usually higher than DMI of cattle on pasture-only diets (Bargo et al., 2002). If cattle are already experiencing heat stress, the heat of fermentation adds to the animals’ total heat load and increases the energy expenditure needed for heat dissipation (Mader et al., 1999), partially offsetting the productive benefits of supplementary feeding. To our knowledge, no studies have explored the effects of the interaction between supplementation and the availability of shade on animal performance and the risk of heat stress on growing cattle in grazing systems. We chose to explore this interaction, in part, because only 21% of farmers in Uruguay have shade in all the grazing paddocks (Pravia et al., 2013), and guidelines are needed about the impact of supplementation with or without shade.

Elevated body temperature and increased respiration rate are early signals of heat stress in cattle (Eigenberg et al., 2005; Godyn et al., 2019). These physiological parameters can start to abruptly rise at some environmental thresholds, which are commonly defined by the temperature-humidity index (THI). Recent studies have suggested the critical THI thresholds for dairy calves and cows (Dado-Senn et al., 2020; Kovács et al., 2020; Ouellet et al., 2021). To our knowledge, there are no published THI thresholds based on physiological indicators of growing beef steers exposed to nutritional (supplementary feeding) and management (provision of shade) technologies in grazing systems. Identifying critical THI could help farmers to make timely decisions to maintain animal productivity and welfare (Yan et al., 2021).

Therefore, the objective of the current study was to evaluate the effects of supplementary feeding (yes vs. no) and the provision of shade (yes vs. no) during summer on performance and physiological response of grazing cattle in a subtropical humid climate. The hypothesis tested was that supplementing beef steers in summer would improve animal performance while the provision of shade would lessen the physiological response to heat stress. Based on this rationale, both strategies would be complementary aligning with the concept of sustainable livestock intensification, which aims to enhance animal source foods to feed a growing population without causing additional animal stress.

Experimental design

This study was carried out from 18 December 2024 to 13 March 2025 (86 days) at the INIA-Treinta y Tres Research Station (latitude: 33°14’S, longitude: 54°15’W) in Uruguay. The climate is subtropical humid with a mean daily temperature of 23 °C and 11 °C in summer and winter, respectively, and a mean annual rainfall of 1300 mm (Pravia et al., 2019). All procedures involving animals were approved by the INIA Animal Use and Ethics Committee (protocol #24.2023). Thirty-two growing 15-month-old Aberdeen Angus steers [average initial body weight (BW) ± standard deviation (SD): 275 ± 25 kg] were randomly assigned to 8 groups corresponding to 4 treatments replicated in 2 blocks (topographic slope approximately 1% and 3%). The experimental design was a 2 x 2 factorial arrangement which consisted of steers provided with or not given access to artificial shade (Shade and Sun, respectively) and supplemented (Suppl) or not supplemented (Control) with a totally mixed commercial ration for growing steers (16.8% CP; 5.1% ADF, 25.1% NDF; 14.0% ashes; DM basis). Each replicate consisted of 4 steers continuously grazing 2 ha of native pastures mainly composed of perennial summer grasses such as Cynodon dactylon, Paspalum notatum and Axonopus affinis, with free access to water trough supplied by underground well water. Groups of animals were randomly assigned to treatments within blocks. Artificial shade structures (length: 6 m; width: 4 m) consisted of a black woven polypropylene cloth with 80% blockage of solar radiation mounted on 4.0-m-high wooden posts corresponding to an average of 4 m 2 per animal. Supplemented steers were group-fed in the grazing paddock early in the morning (06:00-07:00 h), 5 days per week (Monday through Friday), at a level of 1.1% body weight each day (DM basis). No orts were observed.

Data collection and processing

Precipitation was measured using a rain gauge placed on the research unit. Accumulated rainfall during the experimental period was 290 mm distributed over 2 mm in December (18/12 – 31/12/2024), 125 mm in January, 124 mm in February and 39 mm in March (01/03 – 14/03/2025). An automated HOBO device (Onset R Computer Corporation, Bourne, MA, USA) exposed to direct solar radiation and placed at a height of 150 cm was used to obtain dry bulb temperature (T db) and relative humidity (RH) readings at 1-h intervals. Later, both values were used to calculate the temperature-humidity index (THI) with the formula: THI = (0.8 x T) + [(RH/100) x (T – 14.4)] + 46.4 (Thom, 1959). Mean climatic conditions are summarized in Table 1. THI values were interpreted according to the Livestock Weather Safety Index (LCI, 1970) for heat stress potential as: normal, ≤ 74; alert, 74< THI< 79; danger 79 ≤ THI ≤ 84; and emergency, THI ≥ 84 (Figure 1). Most of the hourly THI values were in the normal category (57%), while alert, danger and emergency conditions for heat stress existed during 25%, 13% and 5% of the time, respectively.

Pastures were sampled to determine total herbage mass, sward height, green:dead forage ratio and forage nutritive value on days 0, 26, 54, and 84, corresponding to December, January, February and March, respectively. At each sampling time, forage clippings were made at ground level within a 0.2 x 0.5 m quadrant (4 quadrants per 2-ha paddock). Pasture height was measured in a zigzag pattern through the paddocks using a centimeter ruler, with 25 points per paddock. Fresh forage samples were transported to the laboratory and were weighed individually. Then, individual samples from the same paddock were pooled in a composite sample and two subsamples of 100 g each were taken. In one subsample, green and dead fractions were manually separated to estimate the proportion of green and dead forage (DM basis), while the second subsample was used to estimate the overall DM content of the pasture. All samples were put into an air-forced oven at 60 °C for 72 h, after which they were dry weighed, ground at 1- mm sieve, and sent to the laboratory of animal nutrition of INIA (La Estanzuela, Colonia, Uruguay) for determination of crude protein (CP; AOAC, 2007), neutral and detergent fiber (NDF and ADF; Van Soest et al., 1991), and ashes (AOAC, 2007). The same analysis of chemical composition was performed in a sample collected from the supplement.

Steer BW were collected on days 0, 27, 57 and 86 after 18 h of fasting. Average daily gain (ADG) was calculated by the linear regression of BW on days of study. Respiration rates (RR) were taken by counting flank movements by the same trained observer for 30 s and doubled to obtain breaths per minute. Readings were obtained for a total of 25 random days during the experimental period and were taken for all steers before supplementation (06:00 h) and again later in the morning (11:00 h) and afternoon (15:00 h). Subcutaneous body temperature (BT) was measured automatically at 1-h intervals in one animal per replicate using button-shaped digital thermos-loggers (iButton DS1921H-F5, Maxim Integrated, San Jose, CA, USA) surgically implanted into the lateral neck of the animals following the protocol published for implantation, recovery, and data retrieval (Lee et al., 2016).

Statistical analysis

Data were analyzed as a 2 x 2 factorial arrangement using a mixed effects model in R (v.4.0.5) for a completely randomized block design. For the productive variables, significance was determined at P< 0.05. When significant effects were found, the Tukey test was used to establish pairwise differences. The group of 4 animals in each 2-ha paddock was considered the experimental unit. The model equation for pasture attributes, BW and ADG was:

where:

Y ijk = observation k in level i of factor α and level j of factor β in block k.

μ = the overall mean.

α i = the fixed effect of supplementation (Suppl, Control).

β j = the fixed effect of shade (Shade, Sun).

(αβ) ij = the interaction between supplementation and shade.

γ k = the random effect of block (topographic slope 1% and 3%).

ξ ijk = random error.

In addition, the fixed effect of time was included for the analysis of RR (06:00, 11:00 and 15:00 h) and BT (24 readings per day at 1-h interval) using corAR(1) as an autoregressive error correlation structure in the nlme R statistical packages to model BT over time. Two and three-factor interactions between time and the main effects (supplement and shade) were analyzed using the emmeans package in R to perform pairwise comparisons (Lenth and Piaskowski, 2026). Segmented regressions were conducted using the statistical package segmented (Muggeo, 2008) to detect THI breakpoints at which physiological variables (RR and BT) begin to significantly rise.

Productive variables

Effects of the main factors or the interaction term were not detected for pasture attributes (P > 0.05). Across all treatments, herbage mass and sward height averaged (± SEM) 4024 ± 202 kg DM/ha and 10.4 ± 0.3 cm, respectively. Green forage represented 53 ± 2% of the total herbage mass on a DM basis (min. 48% in January, max. 67% in March). The chemical composition of the pasture averaged CP = 7.4 ± 0.2% (min. 6.7%; max. 8.3%), ADF = 46.6 ± 0.6% (min. 43.8%; max. 49.9%), NDF = 70.0 ± 0.5% (min. 66.9%; max. 72.0%) and Ashes = 10.7 ± 0.3% (min. 9.6%; max. 11.5%). Effect of supplementation (P < 0.05) but no effect of shade (P > 0.05) was detected for final BW and ADG of steers (Table 2). Supplementary feeding to grazing steers promoted an additional gain of 0.53 kg/a/d (95% confidence interval [CI]: 0.24 – 0.82 kg/a/d). As a result, cattle in Suppl treatments were 38 kg/a heavier (P < 0.05) at the end of the experiment than unsupplemented controls (95% CI: 11–65 kg/a). On average, cattle required 5.2 kg (Sun) and 4.0 kg (Shade) of supplement (DM) for every one kg of BW gain (P > 0.05). The provision of shade improved ADG by 26% (Control) and 33% (Suppl), although it was not statistically significant (P > 0.05) with a 95% CI of (– 0.43, 0.70 kg/a/d) for the difference between Shade and Sun treatments.

Respiration rate

Among the main factors, the provision of shade (Sun: 53 ± 1; Shade: 48 ± 1 breaths/min) and supplementation (Control: 49 ± 1; Suppl: 52 ± 1 breaths/min) had no effect on average RR (P > 0.05). On the other hand, interaction between shade x time was detected for RR (P < 0.05). Sun steers exhibited elevated (P < 0.05) RR at 11:00 h (57 ± 1 breaths/min) and 15:00 h (66 ± 1 breaths/min) compared to Shade steers (52 ± 1 and 59 ± 1 breaths/min, respectively), but no differences (P > 0.05) between treatments were observed at 06:00 h (34 ± 1 breaths/min) (Figures 2a, b). The significant decrease in RR associated with the provision of shade was 5 breaths/min at 11:00 h (95% CI = 1–8 breaths/min) and 7 breaths/min at 15:00 h (95% CI = 3–10 breaths/min). Interaction between shade x supplementation x time of the day was not detected (P > 0.05).

Body temperature

Figure 3 shows the variation in BT in cattle and THI across hours of the day. Overall, maximum BT occurred at 15:00 h for Sun steers (Control: 41.5 °C; Suppl: 40.7 °C), and at 17:00 for Shade steers (Control: 39.2 °C; Suppl: 39.3 °C). Minimum BT occurred between 02:00 and 04:00 h for Control-Sun steers (38.6 °C) and at 06:00 h for the rest of the treatments (37.7 - 38.3 °C). Figure 3 also shows that peak THI was at 14:00 h (THI = 78.7), while the minimum was observed at 05:00 h (THI = 62.0). Interaction between shade x supplement x time was detected for BT (P < 0.05). During the hottest hours of the day (09:00 – 18:00 h), Control-Sun steers showed the greatest BT (40.8 ± 0.12 °C), followed by Suppl-Sun steers (40.1 ± 0.11 °C), with the lowest BT observed in Shade steers (Control: 38.7 ± 0.07 °C; Suppl: 39.0 ± 0.06 °C). Shaded steers had similar BT (P > 0.05) throughout the day. There were no differences (P > 0.05) in cattle BT among treatments during the night and early morning hours (19:00 – 08:00 h).

Physiological indicators and heat stress

Figures 4, 5 illustrate the difference in RR and BT patterns in each heat stress category. Overall, both physiological indicators increased as the heat stress level progressed. The main effects of supplementation and shade had a significant effect on RR in the heat stress categories (P < 0.05). Supplemented steers exhibited elevated RR in alert and danger heat stress conditions (57 and 72 breaths/min, respectively) compared to Control steers (49 and 64 breaths/min, respectively). On the other hand, cattle without access to shade had higher RR during alert (57 breaths/min), danger (73 breaths/min) and emergency (72 breaths/min) categories compared to Shade cattle (50, 64 and 65 breaths/min, respectively).

Differences in BT between Sun and Shade treatments were significant (P < 0.05) and increased in magnitude from alert (Sun: 39.3 °C; Shade: 38.7 °C) to danger (Sun: 40.6 °C; Shade: 39.3 °C) and emergency levels (Sun: 41.1 °C; Shade: 39.5 °C). Interaction Shade x Suppl (P < 0.05) was detected for BT in normal, danger and emergency categories where steers in the Control-Sun treatment had the highest BT (normal: 39.2 °C; danger: 40.8 °C; emergency: 41.3 °C), followed by Suppl-Sun (normal: 39.0 °C; danger: 40.4 °C; emergency: 40.9 °C), with Control-Shade and Suppl-Shade treatment being lowest (normal: 38.5 °C; danger: 39.3 °C; emergency: 39.5 °C).

THI thresholds were established by detecting rapid changes in steers RR and BT. For RR, steers had a THI breakpoint of 71.0 (Control-Sun), 71.6 (Control-Shade), 66.0 (Suppl-Sun) and 71.0 (Suppl-Shade), whereby RR began rising at a rate of 2.65, 1.96, 2.30 and 2.46 breaths/min for every unit increase in THI, respectively (Figure 6). The respective breakpoints for BT were 71.5 (Control-Sun), 67.8 (Control-Shade), 71.6 (Suppl-Sun), 66.5 (Suppl-Shade), whereby BT began rising at a rate of 0.22, 0.09, 0.18 and 0.10 °C for every unit increase in THI, respectively (Figure 6). THI breakpoints 95% confidence intervals (CI) for the different treatments are summarized in Figure 7. THI breakpoints CI for BT had narrower limits and did not overlap between Sun (70.8 – 72.3) and Shade (65.6 – 68.8) treatments compared with CI breakpoints for RR which had broader limits.

Nutritional strategies to increase animal performance and management strategies to mitigate the effects of heat stress in grazing cattle during summer are rarely evaluated together. The results of this study offer essential insights into the effectiveness of summer interventions, specifically supplementary feeding with an energy-protein TMR and the provision of shade on the growth performance and thermo-physiological responses of grazing cattle. In support of our hypothesis, the findings from the present study suggest that under intermittent environmental conditions predisposing to heat stress cattle with restricted supplementation (0.8% BW) increases ADG in summer whereas the provision of shade improves physiological responses (RR and BT) to cope with heat stress, especially during the hottest hours of the day. On the other hand, shade and supplementary feeding have a moderate impact on ADG and the risk of heat stress, respectively. Nonetheless, these results must be interpreted with caution due to experimental limitations of the study. The absence of significant results, despite a moderate effect size, may reflect limited replication rather than robust biological evidence. The results of the present study are crucial to align intensification of livestock production with an increased concern for animal heat stress in a scenario of global warming (Brandt and Bjerg, 2024). To the best of our knowledge, the findings are novel and have important implications for cattle management in subtropical humid climates.

Animal performance

This study demonstrated that, regardless of access to shade, summer supplementation significantly improved ADG of grazing cattle. Supplemented beef steers produced an additional 0.45 and 0.61 kg/a/d than Control (unsupplemented) steers in unshaded and shaded paddocks, respectively. In paddocks with shade structures, Verdaguer and Rovira (2026) previously reported that supplemented steers had an ADG 0.45 kg/a/d higher than unsupplemented steers averaging three summers in Uruguay due to the improved nutritional status of cattle. The results of the present study demonstrate that the productive benefit of supplementation was maintained even in the absence of shade in the grazing paddock, although Suppl-Shade steers had numerically higher ADG (0.85 kg/a/d) and better feed conversion (4.0 kg DM supplement per kg of added LW) than Suppl-Sun steers (0.64 kg/a/d and 5.2 kg DM/kg LW, respectively) suggesting a moderate production benefit of providing shade in supplemented cattle. This trend was also observed in Control cattle averaging 0.19 and 0.24 kg/a/d in Sun and Shade steers, respectively. Monn et al. (2018) and Silva et al. (2021) reported the provision of shade to grazing beef heifers promoted an additional gain of approximately 0.20 kg/a/d in both experiments. Although the positive biological impact of shade on animal performance (+26 - 33% increase in ADG), statistical significance was not detected in our study eventually due to limited replication. However, the consistent magnitude and directional patterns observed in Control and Suppl steers indicate that the provision of shade appeared to be associated with a tendency toward higher ADG. The lack of shade increases energy requirements for thermoregulation and nutrients that could be used for growth are redirected toward maintaining homeothermy resulting in reduced ADG (Beede and Collier, 1986; Collier et al., 2019; Silva et al., 2021).

Respiration rate

In the present study, the average RR of cattle exposed to the different treatments was within the range of low risk of heat stress (Silanikove, 2000; Pacheco et al., 2020). However, we found a significant reduction in RR among grazing steers with access to shade between 11:00 - 15:00 h, underscoring the effectiveness of this intervention in alleviating heat stress. These results agree with those from previous studies that have found that shaded cattle exhibit lower RR than their unshaded counterparts, especially during the hottest hours of the day (Rovira and Velazco, 2010; Rovira, 2014; Silva et al., 2023). The benefits of shade for animals come from reducing radiant heat gain, body heat storage, evaporative cooling through panting and sweating and by increasing the frequency of beneficial behaviors (e.g., lying down and ruminating) (Mitlöhner et at., 2001; Gebremedhin et al., 2011; Maia et al., 2015; 2023). The fact that daily average RR was not affected by shade suggests that unshaded cattle were able to cool at night to keep 24-h RR near those of shaded cattle (Mader and Johnson, 2010). For dairy cows, studies reported that despite high ambient temperatures during the day a cool period of less than 21 °C for 3 to 6 h will minimize the decline in milk yield (Igono et al., 1992; West, 2003). In our study, 26 out of 86 nights (30%) recorded temperatures above 21 °C during nighttime hours (20:00 - 05:00 h). In those “warmer” nights the temperature averaged 20.6 °C (min. 14.3 °C; max. 25.8 °C) with 7.1 h per night with temperatures below 21 °C (min. 5 h; max. 9 h). This suggests sufficient night cooling conditions even in night periods with temperatures above 21 °C allowing cattle a greater opportunity to recover for high daytime temperatures (Renaudeau et al., 2012).

We did not detect that supplementary feeding affected RR of cattle. Contrary to our results, Verdaguer and Rovira (2026) reported that supplementation in the morning increased RR of grazing steers during the afternoon in the summer months. The lack of effect of supplementary feeding on RR observed in the present study may be attributed to the time and level of supplementation. Although they used a similar TMR to that used in the present experiment, the supplement was delivered 2 hours later (08:00 h) and at a higher level (1.9% BW) compared to our study (06:00 h and 0.8% BW, respectively). Managing heat load by altering timing and/or amount of feed delivered has improved growth performance (Brosh et al., 1998). For example, feeding cattle early in the morning or in the evening offsets the occurrence of peak heat increment with minimal environmental heat load (Brosh et al., 1998; Barajas et al., 2013). Increased RR is an early sign of increasing heat loss from the body, and it is triggered to help return BT to within normal ranges (Kadzere et al., 2002; Dunshea et al., 2013; Idris et al., 2026).

Body temperature

Across treatments, BT followed a circadian rhythm, lower in early morning and greater in late afternoon (Kendall and Webster, 2009), mirroring circadian changes in THI. The BT of steers without access to shade was affected by hot summer conditions. Between 09:00 and 18:00 h, the BT of unshaded steers was on average 41 °C. The increase of BT was reduced by having access to shade (39 °C), in line with the results reported by several authors (Kendall et al., 2006; Tucker et al., 2008; Van Iaer et al., 2015). In Sun treatments, steers with supplementary feeding had lower BT than Control steers between 09:00 and 18:00 h. This result was unexpected but must be considered with caution. The limited number of biological replicates in our study (2 steers with temperature loggers per treatment) may overestimate the magnitude of the effect of supplementary feeding on BT. The effect of summer supplementation on BT of grazing steers needs further investigation to understand the underlying mechanisms associated with it.

Minimum BT of steers was similar in all treatments (37.7 - 38.6 °C). Shephard and Maloney (2023) suggested that the minimum, and not the maximum BT, is potentially more useful for monitoring cattle homeothermy under heat exposure. Alternatively, the daily maximum and minimum BT, and thereby the range, may be a more useful way to assess homeothermy than the comparison of a single value against a defined distribution (Maloney et al., 2019). In our study, unshaded steers had greater BT range (Control: 2.9 °C; Suppl: 2.4 °C) than shaded steers (Control: 1.5 °C; Suppl: 1.3 °C). Although core BT has a natural daily rhythm oscillating by less than 1 °C each day, the subcutaneous BT usually displays a larger amplitude than the true core BT as it is measured near the periphery of the body (Maloney et al., 2019). Regarding supplementation, the results showed that supplementary feeding decreased the BT range by 0.5 °C and 0.2 °C in Sun and Shade steers, respectively.

Physiological indicators and heat stress

One of the objectives of the study was to investigate physiological variables as indicators of heat stress. Because we wanted to ensure broad applicability across similar regions and management systems, we chose to use a broadly applicable THI calculation to identify the risk of heat stress in animals (Marett et al., 2025). In addition, this index can be easily forecasted and transformed into early warning messages to mitigate the impact of heat stress on livestock in commercial farms (Ouellet et al., 2021; Mohad et al., 2025). It is limited, however, in that it does not account for solar radiation or wind speed, which are both known to be key variables contributing to heat stress (Brown-Brandl, 2018). In our study, the provision of shade was the most efficient strategy to reduce the RR of cattle in the THI-heat stress categories. Respiration rate was reduced by 10-12% with shade in the alert, danger and emergency heat stress levels compared with unshaded cattle. Supplementary feeding increased RR of steers in alert and danger heat stress levels. This rise in RR may represent the animal’s response to the increased heat load associated with supplementary feeding (Brosh et al., 1998.; Tresoldi et al., 2025), combined with heat load from the environment.

Similar THI thresholds where RR began rising at a higher rate were detected for Shade vs. Sun and Control vs. Suppl steers. However, the wide 95% confidence intervals observed in Figure 7b suggest uncertainty around THI breakpoints for RR. Limited replication, individual variation and the limitations of the THI previously mentioned may impact these associations between animal response and environmental conditions. Overall, THI values at which an abrupt change in RR and BT occurred ranged from 66.0 to 71.6. Previous studies established THI thresholds for thermoregulatory responses between 65 to 78 in dairy cows (Da Costa et al., 2015; Pinto et al., 2020; Ouellet et al., 2021), and of 69 and 74 for dairy calves and heifers (Dado-Senn et al., 2023; Daniels et al., 2025). Therefore, our results provide novel information on THI breakpoints for growing beef cattle. These results are generalizable to Bos taurus breeds, with emphasis on Aberdeen Angus cattle, since Bos indicus breeds and their crosses usually tolerate higher THI thresholds before showing signs of heat stress (Fedrigo et al., 2021). Shaded steers experienced less radiant solar load (DeBord et al., 2024), which lowered the BT increase per unit of additional THI before and after THI breakpoints (b 1 and b 2 slopes in Figure 6) compared to steers permanently exposed to direct solar radiation. Lower slopes mean that shaded cattle would attain a high BT at higher THI levels compared to unshaded cattle. This is confirmed by the lower BT of shaded steers compared to unshaded steers in the danger (79 ≤ THI ≤ 84) and emergency (THI > 84) categories of heat stress between 09:00 and 18:00 h (Figure 5). However, we found that cattle in Shade treatments had lower THI breakpoints (mean: 67.1) than cattle in Sun treatments (71.5) irrespective of supplementary feeding (Figure 7), which is counterintuitive considering that they had access to a heat stress mitigation strategy. Further studies should be undertaken to explore the relationships between environmental conditions and physiological indicators in animals using more thoroughly thermal indices (i.e., heat load index, Gaughan et al., 2008).

In summary, supplementary feeding significantly increased BW and ADG of steers grazing native pastures during summer, whereas the provision of shade significantly reduced RR and BT of steers during the hottest hours of the day. While these results supported the main hypothesis, future research with robust experimental designs is needed to unveil the combined effects of shade and supplementation on physiological indicators and animal performance during summer. In addition, this study identified THI thresholds for RR and BT in growing beef steers for the first time, which could be used as a guideline to take management decisions under commercial conditions. However, studies with more biological replicates and more comprehensive environmental indices are needed for a better estimate of critical climate thresholds.