## 3 Results and Discussion
### Behavior  
Behavioral changes are the animals' first response to heat stress [66]. The main behavioral changes reported are shown in Figure 3. Overall, cows decrease their time spent lying down and decrease their feeding events; in contrast, they increase their walking behavior and water trough visits. Of the seven studies included in this review, four reported a decrease in feeding events. However, among the studies evaluated, no study reported whether there was a reduction in dry matter intake (DMI) or production losses in the animals. However, increases in air temperature negatively impact the appetite center of the hypothalamus, reducing the DMI of dairy cows [67]. The reduction in the DMI of lactating cows begins at air temperatures between 25 and 26 \({}^{\circ}\)C [68]. The feed intake of cows results in the production of metabolic heat generated during rumination and feed digestion. These metabolic processes increase the body temperature of lactating cows [69]. A reduction in the DMI by heat-stressed cows was observed after two days of heat stress exposure [63]. Heat stress reduced the DMI by 28-34% [70] and reached up to 40% when the air temperature exceeded 40 \({}^{\circ}\)C [68]. In this context, it would be interesting to analyze the cumulative heat load in animal responses over several consecutive days. [PERSON] et al. [71] determined a heat-load threshold (THI 67) that led to changes in different activity traits in high-yielding cows during lactation. Furthermore, this study demonstrated  
Figure 3: Main findings of the studies included in this review (n = 7) on the effects of heat stress on the behavior of lactating cows raised in compost barns. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Reference** & **THI Equations** & **Indicators of Stress** & **Studies** \\ \hline
**[PERSON] et al. [63]** & THI = td \(-\) (0.55 \(-\) 0.55 RH) (td \(-\) 58) & Not informed & [48] \\
**[PERSON]. [64]** & THI = Ta + (0.36 \(\times\) Tdp) + 41.5 & Normal 74; alert 75\(-\)78; danger & [49; 50; 51; 52] \\  & & 79-83; emergency 84 & [49; 50; 51; 52] \\
**[PERSON] et al. [65]** & THI = (0.8 \(\times\) Tdb) + [(RH/100) \(\times\) (Tdb \(-\) 14.3)] + 46.4 & Normal 69; alert 70\(-\)74; danger & [53] \\
**[PERSON]. [56]** & THI = Ta + 0.36 \(\times\) (Tdp) + 41.5 &
\begin{tabular}{c} Normal 69; alert 70\(-\)75; danger \\ 75\(-\)79; emergency 80 \\ \end{tabular} & [54] \\ \hline \hline \end{tabular}  
* THI—temperature-humidity index; Tdp—dew-point temperature; RH—relative humidity; Tdb—air temperature.  
\end{table} 
Table 3: Temperature humidity index (THI) equations that were used in the studies included in this review to determine the thermal environment for lactating cows raised in a compost barn system. that heat-load duration and accumulation should be considered when assessing the heat load in lactating cows. As lactating cows show a time delay in their response to heat stress [15], and according to [72], the time average was explained over three days. Thus, the daily heat-load duration can assist in assessing the time a lactating cow needs to recover from heat stress [71]. The behavior of high-production cows (milk production of more than 30 kg/day) was altered when the THI was greater than 68 [60]. In a recent study, the author of [14] observed that mild heat stress (THI = 73) altered the feeding behavior of lactating cows housed in tie-stall barn systems. Among the behavioral changes, decreased termination and decreased bolus were observed. Decreased rumination affects rumen health, saliva production and the DMI of lactating cows [73; 74]. The reduction in the DMI of lactating cows limits the amount of nutrients to the mammary glands (MGs). Reducing nutrients to the MGs due to heat stress can reduce milk production from 25 [75] to 40% [76]. In a study conducted over 10 years (from 2008 to 2018) in the US, it was observed that cows under heat stress conditions had reduced milk production by 21% during lactation [77]. Heat stress influences the time that cows spend resting.