Animal welfare can be negatively affected when dairy cattle experience heat stress. Managing heat stress has become more of a challenge than ever before, due to the increasing number of production animals with increased milk yield, and therefore greater metabolic activity. Environmental temperatures have increased by 1.0°C since the 1800s and are expected to continue to increase by another 1.5°C between 2030 and 2052. Heat stress affects production, reproduction, nutrition, health, and welfare. Means exist to monitor and evaluate heat stress in dairy cattle, as well as different ways to abate heat, all with varying levels of effectiveness. This paper is a summary and compilation of information on dairy cattle heat stress over the years.

The objective of this study was to compare weekly mean lying time (LT), neck activity (NA), reticulorumen temperature (RT), and rumination time (RU) among 3 breed groups, milk yield (MY), and temperature-humidity index (THI). Cows (n = 36; 12 Holstein, 12 crossbred, and 12 Jersey) were blocked by parity group (primiparous or multiparous), days in milk, and MY. Lying time, NA, RT, RU, and MY were recorded and averaged by day and then by week for each cow. For study inclusion, each cow was required to have 10 wk of LT, NA, RT, and RU data. Maximum THI were recorded and averaged daily. Mean (±SE) days in milk, LT, MY, RT, RU, NA, and maximum THI were 159.0 ± 6.0 d, 11.1 ± 0.1 h/d, 28.7 ± 0.5 kg/d, 38.8 ± 0.0°C, 6.4 ± 0.1 h/d, 323.8 ± 3.8 activity units, and 56.5 ± 0.6, respectively. The MIXED Procedure of SAS (SAS Institute Inc., Cary, NC) was used to evaluate fixed effects of breed, MY, parity, THI, and their interactions on LT, NA, RT, and RU with cow nested within breed as subject. All main effects remained in each model regardless of significance level. Stepwise backward elimination was used to remove nonsignificant interactions. The interactions of breed × parity group and maximum THI × parity group were associated with RT. Increasing THI coincided with increasing RT. Least squares means LT for multiparous cows was significantly greater than LT for primiparous cows (11.4 ± 0.3 and 10.5 ± 0.5 h/d, respectively). Least squares means NA for primiparous cows was greater than for multiparous cows of all breeds (372.1 ± 10.9 and 303.4 ± 7.8, respectively). The CORR Procedure of SAS was used to evaluate relationships among RT, RU, LT, NA, and MY. Rumination time was positively correlated with MY (r = 0.30) and negatively correlated with LT (r = -0.14). Reticulorumen temperature was negatively correlated with MY (r = -0.11). Rumination time was positively correlated with NA (r = 0.18) and negatively correlated with LT (r = -0.14). Lying time and NA were negatively correlated (r = -0.43). Neck activity was positively correlated with MY (r = 0.14). Lying time was negatively correlated with MY (r = -0.25). Milk yield was associated with RU, which may be related to cows with greater MY also having a greater feed intake. Lying time increased and NA decreased with increasing parity, which may be effects of social hierarchy, where primiparous cows are more susceptible to being pushed away from the feed bunk and freestalls. Milk yield was positively associated with RU. Greater milk production requires greater feed intake, which may result in longer RU than for low-yielding cows. Lying time decreased as milk yield increased. The behavioral and physiological differences observed in this study provide new insight into the effects that breed, parity, MY, and THI have on cows.

The southeastern United States experiences an extended hot season with a high environmental temperature and relative humidity. With increasing global temperatures, managing dairy cattle in regions with tropical, subtropical, and Mediterranean climates is becoming an increasing challenge. Heat-stressed cows will decrease feed intake, decrease productivity, and increase respiration rate in an attempt to maintain internal body temperature. Temperature-humidity index (THI) is a unitless value that has been used to measure the magnitude of heat stress on dairy cows. Many researchers have studied the THI threshold at which dairy cattle begin to experience heat stress. When housing cows in a confinement setting, a pasture-based setting, or a combination of the two, the appropriate heat abatement should be implemented to allow cows to perform to their potential and to improve overall animal welfare. This review summarizes heat abatement strategies that have been studied to reduce the negative effects of heat stress.

The objectives of the study were to use a heat stress scoring system to evaluate the severity of heat stress on dairy cows using different heat abatement techniques. The scoring system ranged from 1 to 4, where 1 = no heat stress; 2 = mild heat stress; 3 = severe heat stress; and 4 = moribund. The accuracy of the scoring system was then predicted using 3 machine learning techniques: logistic regression, Gaussian naïve Bayes, and random forest. To predict the accuracy of the scoring system, these techniques used factors including temperature-humidity index, respiration rate, lying time, lying bouts, total steps, drooling, open-mouth breathing, panting, location in shade or sprinklers, somatic cell score, reticulorumen temperature, hygiene body condition score, milk yield, and milk fat and protein percent. Three different treatments, namely, portable shade structure, portable polyvinyl chloride pipe sprinkler system, or control with no heat abatement, were considered, where each treatment was replicated 3 times with 3 second-trimester lactating cows. Results indicate that random forest outperformed the other 2 methods, with respect to both accuracy and precision, in predicting the sprinkler group's score. Both logistic regression and random forest were consistent in predicting scores for control, shade, and combined groups. The mean probability of predicting non-heat-stressed cows was highest for cows in the sprinkler group. Finally, the logistic regression method worked best for predicting heat-stressed cows in control, shade, and combined. The insights gained from these results could aid dairy producers to detect heat stress before it becomes severe, which could decrease the negative effects of heat stress, such as milk loss.

The objective was to evaluate the influence of cell-mediated immune response classification on performance of dairy cattle experiencing conditions of heat stress on pasture. On d 0, pregnant, lactating dairy cows (n = 27; 18 Holstein and 9 Jersey) received a subcutaneous sensitization dose of killed Candida albicans (CA; 0.5 mg) with Quil-A adjuvant (0.75 mg) in 2.5 mL of PBS. Tail skin fold thickness (SFT) was measured with a Harpenden caliper on d 14 immediately following an intradermal injection of killed CA (0.1 mg in 0.5 mL of PBS) in the right tail skin fold and a control injection (PBS) in the left tail skin fold. On d 15, change in SFT was measured and used to assign cell-mediated immune response classifications (CMIR) as High (HR; >0.5 SD above the mean; n = 9), Intermediate (IR; within 0.5 SD above or below the mean; n = 10), or Low (LR; >0.5 SD below the mean; n = 8). Cows from each CMIR were assigned to 1 of 3 environmental conditions (EC): No Abatement (no heat abatement provided; n = 9), Shade (portable shade structure; n = 9), or Sprinklers (polyvinyl chloride sprinkler system; n = 9) for a 38-d period (July 1, 2019 to August 8, 2019). Daily Milk Yield (DMY), Reticulorumen Temperature (RT), Heat Stress Score (HSS; 0 = respiration rate of < 80 breaths/min to 4 = moribund, labored breathing), and instances of Mouth Open (MO), Tongue Out (TO), and Drool were recorded daily. Respiration rate (RR; breaths/min) was recorded 3 times/day (morning, midday, and late afternoon) and averaged on Monday, Wednesday, and Friday weekly. Body Condition Score (BCS; 1 = emaciated to 5 = obese) was recorded weekly. Data were analyzed using MIXED procedures of SAS specific for repeated measures with CMIR, EC, Breed, Time and 2-way interactions as fixed effects and Pen as random. Low exhibited the lowest HSS and BCS, but the greatest MO and TO instances (P < 0.02). High exhibited the greatest instances of Drool (P < 0.02). Heat stress score, MO, and Drool were greatest in the No Abatement group (P < 0.01). Respiration rate was greatest in the No Abatement and Shade groups relative to Sprinklers (P < 0.01). Daily milk yield, RT, and Drool were increased whereas HSS and RR were decreased in Holstein relative to Jersey cows (P < 0.01). Interactions (P < 0.05) included: CMIR by EC for DMY, HSS, RR, MO, TO, and Drool; CMIR by Breed for RT, DMY, and BCS; EC by Breed for RT, DMY, HSS, and RR; and EC by Time for DMY, HSS, BCS, RR, MO, and TO. There was an effect of Time for all variables (P < 0.05). Preliminary results suggest an influence of cell-mediated immune response classification on performance of dairy cattle experiencing conditions of heat stress and warrant future studies with an increased number of animals.