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Most feedlot animals in Australia experience 2 to 3 moderate heat waves during summer. This study aimed to gain understanding of the physiological drivers in response to and during recovery from such events with a view to designing strategies to ensure rapid and safe recovery. Two hypotheses were tested during thermal challenge and recovery in climate-controlled rooms (CCR): firstly, the feedlot steer on a grain-based diet mounts appropriate physiological responses during moderate heat load and in recovery so that its performance and physiology state after recovery is not different to the feed restricted thermoneutral (FRTN) steer. Secondly, commonly used indicators of increased heat load, e.g. respiration rate (RR), panting score (PS), body surface temperatures (ST), and water consumption (WC), reflect rumen temperature (RT) during thermal challenge and recovery at the level of daily means. In this study, 36 Angus steers (live weight (LW) 451.5 ± 22.6 kg) made up 3 cohorts of 12 animals that sequentially underwent the CCR phase. For this 18-d phase, the steers were allocated to either a moderate heat load treatment (thermally challenged, TC, n = 18) or a FRTN treatment (n = 18). The TC group underwent 3 periods, Pre-Challenge (4 d, temperature humidity index (THI) range of 68 to 71), Challenge (7 d, THI 73 to 84 with diurnal cycling), and Recovery (7 d, THI 68 to 71). The FRTN group were held at thermoneutral conditions in the CCR (THI 66.9 ± 0.3), and each animal was offered an amount of feed was based on the feed intake of its LW matched TC pair. Thus, as DMI fell in the TC group during Challenge, feed restriction was imposed on the FRTN group. The data were collected by trained observers were DMI, RT, RR, PS, body STs (forehead, shoulder, leg, rump), and WC. Challenge induced a heat stress response in the TC group with reduced DMI and LW, and elevated RT, RR, PS, body STs, and WC (P < 0.001). These measures were unchanged or reduced in the FRTN group (P < 0.001). At the end of Recovery, the TC and FRTN groups had converged on most measures including LW. Daily mean RT of both groups showed strong linear relationships with THI, RR, PS, head ST, and WC (P ≤ 0.0022) but opposing elliptical relationships with DMI; that is, as DMI fell with increasing RT for the TC group, DMI increased with rising RT for the FRTN group. In all, the feedlot steers in this study demonstrated sufficient homeorhetic capacity to adjust to moderate heat load and recover from it. [ABSTRACT FROM AUTHOR]

Simple Summary: Appropriately designed cowsheds could help improve the microclimate within Vietnamese tropical smallholder dairy farms to minimise the risk of heat stress in the cows. Currently, these farmers build cowsheds on whatever land is available based on self-accumulated experiences without careful consideration of heat stress. This study characterised heat stress abatement strategies by identifying the housing parameters most associated with the cowshed microclimate across four climatically contrasting dairy regions of Vietnam. During the daytime, the microclimate inside the cowsheds was found to be relatively hot in highland and very hot in lowland regions. Although there were seven typical cowshed types defined, none were more effective than the others in improving cowshed microclimate. Increasing altitude, eave roof height and floor area per cow, and using the roof soakers together with fans, were most associated with improving microclimate, as indicated by decreasing temperature, decreasing temperature-humidity index and increasing air speed. These cowshed parameters should be prioritised for future research into the amelioration of heat stress of the cows in tropical smallholder dairy farms. In smallholder dairy farms (SDFs), farmers often build cowsheds using local materials and based on self-accumulated experience without due consideration to reducing the risk of heat stress. This study aimed to characterise the heat stress abatement strategies and microclimate within SDF cowsheds from four typical dairy regions of Vietnam (south lowland, south highland, north lowland and north highland) and identify the housing parameters most associated with the microclimate. The study was conducted on 32 SDFs (eight SDFs per region) in autumn 2017. Twelve housing management variables, illustrating cowshed design and heat stress abatement methods of each SDF, were collected. Six microclimate parameters, collected within the cowshed, were temperature (AT), humidity, air speed (AS), heat load index (HLI), Temperature-humidity index (THI) and accumulated heat load units (AHLU) during a day (06:00 h to 18:00 h). Factor analysis and cluster analysis was applied to group cowsheds of SDFs into clusters where SDFs in the cluster had the same cowshed characteristics. Multivariable linear models were applied to define the parameters most likely to inform future research into heat stress mitigation on SDF. Averaged from 08:00 h to 18:00 h, microclimate inside the cowsheds was considered hot (HLI > 79) in the highland and very hot (HLI > 86) in the lowland regions. Cows in the lowland regions accumulated high heat load (AHLU > 50) by 18:00 h. Cowsheds of SDFs varied widely and grouped into seven cowshed types, but no type was more effective than others in reducing heat stress conditions within cowsheds. Using roof soakers together with fans decreased AT and HLI by 1.3 °C and 3.2 units, respectively, at 14:00 h compared to 11:00 h. Each 100 m increase in altitude was associated with decreases of 0.4 °C in AT, 1.3 units in HLI and 0.8 units in THI (p < 0.001). Each meter increase in the eave height of the cowshed roof was associated with decreases of 0.87 °C in AT, 3.31 units in HLI and 1.42 units in THI, and an increase of 0.14 m/s in AS (p < 0.05). The cowshed parameters that should be prioritised for future research into the amelioration of heat stress in SDF cows include using the roof soakers together with fans, increasing altitude, eave roof height and floor area per cow. [ABSTRACT FROM AUTHOR]

Simple Summary: It is well known that the thermal environment has an integral role in maintaining the health and productivity of cattle. Although cold stress has been identified to negatively influence cattle comfort and productivity, the predominant focus herein has been describing the influence of heat stress on bovines. The impact of heat stress is particularly important due to the changing global environment. Global warming is likely to occur, however, the nature and magnitude of environmental changes, both climatic and non-climatic, are difficult to elucidate. Therefore a predominant focus on the impact of hot environments on cattle is warranted. This review provides an overview of the dynamic relationship that exists between the thermal environment and bovines. Heat stress and cold stress have a negative influence on cattle welfare and productivity. There have been some studies investigating the influence of cold stress on cattle, however the emphasis within this review is the influence of heat stress on cattle. The impact of hot weather on cattle is of increasing importance due to the changing global environment. Heat stress is a worldwide phenomenon that is associated with reduced animal productivity and welfare, particularly during the summer months. Animal responses to their thermal environment are extremely varied, however, it is clear that the thermal environment influences the health, productivity, and welfare of cattle. Whilst knowledge continues to be developed, managing livestock to reduce the negative impact of hot climatic conditions remains somewhat challenging. This review provides an overview of the impact of heat stress on production and reproduction in bovines. [ABSTRACT FROM AUTHOR]