Latent Heat Loss of Dairy Cows Bred in a Semiarid Environment

Latent Heat Loss of Dairy Cows Bred in a Semiarid Environment

Severino Guilherme C. Dos Santos1, Edílson P. Saraiva1, Vinícius F. C. Fonsêca1, Edgard C. Pimenta Filho1, Pedro J. Rodrigues Neto1, Raniere S. Paulino1, Antônio C. Pinheiro1

1 Federal University of Paraiba – UFPB – Brazil

It has long been recognised that the ability of an animal to withstand with hot environments is proportional to its ability to eliminate latent heat by evaporation of sweat from the skin or from the respiratory system. Direct determination of the evaporation from the skin and the respiratory tract can be done by ventilated capsules and respiratory masks, but they are extremely difficult to measure under field conditions. In those situations, indirect methods of evaluation of the cutaneous and respiratory evaporation rate would be interesting. Thus, for this study, it was aimed to estimate evaporative losses from sweating and the respiratory tract of dairy cows bred in a semi-arid environment. Thirteen dairy cows from a herd in the city of Caturité, PB, Brazil (07° 25' 13'' S, 36° 01' 38” W, 405 m altitude) were allocated in two groups (n = 15 for group 1, n = 15 for group 2) on the basis of their milk yield: low ( <15 kg day-1) and high (>20 kg day-1); the cows were 7/8 Holstein – 1/8 Zebu and predominantly black. The herd was managed on open confinement, fed silage and concentrated diet. The cows remained in the field exposed to sun between the milkings (3:50 a.m and 3:50 p.m., respectively). The observations (n = 450 observations) on the selected cows were made just after the first milking (7:00, 9:00 and 11:00 a.m.). An infra-red thermometer (Fluke, mod. 568), adjusted for an emissivity of 0.98 was used to determine the hair coat surface temperature (Ts), measured by scanning the dorsal area. The respiration rate (RR) was measured by the counting of flanks movements in the animal. At the same time, environmental variables were recorded: Air temperature (Ta), black globe temperature (Tg), relative humidity (RH) and wind speed (W). The black globe was a standard one, with 15 cm diameter and placed 0.90 m above the ground close to the animals; air temperature and relative humidity were measured with a thermohigrometer (HOBO, mod. U12-013) under the sun, 1.2 m above the ground; wind speed was measured near the black globe using a digital anemometer (LM-8000). The following equations were used in order to calculate the latent heat flux through the respiratory tract: (ER = λ (ΨEXP – ΨATM)/rVR W•m-2). where, ER is the heat flux through the respiratory tract, W m-2; λ (2500.7879 – 2.3737tA [J•g-1]) is the latent heat of water vaporization, ΨEXP is the absolute humidity (g•m-3) of the expired air, ΨATM is the atmospheric absolute humidity, and rVR is the water vapor resistance to the heat loss through the respiratory tract. In order to estimate the heat loss due to sweating, the equation considering the animal's hair surface temperature (Ts) was used: Es = 31.5 + 3.67e(Ts – 27.9)/2.1915 W•m-2, where Es is the rate of heat loss by cutaneous evaporation, W m-2 and Ts is the hair coat surface temperature, ºC. The statistical analyses were based on the Generalized Linear Model (Glimmix Procedure). Environmental conditions during the observations (7:00, 9:00 and 11:00 a.m.), and their standard deviation were as follows, respectively: air temperature (ºC): 25.14±0.9, 27.71±1.2, 30.1±1.4; black globe temperature (ºC): 30.55±0.9, 32.01±0.4, 35.23±1.2; relative humidity (%): 75.52±1.34, 64.60±1.85, 53.71±2.45 and wind speed (m s -1): 1.68±0.34, 2.15±0.28 and 2.95±0.52. The results of analyses of variance for the different periods of observation demonstrated a significant effect on Es and Ts. However, the RR (53±7.5, 61.17±5.5 e 66.84±9.5 breaths min-1) and Er (29.20±0.6, 30.01±0.2 and 30.23±0.72 W.m-2) did not change (P > 0.05). The average value for Ts (34.22±0.4ºC) at 7:00 a.m. differed significantly (P < 0.01) with the observed at 9:00 a.m. (35.55±0.6ºC) and 11:00 a.m. (36.50±0.35ºC). These differences can be explained by the levels of radiation load in the different periods of observation, measured by the black globe temperature. Sweating rate seems to follow skin temperature, therefore, the skin temperature is the primary driving force for sweating. Thus, the cutaneous evaporation at 7:00, 9:00 and 11:00 a.m. were 131,41±12.3, 161.76±15.20 e 224.57±11.34 W.m-2, respectively. In our study, the contribution of cutaneous evaporation for the latent heat losses increased as a function of the environmental temperature (75, 80 and 90% at 7:00, 9:00 and 11 a.m., respectively). The heat loss from the respiratory tract was constant between the periods of observation, results that differ from those reported in the literature, showing that Er increases exponentially with the levels of environmental temperature. Considering the level of milk yield, there was no difference (P > 0.05) in the physiological responses between the groups for both heat losses from the respiratory tract (29.50±2.01 W.m-2 for group 1 and 30.25 for group 2) and cutaneous evaporation (164.03±22.30 W.m-2 for group 1 and 180.40 ± 16.50 W.m-2 for group 2). Milk yield can leads to a metabolic heat production due to the metabolism of a large amount of nutrients, making the high producing cows more vulnerable to heat stress than lower yielding ones. However, the metabolic heat production in cows of both groups may be similar, causing minimal differences in the thermal balance of these animals. Based on the results, we can conclude that the cutaneous evaporation represent up to 90% of the latent heat loss in 7/8 Holstein x 1/8 Zebu crossbred cows in an environment with average temperatures of 30 ° C.