Influence of solar radiation in Nelore cattle thermoregulation

Studies on thermal balance of Nelore cattle in tropical conditions are very limited. Therefore the aim of this study was to evaluate the influence of solar radiation on the thermal balance of Nelore cattle, by addressing the metabolic heat production, latent and sensible heat exchanges.

The study was conducted in the city of Jaboticabal, at São Paulo State University – UNESP, in southern Brazil (south latitude 21° 8', longitude: 48° 11' and elevation of 583 meters).  Five Nelore cattle with an average body weight of 750kg and same age and body condition were evaluated. A 5x5 latin square experimental design was conducted in two environmental conditions, shade and exposed to solar radiation under natural conditions. Two replicates were done in both environments totaling 20 days of data collection.  The animals were evaluated in five different periods during the day (1=08:00h-10:00h, 2=10:00h-12:00h, 3=12:00h-14:00h, 4=14:00h-16:00h, 5=16:00h-18:00h).

The Animal Biometeorology Laboratory developed the System of Physiological Measurement to the continuous measurement of respiratory gases (oxygen; carbon dioxide; water vapour), respiratory functions (tidal volume, respiratory flow and respiratory rate) and body temperatures (skin, haircoat, rectal and expired air). The system used for cattle is composed by: the mask (developed by the Laboratory); oxygen and carbon dioxide analyzers (model FMS-1201-05, Field Metabolic System); two water vapour analyzers (one for the atmosphere and one for the expired air of cattle, model RH-300, Sable System); three pumps (model SS4 sub-sample, Sable System); a dessicant column (Magnesium Perchlorate); spirometer (model ML141, ADInstruments); chamber to the mixture of gases (model MLA246, ADInstruments); two breathing tube; a flow head (model MLT1000, ADInstruments); a probe for the expired air temperature (model MLT415/AL, ADInstruments). To measure the heat loss by cutaneous evaporation was used a ventilated capsule placed in a corporal surface of the animal and maintained manually. The heat flows by convection (q_CONV, W m^-2) and long-wave radiation (q_RAD, W m^-2) were also measured.  In each sampling day the air temperature (T_A, °C) and relative humidity (H_R, %) were measured using a data logger that recorded in regular intervals of 1 second. The mean radiant temperature (M_RT, °C) was calculated according Silva (2000). The solar radiation (R_S, W m^-2) was measured using a Pyranometer (model CMP 22, Kipp & Zonen) in regular intervals of 10 minutes. The statistical model of the study was:

Y_ijklm is the m-th observation of variables (q_MET, q_RE, q_CE, q_CONV, q_RAD, R_R, R_V, T_R, T_S e T_HC), A is the random effect of the i-th animal (1, 2, 3, 4 e 5); M is the fixed effect of the j-th environment (sun or shade); R is the fixed effect of the k-th latin square (1 and 2); I is the interaction between the j-th environment and the k-th latin square replicates; D is the random effect of the l-th sampling day inside the interaction between environment and latin square; CH is the fixed effect of the classes of hours (1=8:00h-10:00h, 2=10:00h-12:00h, 3=12:00h-14:00h, 4=14:00h-16:00h, 5=16:00h-18:00h); e_ijklmn is the residual term and µ is the parametric mean.

During the hottest hours of the day, between 12 and 13 hours, the solar radiation reached more than 800 W m^-2; while before 9h and after 16h the solar radiation averaged 400 W m^-2. The average of metabolic heat production was 233.66±5.92 W m^-2 (P<0.05), with a higher mean at shade (245.25±5.67 W m^-2), than  exposed to solar radiation: 222.03±5.43 W m^-2 (P<0.01). The heat loss by respiratory evaporation (q_RE) represented a little part of the heat dissipation produced by metabolism, with an average of 13.59±0.55 W m^-2. The same was observed for the heat loss by the sensible mechanisms, with an average of 10.23 W m^-2 for the heat flow by convection and 15.81 W m^-2 for long-wave radiation. The respiratory rate did not differ statically between the environments with an average of 20.04±3.0 breaths min^-1. However, the respiratory flow was higher average at shade (6.09±0.13 L breath^-1), while at sun was 5.62±0.12 L breath^-1. The heat loss by cutaneous evaporation (q_CE) was the mechanism that most contributed to the heat loss of Nelore cattle, and it was higher at sun (74.70±3.86 W m^-2) and lower in shade (66.83±3.70 W m^-2). However this mechanism was not enough to the dissipation of the heat metabolic produced. Thus, these animals did not change significantly their physiological answers. This effect could be due to heat storage, because rectal temperature during the 10 sampling hours increased approximately 1°C. We observed a heat storage for these animals around 70 W considering the interval of 10 hours.

Therefore, our results indicate that solar radiation influenced the metabolic heat production. Furthermore, the mechanisms of heat loss were not efficient to the dissipation of all the thermal energy and the heat storage was an important mechanism for the thermoregulation of these animals.