Thermal radiation absorbed by dairy cows in pasture

Abstract

The goal of the present paper was to assess a method for estimating the thermal radiation absorbed by dairy cows (0.875 Holstein–0.125 Guzerath) on pasture. A field test was conducted with 472 crossbred dairy cows in three locations of a tropical region. The following environmental data were collected: air temperature, partial vapour pressure, wind speed, black globe temperature, ground surface temperature and solar radiation. Average total radiation absorbed by animals was calculated as ${R_{abs}} = 640.0 \pm 3.1\, W.{m^{ - 2}}$. Absorbed short-wave radiation (solar direct, diffuse and reflected) averaged 297.9 ± 2.7 W m⁻²; long wave (from the sky and from terrestrial surfaces) averaged 342.1 ± 1.5 W m⁻². It was suggested that a new environmental measurement, the effective radiant heat load (ERHL), could be used to assess the effective mean radiant temperature $\left( {T_{\text{mr}}^* } \right)$. Average $T_{\text{mr}}^*$ was 101.4 ± 1.2°C, in contrast to the usual mean radiant temperature, ${T_{mr}} = 65.1 \pm 0.5^\circ C$. Estimates of $T_{\text{mr}}^*$ were considered as more reliable than those of T _mr in evaluating the thermal environment in the open field, because T _mr is almost totally associated only with long wave radiation.