Reliability of methods to determine cutaneous evaporative water loss rate in furred and fleeced mammals.

We used a high-precision weighing system and flow-through respirometry to quantify cutaneous evaporative water loss rates in woolly sheep (wool thickness, ca. 6.5 cm) and haired goats (coat thickness, ca. 2.5 cm), while simultaneously recording parallel data obtained from (1) a flow-through ventilated capsule, (2) a closed hand-held electronic evaporimeter chamber, and (3) a closed colorimetric paper disc chamber. In comparison to the weighing system and respirometry, used here as a "gold standard" measure of cutaneous evaporative water loss rate, we found relatively good agreement with data obtained from the flow-through ventilated capsules. However, we found poor agreement with data obtained from the closed electronic evaporimeter chambers (underestimated by 60%, on average) and the closed colorimetric paper disc chambers (overestimated by 52%, on average). This deviation was likely associated with a requirement for shaved skin in the closed chamber methods. Our results therefore cast doubt on the validity of the closed chamber methods for measurement of cutaneous evaporative water loss rates in furred and fleeced mammals, and instead show that more accurate values can be obtained using flow-through ventilated capsules.

Comparative methods analysis on rates of cutaneous evaporative water loss (CEWL) in cattle.

Closed colorimetric paper disc chambers and flow-through ventilated capsules are the most employed methods of measuring rates of local cutaneous evaporative water loss in cattle. However, we do not know if these methods show a close agreement with the total rate of cutaneous evaporative water loss derived from the weighing system (i.e., the gold standard method). We therefore combined a high-precision weighing system and flow through respirometry to accurately quantify the cutaneous evaporative water loss rates in shaded heifers, while simultaneously recording parallel data obtained from a flow-through ventilated capsule, and a closed colorimetric paper disc chamber. Least square means of the local surface-specific cutaneous evaporative water loss rate (g m-2 h-1) derived from the colorimetric paper discs and ventilated capsules show close agreement to the total rate of surface-specific cutaneous evaporative water loss (g m-2 h-1) derived from the weighing method. Likewise, fitted linear regression lines also showed that they were well correlated (e.g., R2 = 0.93 and r = 0.96 for ventilated capsule vs weighing method; and R2 = 0.81 and r = 0.91 for colorimetric paper discs vs weighing method). However, the mean square deviation revealed various sources of disagreement between the local measurements and those derived from the weighing method, in which the local rate of cutaneous evaporative water loss derived from colorimetric paper discs showed greater deviation. In conclusion, given the importance of cutaneous evaporative water loss for assessing temperature requirements and heat tolerance of cattle, our findings show large discrepancies derived from the closed colorimetric paper discs chamber when compared with parallel data derived from the gold standard method, which is sufficient to call into question previous findings obtained by employing such methods. Moreover, the flow-through ventilated capsule appears to be the most accurate method to assess the local rate of cutaneous evaporative water loss in cattle.

Respiratory heat loss in the sheep: a comprehensive model.

A model is presented for the respiratory heat loss in sheep, considering both the sensible heat lost by convection ( C(R)) and the latent heat eliminated by evaporation ( E(R)). A practical method is described for the estimation of the tidal volume as a function of the respiratory rate. Equations for C(R) and E(R) are developed and the relative importance of both heat transfer mechanisms is discussed. At air temperatures up to 30 degrees C sheep have the least respiratory heat loss at air vapour pressures above 1.6 kPa. At an ambient temperature of 40 degrees C respiratory loss of sensible heat can be nil; for higher temperatures the transfer by convection is negative and thus heat is gained. Convection is a mechanism of minor importance for the respiratory heat transfer in sheep at environmental temperatures above 30 degrees C. These observations show the importance of respiratory latent heat loss for thermoregulation of sheep in hot climates.