0.05). Johnson et al., (1963) summarized that DMI and milk yield were shown to decrease significantly when maximum THI reached 77, this was later re-assessed and values of 64, 72, and 76 for minimum, average, and maximum THI were given respectively (Igono et al., 1992).
The black globe humidity index may perhaps be a more ideal measurement of heat stress due to the fact that solar radiation is incorporated. When calculating the black globe humidity index from these studies, only four out of the eight studies actually recorded the appropriate values in order to produce the BGHI. Therefore, small numbers of observations are attributed to the lower values and correlations observed were very small. Thus, we did not produce evidence that BGHI was superior to THI for estimating the threshold temperatures for milk yield loss.
The next question is a practical one. What is the cost return to the producer for cooling beginning at an average THI of 68 versus 72? If we use an example employing 100 dairy cows which are being cooled by a Korral Kool cooler we should expect a milk yield gain of 2.2 kg of milk per day beginning cooling at 68 versus 72. For 100 dairy cows, that would equate to a milk yield gain of 4.84 CWT‘s. Using a milk price of $17.00 and a feed price of $14.00 the income above feed costs would be $14.52. The cost of using the coolers is shown in Table 2 from Burgos et al. 2007. Using a variable cost of $0.14 per cwt of milk produced and assuming each cooler would cool 10 cows the total cooler variable cost would be $6.8 producing an income of $7.09 per 100 cows or $0.071 per cow per day. In a herd of 3000 lactating dairy cows the potential income would equate to $213 per day or $1491 per week. This does not take into account any beneficial effects on reproductive performance in these cows.
However, the cost of cooling could be reduced dramatically by using geothermal conductive cooling of cows. Bastian and co-workers 2003 demonstrated that waterbeds filled with chilled water offered an alternative cooling method for dairy cows. However, there was considerable condensation on the surface of the cooled waterbed which would represent a mastitis risk to dairy cows. Recently collaborative work by Agriaire Industries and the University of Arizona has demonstrated that heat exchangers could be buried 12 inches below the surface of a freestall bed and still provide significant conductive cooling to dairy cows. Geothermal cooling would represent significant cost reduction in reducing heat stress on dairy cows and offers the additional opportunity of using the same approach to warm cows during cold winter months in northern dairy locations. Field testing of this concept is currently underway