During hot summer months, the water needed for a cow herd often determines several other management decisions. To best assess the adequacy of water quantities in surface water or from wells or "rural water" supplies, it first is necessary to have an idea of the amount needed for cattle of different sizes and stages of production that you may have during the summer on the ranch.
A University of Georgia publication (Rossi and Pence, revised by Dyer, 2012) lists the estimated water requirements for cattle in different production stages if the daily high temperature is 90 degrees F. They suggest that the amount of water required can be estimated by the production stage and the weight of the cattle. For instance, a lactating cow needs 2 gallons of water per 100 pounds of body weight. A non-lactating cow or bull needs just 1 gallon of water per 100 pounds of body weight. If you are estimating water needs for your cattle, be honest about the weight of the cows in the herd. Many cows today weigh 1200 pounds or more (some a lot more). Therefore expect that most spring calving cows will need at least 24 gallons per day for themselves and another 5 to 10 gallons of water for their calf. Also recognize that some summer days in Oklahoma get even hotter than the 90 degrees used in the Georgia paper. On days with extreme heat, expect the water usage to go up even further.