David L. Morris: Vet Column 1-10-11

David L. Morris, DVM, Ph.D.
Fort Collins, Colo.

Water is well understood as being a critical nutrient for livestock diets. Because of its relative availability and minimal cost compared to dietary protein sources and costs among other dietary considerations in many locations, water is often overlooked as a limiting factor in implementing a livestock production system. Notwithstanding water quality and sustainability issues, just how much water is needed for production needs? Investigators from the University of Nebraska and collaborators from a university in Chile recently assessed environmental factors that affect daily water intake on cattle finished in feedlots.

Ambient temperature has been found in numerous studies to be associated with daily water intake of cattle. During the summer seasons in the United States and Australia, excessive heat load creates negative effects on cattle health and performance. During heat waves, normal heat exchange is often impeded if adequate water is not available. When climatic factors, type of diet, breed, body weight, and physiological status are taken into account, determining daily water intake requirements can be difficult.

Studies of 47 yearling cattle in Oklahoma during the summer season on three differing salt levels in the diet reported an average daily water intake of 9.5 gallons. Another study of 120 yearling steers housed in a confinement barn averaged 9.8 gallons of water consumed per summer day. In yet another study involving 50,000 head of feedyard cattle in the Texas high plains, the daily water intake was 9.4 gallons. Neither of these studies reported the relative humidity nor the solar radiation impact. Relative humidity is known to affect the rate of evaporation from surfaces and, therefore, evaporative heat loss. Solar radiation has been found to influence body temperature and daily water intake.

Under cold conditions, dry matter intake of cattle generally increases and daily water intake decreases. When hotter conditions prevail, the opposite is generally true. Since dry matter intake is influenced by cattle type and body condition as well, predicting daily water intake cannot be solely related to ambient temperature.

Management of livestock, in general, and feedlot cattle in particular, is often guided by the Livestock Weather Safety Index during hot weather. An index is measured by which a threshold is reached at which animals begin to activate physiological mechanisms to cope with the extra heat load received. For this study, 67.2 was the threshold level.

Another measure or indicator used to predict heat stress is the daily minimum temperature. The minimum daily temperature reflects the ability of the animal to dissipate heat during the night. It has been reported that temperatures above 53.6 degrees Fahrenheit at night begin to lessen the ability of cattle to dissipate heat load through convection and conduction.

In summary, when cattle are subjected to heat stress, the consumption of water will be followed by a decrease in respiratory rate and skin temperature. Water supply system design and layout are important to minimizing heat effects on cattle. Monitoring minimum daily temperature and the temperature humidity index are useful in anticipating heat load stress and the resulting potential impact on daily water intake.