Why Study Heat Stress in Farm Animals?
Farm animals have well known zones of thermal comfort (ZTC). The range of ZTC is primarily dependent on the species, the physiological status of the animals, the relative humidity and velocity of ambient air, and the degree of solar radiation. Economic losses are incurred by the U.S. livestock industries because farm animals are raised in locations and/or seasons where temperature conditions venture outside the ZTC. The objective of this presentation is to provide current estimates of the economic losses sustained by major U.S. livestock industries from thermal stress and to outline future challenges as animal productivity is improved. Species (production) considered are: chicken (meat), chicken (eggs), turkey (meat), cattle (meat), cattle (milk), and pig (meat).
What Did We Do?
Financial losses are the summation of:
- decreased performance (growth, lactation, egg production),
- change in feed intake,
- increased mortality, and
- decreased reproduction.
USDA and industry data were used to estimate the population size of each species in each month of the year, for each of the 50 states. Weather data from the National Weather Service from 270 stations over a minimum of 70 years were used to estimate mean daily maximum and minimum temperatures and relative humidity, and their variances.
A model based on an abrupt threshold and linear decrease in performance and reproduction and a linear increase in mortality above and below the ZTC is used for each species. Solar radiation and air velocity are assumed negligible. Probabilities of exceeding minimum or maximum values of ZTC are calculated from means and variances of weather data in each of the 50 states. Four scenarios of losses are estimated.
Total potential losses are calculated as if no thermal stress abatement strategies were used by any of the animal industries. This estimate is biased upwards, as most animal production systems uses some form of active cooling, but it is used to set a ceiling to the magnitude of the actual losses. For each species, losses are also calculated under minimum cooling (fans), intermediate cooling (fans and sprinklers), and extensive cooling (evaporative cooling). For each state and for each species the optimal strategy is the one that among the four results in the minimum financial losses under Monte Carlo simulations (1000 year replicates).
What Did We Learn?
At current production levels, the optimum cooling system varies considerably across species and states. Nationally, heat stress results in total economic losses ranging between $1.9 and $2.7 billions per year. Although projected increases in ambient temperatures (+ 1.2 to 1.3 °F by 2050) will result in additional financial losses, the additional metabolic heat resulting from the projected increase in animal productivity will have far greater impact, between 2 and 4 times as much as global warming. Considering that all moderate to intensive animal cooling system currently in use require substantial amounts of water and are approaching their maximum cooling capacity, technical innovations that will be both water and energy efficient will be badly needed before 2050.
Normand R. St-Pierre, The Ohio State University, Columbus, OH-43201
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