Northeast Pasture Research and Extension Consortium
Ramada Conference Center and Inn
State College, Pennsylvania
February 1-2, 2011

Session 3
What Will Climate Change Mean to Grazing Animals?
Dr. Larry Chase, Professor of Animal Science-Nutrition from Cornell University

All animals have a zone of thermoneutral temperatures conducive to normal function. The upper critical temperature is where the effects of heat stress start to appear. Heat stress is when the heat load of cow is greater than the cow’s ability to dissipate heat. More generally, it is the inability to maintain /stabilize body temperature within very narrow limits. Without proper maintenance of body temperature, there is a drop in biochemical reactions and physiological processes, and metabolism is abnormal. The thermoneutral zone or comfort zone is the combination of temperature and humidity where dairy cattle are comfortable and not under heat or cold stress.

The temperature humidity index (THI) is a way to measure this combination of humidity and heat to alert one’s self to an unhealthy heat episode. It includes both temperature and humidity in determining the point at which heat stress may occur. A THI > 72 is considered to indicate heat stress in dairy cattle. The table below illustrates what temperature and humidity level equals 72. In actuality for higher producing cows, the THI value is lower based on some recent work at Arizona. This work indicates that a THI of 68 rather than 72 should be used for cows producing > 75-80 lbs of milk per day. The older data used to set the THI at 72 was from lower producing cows.

All of these = THI of 72


Temperature, º F Humidity, % 84 15 80 30 77 50 75 65 72 95


Heat exchange between a cow and its environment is accomplished by a number of phenomena. Heat dissipation occurs when the cow pants or sweats. The sweat evaporation causes cooling. Heat can also escape by means of convection (wind or fans). Sun radiation causes a gain in heat. Conduction with the ground can be either a gain or loss in heat from the cow.

Nonevaporative cooling makes up more than 80% of the heat dissipation in cows up to 40º F. It then declines quickly in effect thereafter as temperatures rise to 95º F, accounting for only 7% of the heat dissipation at that temperature. Evaporative cooling starts to kick in around 40o F, and at 95º F, it accounts for 70% of the heat dissipation. The other 23% of heat dissipation at 95º F is accomplished by evaporative cooling in the respiratory tract of the cow. This last source of cooling is at a very low level, less than 5%, until 75º F is reached and then it begins to become a more important means rapidly.

Once the THI reaches 68 or 72 depending on the cow’s milk production level, energy requirements for maintenance increase by 20-30%. Their respiration rate increases. More blood flows to the skin, thereby less blood flows to the body core. Energy requirements for maintenance at 104º F are 130% of the maintenance energy required at 68º F.

When dairy cows undergo heat stress, their dry matter intake (DMI) drops off quickly within 2 days and then declines more slowly if the heat continues over the next 7 days (Rhoads et al., 2007). In the Rhoads study, the DMI decline of heat stress was mimicked by underfeeding another set of cows. When these two herds were compared for loss of milk production, the heat stressed cows produced 45% less milk by day 7 than before they became stressed. The underfed cows’ milk production only declined 19%. Thus, feed intake only accounts for 50% of the reductions in milk yield observed in heat stressed cows.

A number of factors determine the severity of heat stress. They are:

  • Actual temperature and humidity
  • Length of the heat stress period
  • Degree of cooling that occurs at night
  • Ventilation and air flow
  • Cow factors (size, milk production)
  • Water availability for drinking, or for spraying or sprinkling on cow (or for even standing in)
  • Hair coat depth

The genetics of the dairy cow plays are large role on when heat stress becomes a factor and how severe it is on the individual animal. These traits are:

  • Range of -1 to +0.84 lbs milk per unit THI> 72
  • Top 100 bulls with resistance to heat stress - better pregnancy rates, lower milk
  • Slick hair gene = improved heat tolerance
  • Coat color: white better than black, light color better than dark color, darker coats absorb sun’s heat. Light color tends to reflect sun rays.
  • Will crossbreeding help?

Looking at breed differences:

  • Jerseys seem to tolerate heat stress better than Holsteins.
  • At the same THI, core body temperature will be 0.5 to 1.0 lower in a Jersey compared with Holsteins.

Heat stress effects on reproduction:

  • Usually decreases in heat stress situations
  • Estrus intensity and length decrease
  • Fertility rate decreases
  • Ovarian follicle growth decreases
  • Increase early embryonic death
  • May affect fetal growth

To minimize heat stress in cattle, there are 2 basic approaches: Adjust the feeding ration and adjust the environment in which the cow lives. Ration adjustment to keep the cows healthy are feeding higher quality forages and more digestible feed ingredients (lower heat production during digestion) and consider added fat. It is a concentrated energy source and has a low heat of digestion. Rumen modifiers might also be considered.

Drinking water is extremely important in times of heat stress. Intake may increase by 20 to >50% during heat stress. Clean, fresh water must always be available close at hand. This is done by having adequate watering devices or space (minimum of 2-3 inches per cow) on hand, making sure pressure is adequate to refill waterers (quickly for light plastic tubs), and providing more water sources in the pasture if travel distances or sight-distance issues arise.

Other potential adjustments include providing portable shades or sprinklers in the pasture and grazing cows only at night and keep them in the barn during the day. If kept in the barn during the day, the barn will need cooling equipment, such as fans and sprinklers.

There are benefits of cooling dry cows during hot, humid spells. Five to six studies have examined this issue. Increased milk in the next lactation ranged from +3 to +16 lbs/cow/day (most were in the 3 – 7 lb. range), so it is very important to keep dry cows cool as well.

Forages fed to cows also are stressed by heat. Generally, forages produced in hotter conditions have more lignin and a lower digestibility. First cutting hay feeds better than second cut hay in the Northeast since it is produced in cooler weather. We may need to consider shifting pasture forage species to more warm weather type grasses (Ed. note: Or, at least use more drought tolerant cool season forages and forage varieties able to withstand diseases more prevalent in hot, humid weather).

In summary, as we get more days of hot weather and/or more prolonged periods, we can expect less milk production and growth in animals. We can make some adjustments in rations and animal management strategies. However, these will probably not be adequate to totally counteract the detrimental effects of heat stress. It will most likely take some additional changes to assist in minimizing the effects of heat stress, such as changes in cattle breeds used, genetic selection within breeds or cross-breeding, coat color of cow, the animal’s environment, times of day for grazing to the cooler periods, and forage types and rotations.