How are your cows feeling?. . . . How are they milking?

By Dr. Jim McFarlane, Animal Environment Specialists, Inc.

For as laid-back as dairy cattle are, it doesn’t take much to get them stressed—heat stressed that is. 

     Lactating dairy cows produce metabolic heat at roughly the same rate as sixteen 100-watt light bulbs.  That’s a lot of heat and it needs to be dissipated if internal body temperature is to remain around the normal 101.5 F.  The warmer the surrounding environment, the harder it is to lose that heat and the greater a cow’s thermal discomfort.  The more uncomfortable she feels, the more apt she is to invoke coping mechanisms to help “keep her cool”.  Of greatest concern to dairy producers is her tendency to reduce feed intake, since lower milk yield is a likely result.

     Dry matter intake (DMI) decreases by about 0.17 lb for every 1 degree above rise in environmental temperature over 68 F--and every 1 lb drop in DMI generally translates into a 2-lb loss in milk yield ((NRC, (1).  Other research indicates that milk yield will drop about 0.6 lb/day for every one unit rise in THI over 70 ((Johnson, 1963) (2).

     Milk yield in severely heat stressed cows may drop by as much as 25 to 30%.

     Heat stress seriously impacts reproduction, too.  One Florida study demonstrated that conception rate dropped from about 52% during cool months to 30% for the hot months of June through September.  Conception rate in commercial dairies has been known to decline to less than 10% during hot weather.  Even when conception is successful, heat stress can reduce calf birth weight, and reduce milk yield during the subsequent lactation (3). 

     Further, heat stress compromises cows’ immune function and can cause a drop in milk fat content (1).

.....Figure 1.  Temperature Humidity Index (THI) for dairy cows.  Modified from Dr. Frank Wiersma (1990), Dept. Agricultural Engineering, University of Arizona, Tucson

     Monitoring THI around the clock gives you a much more accurate picture of when, how long, and to what degree your cows are heat-stressed.  More on data logging THI

     Information such as the above is useful to dairy managers in a couple of ways: (1) a guide for determining how cost-effective cooling systems such as stir fans can be, and (2) deciding the best temperature at which stir fans should come on.     

     Dairy Specialists at Mississippi State University suggest these nine points (8):

     A carry-over from the poultry and pork industry, tunnel ventilation is proving its worth in dairy units, too.  Excellent information on the principles of tunnel ventilation, economics, and design considerations may be found at the Cornell University Pro-Dairy website (9) .  

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     New to the dairy market in the last couple of years are the high volume low speed (HVLS) fans that put a lot of air across cows for considerably less electricity than traditional stir fans. 

     Commonly referred to as “big ass fans” HVLS paddle fans have 10 and measure up to 24 feet in diameter.  They are driven by a 3/4 to 1 HP motor at an rpm as low as 50.

     A recent Wisconsin field study (10) indicates that dairies are generally placing 24-ft diameter HVLS fans in a single row over the center drive lane in 4-row or 6-row free stall barns.  In 3-row barns, the fans are placed over the centerline of the free stall platforms.  Fans within a row are spaced 60 to 70 feet apart o.c. and are mounted at a height of 16 to 18 ft off the floor.    

     The effective stall coverage appears to be 45 stalls/fan in a 3-row barn, 53 stalls/fan in a 4-row, and 88 stalls/fan in a 6-row barn.  The fan systems, installed as described, put approximately 200-300 fpm airspeed over the feed bunk in a 4 or 6 row barn, with a low-end velocity of slightly less than 100 fpm at the outer row of free stalls. 

     The 24 ft diameter fans cost approximately $4000 to $5000 each, including controls and installation labor.  Based on an electricity charge of $.07/kWh, the cost to continuously operate a 24 ft HVLS fan is approximately $1/fan/day, compared to nearly $18/day for operating a bank of traditional stir fans moving the same volume of air.   

     Fog or mist systems (having very fine sprays) are the evaporative cooling method of choice in the air southwest United States.  Such systems cool the air, which in turn cools the cows—unlike sprinkler systems (generally used in more humid climates) that directly wet and cool the cows’ hides (11).  

     Results of a recent Ohio State University study suggest that, based on net return, a combination of fan and sprinkler set-up is the best cooling system for producers across most of the United States (12).  

     Several companies offer stir fans that oscillate in order to increase the effective cooling area.  One of note (13) uses a special controller to program the movement of a bank of stir fans to track along with the movement of shaded areas as their location changes with sun position over the course of a day.  In this way, cows are always befitting from the combined cooling effect of the shade and the air movement.   

     Fog or mist systems are often combined with stirring fans in order to increase the efficiency of the evaporative process.  Combination fog/stir-fan systems are available from several manufacturers.