It’s easy to quantify the effect of heat stress on milking cows, but how can we really know its effect on dry cows? Does it even have an effect?
During the dry period, the udder cells are recovering from the previous lactation and preparing for the next. This is the time to replace the old milk secretory cells and grow new cells that will support milk production in the next lactation. Heat stress during this important time of cell turnover can dramatically impact transition into the subsequent lactation.
For instance, researchers at the University of Florida evaluated the effects of heat stress on dry cows. At dry-off, cows were moved to a freestall barn and exposed to either heat stress or cooling with fans and sprinklers. Both treatments received the same dry cow ration from dry-off until calving.
After calving, the animals from both treatments were moved to a sand-bedded freestall with fans and sprinklers that turned on automatically whenever the ambient temperature exceeded 70ºF. It’s important to note the cows were heat stressed only during the dry period and the milking cow ration was similar for both groups. The experiment was repeated for three consecutive years.
Heat stress and milk production
Cows exposed to heat stress tended to have a dry period that was seven days shorter than those cooled (38 days versus 45 days, respectively).
Bodyweight was 28.7 pounds lighter for calves born from cows under heat stress compared with those born from cooled cows (68.3 versus 97 pounds, respectively).
Rectal temperature on heat-stressed cows was higher than cooled cows (102.5º versus 101.3ºF, respectively).
Milk production up to 30 weeks of lactation was greater for cows cooled during the dry period compared with those that were heat stressed. The average increase of 3.5 percent fat-corrected milk (FCM) in the cooled cows was 18.7 pounds of milk per cow per day compared to those that were heat stressed.
Remember that the cows were heat stressed only during the dry period. The magnitude of response in the subsequent lactation is related to heat stress load (how hot it is) and intensity (duration of the heat).
For example, last year we experienced temperatures above 90ºF (heat stress load) in the upper Midwest for several consecutive days (intensity). This would have bigger impact on milk production than a few single hot days spread throughout the summer.
Heat stress and immune function
This second experiment was similar in design to the previous experiment and evaluated immune function in addition to milk production and components.
Rectal temperature was increased in heat-stressed cows compared with those cooled (102.4º versus 101.8ºF, respectively).
In addition, heat-stressed cows had greater respiration rates (78 versus 56 breaths per minute, respectively), shorter dry period (39 versus 46 days, respectively) and lighter calves (87 versus 98 pounds, respectively).
Neutrophils, specialized immune cells responsible for fighting infection, did not function well in heat-stressed cows. The Florida researchers measured the ability of the neutrophils isolated from heat-stressed or cooled cows to “eat” bacteria (neutrophil phagocytosis) and “kill” bacteria (neutrophil oxidative burst).
Cows exposed to heat stress during the dry period had reduced ability to “kill” bacteria 20 days in milk compared to those cooled. In addition, the ability of neutrophils to “kill” bacteria was reduced in heat-stressed cows at two and 20 days in milk. That means the heat stress imposed only during the dry period carried over during the lactation and compromised the animal’s ability to fight infection.
Another important part of the immune function is called humoral immunity. Through this process, specialized cells called B lymphocytes are stimulated by antigen (foreign molecules entering the body). The B lymphocytes secrete antibodies to “fight against” the antigen.
In this experiment, heat-stressed cows produced less antibodies against antigen during the dry period compared to those that were cooled.
The average increase in 3.5 percent FCM in the subsequent lactation was 10.3 pounds per cow per day in cows cooled during the dry period compared to those that were heat stressed.
Heat stress and mammary cell proliferation
The third-year experiment was similar to the previous two in design and evaluated mammary gland cell proliferation in addition to milk production.
Cows exposed to heat stress during the dry period produced on average 11 pounds less milk than those cooled.
To understand the reason for the low milk production in heat-stressed cows, this study evaluated the mammary gland cell proliferation of cooled or heat-stressed cows. Heat stress decreased mammary epithelial cell proliferation (creation of new cells), which may be a contributing factor to the low milk yield.
The value of cooling dry cows
Heat stress abatement in dry cows has shown to improve transition into lactation. With better mammary gland cell proliferation in cows cooled during the dry period, milk production is increased in the subsequent lactation. The magnitude of milk production increase in the next lactation is correlated with the heat stress load and intensity during the dry period.
Furthermore, heat stress abatement for dry cows improves immune function during and after the dry period. Although it can mean a significant investment in equipment, minimizing heat stress in dry cows is a valuable management tool to improve performance in the next lactation. PD
Amaral has a Ph.D. in dairy nutrition from the University of Florida, where he also conducted post-doctorate research in dairy physiology and management.