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Don't Think Weather Worries Are Over

20 May 2014

As the US waves goodbye to one of the toughest winters in memory, farmers are reminded that it is summer that can challenge cows most.

Kevin Spurlin, dairy extension expert with Virginia University has reminded dairymen that cows optimum temperature for cows is 59 degrees fahrenheit (15 celcius).

This figure applies to dry and lactating cow, writes Mr Spurlin.

Above this point, minor productivity loss may occur, but intervention is not justified. Once environmental conditions reach a threshold of both heat and humidity which overwhelms the cow’s ability to dissipate her internal heat production, intervention must occur.

"Dry cows, close-up heifers and calves are also affected."

Of course, this critical point is reached sooner for high producers than for lower producers and dry cows, writes Mr Psurlin.

University of Arizona data suggests that this critical point is when the minimum daily Temperature Humidity Index (THI) exceeds 65. Traditionally, a daily THI threshold of 72 was used to initiate supplemental cooling in order to maintain cow productivity and well being.

For information about how to offset heat stress in lactating cows, refer to the Dairy Pipeline article titled “Five Simple Tips to Reduce the Negative Impacts of Hot Weather on Dairy Cattle.” (July/August 2013).

While most dairymen appreciate the negative effects of hot weather on lactating cows such as yield losses, difficulty getting cows bred, and increased mastitis, lactating cows are not the only ones affected on the dairy. Dry cows, close-up heifers and calves are also affected.

Several research studies from University of Florida and Israel addressed the impact of heat stress during the dry and close-up period. Consistently, pre-partum cows and heifers receiving some form of supplemental cooling during times of heat stress:

  • Gave birth to larger calves;
  • Had higher IgG in colostrum;
  • Produced more colostrum;
  • Milked more in early lactation.

Calves from heat stressed cows were lighter and born earlier in gestation. A body weight difference was seen through weaning, but had been corrected by 7 months of age.

Calves also had greater failure in passive transfer of immunity due to both reduced immunoglobulins (IgG and IgA) from the colostrum, and less absorption even when fed similar levels of immunoglobulins. Calves born during heat stress, particularly if dams are not cooled, may need more attention during the summer to keep them healthy and growing.

Finally, as herd genetics and management continue to push productivity higher, remember that all the extra metabolic heat will continue to challenge our current environmental modification strategies.

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