Expert Answers - April 18, 2008

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Q: What changes should I make in my feeding program to reduce the negative effect of heat stress on milk yield and composition?

A: The lactating dairy cow experiences heat stress when the combined temperature and relative humidity exceed 72. The effects are more pronounced at night when the relative humidity increases and temperatures remain above 70 degrees F and the cow does not have an opportunity to cool off. In response to heat stress, body temperature increases and the cow pants in an attempt to cool herself. Depending on the severity of heat stress, panting may increase two- to three-fold, resulting in more carbon dioxide exhaled than produced, reducing bicarbonate available for buffering the blood or recycling into the rumen to maintain buffering capacity. Lower buffering contributes to lower ruminal pH and fermentation efficiency, which contributes to lower milkfat and protein concentrations in milk. As respiration increases, additional energy is required to meet maintenance requirements, reducing energy available for milk production. As body temperature increases, dry matter intake decreases, which further reduces energy available for milk production.

  • Assuming that adequate supplemental cooling is provided, the following changes in rations should be considered to maintain energy intake in support of milk production and prevent sub-ruminal acidosis.
  • Maintain dietary NDF concentrations to maintain ruminal buffering. High-quality forage that is free of molds will help maintain intake and maintain ruminal buffering.
  • Feed a combination of non-protected and ruminally protected fats to increase dietary energy density and maintain fiber digestibility. Feeding additional fermentable grain decreases ruminal pH and may actually increase rather than decrease heat load, especially if dietary fiber is lower than recommended by NRC.
  • Avoid feeding excess protein, especially degradable protein, which produces additional heat and requires energy to convert the excess ammonia to urea for excretion.
  • Use potassium carbonate to increase K to 1.5 percent to 1.6 percent of dry matter, sodium bicarbonate or sesquicarbonate to increase Na to 0.45 percent to 0.60 percent of DM, and magnesium oxide to increase Mg to 0.35 percent to 0.40 percent of DM. These adjustments help compensate for additional losses through panting and sweating.
  • Supplemental yeast culture or fungal additives have been shown to maintain or improve fiber digestibility and improve the efficiency of nitrogen utilization which contributed to improved milk yield and composition.
  • Provide easy access to plenty of fresh, clean water. Water intake increases 30 percent or more because of increased sweating and respiration. Adjust floats if necessary to keep water from heating up and clean tanks to prevent algae growth.


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