Heat induces lethargy in dairy cows, which results in reduced feeding time and reduced dry matter (DM) intake. And with every 1kg reduction in DMI reducing milk yield by two litres, taking steps to counter heat stress pays its way.

Excess heat triggers a drop in milk yield and butterfat percentage and can cause infertility and early embryo death. Immune systems come under pressure, leading to mastitis and high cell counts. Last year also saw reports of IBR and BVD breakdowns, even in vaccinated herds, caused by high temperatures.

Severity of heat stress is quantified using a temperature humidity index (THI), where both ambient temperature and relative humidity are used to calculate stress level. Signs of heat stress become evident in dairy cows when the THI exceeds 72

When a humidity meter is unavailable, symptoms of heat stress would be visible when more than seven out of 10 cows have respiration rates of above 80 breaths a minute or when DMI and milk yield decrease by 10%.

Steps to reduce heat stress

  • Ensure plentiful supplies of cool, clean water are available, particularly immediately after milking. Ensure at least 0.61m (2ft) of trough space for each cow in a batch, and that water flow is sufficient. Minimum water depth of 8cm (3in) is required to allow for cows’ muzzles and it is vital that water troughs are never totally emptied. Careful siting of troughs is vital to stop cows having to walk across open fields in the hot sun.
  • Providing shade, particularly for high yielders and dry cows, which will have faster metabolisms and hence generate more heat, also helps. Keep cows out of the sun during peak heat periods in the day and allow them out at night.
  • Ensure there is adequate ventilation in the collecting yard and consider grouping cows in smaller numbers. Crowding cows in the collecting yard is similar to putting several large furnaces into a small area with their thermostats set to maximum. Try opening the sides of the collecting yard to allow adequate airflow ensuring that hot air can escape through roof outlets.
  • Likewise, cattle housing needs adequate ventilation and air movement, particularly when buffer feeding. Excessive cobwebs on the underside of roof sheets and the smell of stale air are both indications of poor air movement. When airflow is poor, consider removing part of the side panels of the shed, such as every third Yorkshire board, and removing the roof ridge to increase air exit points.
  • An alternative method would be to remove the side covering of the shed and replace with a movable curtain that can be raised or lowered depending on air movement and temperature.

Diet Considerations

  • Careful diet management can greatly affect heat stress. Reducing the forage:concentrate ratio increases the energy density of the ration and mitigates the effects of a reduction in DM intakes, alongside the option of adding fat to the diet. Potassium, sodium and magnesium levels in the diet should ideally be increased to counter loss of salts through sweating and increased breathing rates.
  • Furthermore, adding buffers such as bicarb, biosaf yeast or Acid Buff can minimise sub-acute rumen acidosis. Remember, feeds can heat up quickly, so feed little and often and consider using additives in mixes to prevent spoilage.

Fan Installation

  • When installing fans in the collecting yard, they should be positioned to direct air away from the parlour. If there is limited space, meaning fans cannot be placed above cows, locate them instead along the sidewalls to blow air laterally across the pen in the direction of the prevailing summer wind. Do not blow air from the collecting yard into the parlour, as this can be unsanitary and moves hot, humid air into the milking area.
  • A priority would be to install fans over the feed barrier, followed by the inner row of cubicles and last over the outer row of cubicles. Fans should be installed longitudinally down the shed with a spacing of no more than 10 times blade diameter. One-metre (3ft) fans should be spaced no more than 9m (30ft) apart, while 1.2m (4ft) fans should not be placed any more than 12m (40ft) apart.
  • Fans spaced more than 10 times their diameter apart lose their effective flow velocity and as a result all cows will not adequately be cooled. Fans should be tilted approximately 15-20deg down from the vertical so they are aimed at the bottom of the next fan down the line.
  • Using a sprinkler or misting system to provide heat stress relief is also effective, but should only be used in sheds with proper ventilation, as any extra moisture generated in the shed must be removed by air exchange in order to provide a quality environment.
  • After hair is soaked, water evaporates through the use of fans. It is critical that hair is soaked and not just covered with a light mist, as application of a light mist will result in the outer portion of the hair only to be covered creating an insulating layer, actually impeding heat loss.
  • However, it is critical to ensure that soaking is not to the extent that water runs down to the udder, potentially causing contamination and also resulting in water wastage.
  • For more information on managing heat stress, visit the Kite Consulting website (www.kiteconsulting.com)

PRIORITIES FOR REDUCING HEAT STRESS
  • Supply adequate clean, fresh water
  • Provide shade in housing areas and collecting yard
  • Reduce walking distance to the parlour
  • Reduce time spent waiting in the collecting yard
  • Improve collecting yard ventilation and then feeding area/cubicle shed ventilation
  • Add collecting yard cooling and parlour exit cooling
  • Cool pre-calvers and freshly calved cows