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Published: 12 Apr 2019

Discussion Group 1

Symon Jones, TIA

Irrigation management and heat stress impact on cows have been two of the topics discussed at discussion group meetings in January and February.

Irrigation management

January 2019 was Tasmania’s warmest and driest on record ( This resulted in high evapotranspiration rates, sometimes exceeding 7 mm/day, which really tested farmers’ irrigation system capacity.

David McLaren from TIA spoke to discussion group attendees about how to irrigate effectively and avoid the ‘green drought’. This is when not enough water is being applied to keep-up with evapotranspiration rates. When this happens, pasture remains green but with very low pasture growth rates. This is costly to a farm business.

David also spoke about the irrigation specific discussion group established in the Meander district as part of the new Beyond Water Smart project (funded by Dairy Australia). This is a pilot discussion group this season but there are plans to form similar groups in other regions in the next irrigation season.

If you are interested in being part of an irrigation-specific discussion group, contact Sam Flight on 0409 801 341 or Symon Jones on 0418 876 089.

One of the activities undertaken by the Meander irrigation discussion group was the uniformity testing of their irrigation systems. This is where catch cans are used to check that sprinklers are applying the correct amount of water. The results of the tests were presented at the discussion group meeting as a uniformity test graph. The red line on the graph (below) is the amount of water the irrigator was set to apply, in this case, 8 mm. The blue dots show the amount of water collected in the catch cans, which were placed along the length of the pivot (the left side of the graph is closest to the centre). The graph shows a few of the catch cans received more than the target amount but most were below target. Of particular concern is that the amount of water applied decreases towards the end of the pivot (the right of the graph). This end of the pivot irrigates the largest area, so under-application of water by these outer spans will have a large impact on pasture growth.  

Uniformity graph

The group discussed the factors that affect uniformity – water flow, water pressure and sprinkler spacing – and how to check these.

Heat stress

The hottest day in Tasmania this year was January 25 when it reached 40.1°C (in Hobart). While that is a hot day for humans, did you know cows feel hot before we do? The comfortable temperature range for a cow is 4-20°C, 10 -15 degrees lower than the human comfort zone.

A cow needs to maintain her core body temperature between 38.6 and 39.3°C.

A cow must ensure it stays within its optimal temperature range through thermo-regulation or evaporative cooling. This means balancing the metabolic and the absorbed environmental heat using a range of strategies such as increased breathing rate and sweating. This is why during summer months cows’ tongues may be hanging out of their mouths. This is an attempt to increase the volume of air passing through the airways, maximizing the exchange of heat with the environment.

How do cows handle hot days?

Cows only perspire at 10% of the rate that we do and mainly sweat through their noses. They further dissipate heat through:

  • breathing
  • transferring heat through physical contact
  • standing up to increase airflow either in the shade or water or where there is a breeze - cows rarely lie down when heat stressed
  • positioning themselves away from the sun if there is no shade
  • seeking shade from other cows by dropping their head under other cows

A cow’s core body temperature will be increase throughout the day however she can generally cope by dispersing heat during the night, if night time temperatures are low enough.

Friesians will start to feel the effect of heat stress when temperatures are greater than 21°C and relative humidity is greater than 70%. Jerseys are more tolerant of heat – they don’t tend to suffer from heat stress until the temperature increases above 25°C.

Feed intake will decline by around 10–20% when the air temperature is more than 26°C. Twenty to thirty percent more energy can be required for maintenance to compensate for cooling requirements. Blood hormone concentrations also change and blood flow distribution to the gut and uterus is also altered. This will result in a decrease in milk production of 10-25% during the heat stress period.

Conditions that increase the risk of heat stress

Impact on cows

High daytime temperature – two or more days

Back to back hot days mean heat load accumulates, gradually rising each day of the heat wave

High overnight temperatures

Limits the amount of heat a cow can off load overnight

High relative humidity

Limits the effectiveness of a cows evaporative cooling

Recent rainfall

Can increase humidity

Little or no cloud cover

Increases the amount of solar radiation a cow is exposed to during the day

Little or no air movement

Limits the effectiveness of evaporative cooling

A sudden change from cool, mild weather to hot conditions.

If this occurs in late spring/early summer cows with no previous exposure to heat can be vulnerable

Strategies to keep cows cool

Discussion group participants discussed ways to keep cows cool during the high temperature period. These included:

  1. Ensure cows have access to plenty of cool clean water. A cow requires 30 -40 litres of water per day plus 4 litres for every litre of milk produced. E.g. a cow producing 20 litres requires 40 litres for maintenance plus 80 litres for milk production (20 x 4 litres = 80 litres). This is a total of 120 litres per day. Water requirements can increase by more than 50% in very hot conditions.
  2. Offer the best feed in the early morning and night when the temperatures are cooler.
  3. Keep cows closer to the dairy during the day. Reduce walking distance and speed.
  4. Provide shade, if available, through the day.
  5. Provide sprinklers in the cow yard.
  6. Milk later in the afternoon, when it is cooler.

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