Managing in the heat

Review and re-plan

1. What impacts did excessive heat load events have?

• Look back through your records. How many times did the THI get over 72?
• How did your cows cope furing these periods? (i.e. how many times did their breathing rate exceed 60 per minute)?
• What impacts on milk production, milk composition and reproductive performance did you see?
• How did your cows respond to your management strategies during excessive hear load events?

Assessing the impact on milk production

There are two sources of data readily available which you can use to look back at the impacts of heat stress on your herd's performance over the past hot season:

• Daily (or skip-a-day) milk production data from your dairy company. Most dairy processors provide historic milk production for any given farm via an intranet system
• Daily weather data from the Bureau of Meteorology. The Bureau of Meteorology provides daily weather data for the past 12 months from a comprehensive list of sites across Australia.

The following graphs show the Temperature Humidity Index (THI) at a location in norther NSW during the spring of 2007 against the average milk protein concentration and the milk yield per cow on two nearby farms. Both of these farms calve all-year round with a seasonal bias towards winter/early spring. Spring is a time of the year when herds in this region are susceptible to heat stress as cows are unadjusted to rapid increases in temperature and humidity which can often occur at this time of the year. Herds are also on a high plane of nutrition during this season and consequently are generating considerable metabolic heat.

While there is a lot of "noise" in the milk production data caused by varying planes of nutrition and days in milk, the graph shows two periods during spring when the maximum daily THI exceeded 78 which corresponded with significant falls in milk protein and milk yield.

Such graphs also allow an estimation of the immediate milk production cost of heat stress. On these two farms, milk yieild dropped about three litres per cow per day in the first period (14 days) and 1.5 litres per cow in the second period (10 days). At a milk price of 45¢ per litre, this equates to a loss of $19 per cow over these two periods combined. Similarly milk protein drops 0.15% units during each heat stress period. This cost equates to around 1¢ a litre - a further $9 per cow loss over this period.

Assessing the impact on reproductive performance

You can also look back at the impacts of heat stress on your herd's reproductive performance over the past 12 months using your herd reproductiong records.

Some things to look at are:

• Submission rates – what percentage of cows were inseminated during these hot periods versus cooler periods?
• Conception rates – of the cows that were inseminated, what percentage of these resulted in a pregnancy?
  • If you are a year-round calver, compare conception rates between inseminations completed in summer/autumn and winter/spring
  • If you are a split-calver, compare conception rates of cows calving just before or during summer months to that of cows calving in autumn/winter months
  • A difference in conception rate of more than 10 - 12% indicates poorer performance in hot months if at least 100 inseminations were performed in each period.
  • Be sure to rule out other causes of poorer performance between seasons, such as changed AI practices or differences in feed quality
  • As seasonal calving herds do not have two seasons to compare, it is difficult to reliably measure the effects of heat stress in these herds
• Calf size and viability:
  • Were calves born over the next few months lighter than usual?
  • Was there greater calf mortality during this period?

Dairy Australia’s InCalf program can provide tools to assist with these analyses (InCalf Fertility Focus Report and Herd Assessment Pack). You can also access InCalf resources and tools via the Dairy Australia website.