Our Farms Project Final Update - Heat stress and agroforestry for dairy welfare

Key results

• Heat stress occurs in cows at lower temperatures than farmers might expect, at 21 degrees and above. Monitoring their behaviour and physiological signs is an effective way to identify this.
• Grazing systems without adequate shade can incur milk yield losses of up to 180 l/cow over the summer. A financial impact of the milk yield losses of £77 alone. The financial impact would double when impact on fertility, lameness and dry cows is taken into account.
• Trees can provide very good protection and relief from heat stress. Mitigation tips:
  • Ensuring areas such as collecting yards, parlours and holding yards are shaded is crucial.
  • Consider housing and buffer feeding cows during the hottest part of the day and grazing in the morning and evening and provide ample water.
  • Initial results from intraruminal temperature sensors indicate a better cow focused way of assessing heat stress than external DHLI sensors.

Background

Heat stress in dairy cows is a growing problem and causes significant reductions in production, profitability and sustainability as well as being a significant welfare problem. Historically much of the heat stress research has been conducted with housed cows in hot areas (e.g. southern states of USA) with less research around grazed cows in temperate conditions such as those found in Wales. This project sought to assess heat stress risks in grazing conditions on 450 cross bred cows, autumn calving, summer grazing, 8100 l/cow/year herd, and to assess the value of trees in providing shade and shelter for grazing stock.

Purpose of the work

1. Assess the level of heat stress on a typical Welsh dairy farm
2. Quantify potential milk yield losses and financial impact from heat stress
3. Assess the impact and value of shade from in-field tree planting

What we did:

• 2 heat stress monitoring devices were installed in the cow sheds, 2 under the shade of a tree on the grazing platform, and 2 in full sun on the fence line. These measured humidity, temperature and radiant heat load measurements every 20 minutes using a LoRaWAN network. https://businesswales.gov.wales/farmingconnect/sites/farmingconnect/files/documents/Llyfryn%20Saesneg%20Lorawan%20A5_1.pdf
• Working with Dr Tom Chamberlain, standard heat stress indexes were calculated for the period 5 May to 1 Oct 2025 (150 days).
  • Temperature and Humidity Index (THI) was calculated for housed conditions using a standard USA equation.
  • Dairy Heat Load Index (DHLI) was calculated for grazing conditions using Queensland, Australia equation
• Impact of heat stress on milk production was calculated from heat index figures and cost of milk losses quantified per cow and for the herd
• All cows were fitted with Smaxtec rumen boluses that record rumen temperature and accelerometer data every 10 minutes
• Smaxtec rumen temperature data (corrected for drinking effects) were also extracted and assessed as an hourly average across 100 selected cows.

Outcomes:

• High levels of heat stress were observed on-farm under grazing conditions (Figure 1). Over the 150-day summer period heat stress was recorded on 30% of days in the full sun sensor and only 4% of days in the sensor under the shade of a tree. 

 Figure 1. Heat stress events in grazing conditions assessed by DHLI score

• Where cows are housed, the predicted potential milk yield losses were estimated at 50 l/cow over 150 days of summer period (Table 1). In un-shaded grazing conditions such as in paddocks with full sun, the milk losses equated to 179 l/cow, whilst in shaded conditions this was only 13 l/cow.

Table 1. Predicted milk yield losses and financial impact (at an average farmgate milk price of 43.12ppl)

For a herd of 450 cows the total milk losses are predicted to be worth £34,600 (calculated at a milk price of 43.12ppl) for full sun and this figure doubles when the impact of heat stress on animal health and welfare is taken into account compared to £2,438 where shade was available.

Analysis of temperature data from rumen boluses indicates potential technology advances in cow-based methods of assessing heat stress, as the data is showing long term changes in rumen temperature similar to the heat stress patterns (Figure 1 and figure 2). 
 

 Figure 2. Reticulum temperature (median of one hundred animals) over summer period.

How to apply on your farm

1.  Consider heat stress a growing risk to your business
2. Providing shade is a major mitigation
3. Ensure ample water supply
4. Where trees are available, protect and use them. Where possible plant and protect trees on as many field boundaries as possible
5. Ensure collecting and holding yards etc. are shaded.
6. Consider housing cows 12 noon until afternoon milking if cubicles are the best source of shade. Allow to graze in cooler parts of the day, and overnight.
7. Consider fitting fans in major pressure points such as collecting yards, parlour and holding areas. Simple water sprays can complement and improve the effect of fans.      
8. Consider feed additives and supplements to support the rumen function which can be compromised during bouts of heat stress.      

Please contact timtechnegolcff@mentera.cymru if you would like to receive a copy of the full final report for this project.