05 September 2025
New data shows the impact of heat stress induced by an absence of shade during hot weather on dairy cows, with a milk yield loss of 117.3kg/cow estimated over an 87-day period compared to just 9.1kg/cow in animals sheltered from the sun.
The Evans family, who produce milk from a herd of 470 Holstein/Norwegian Red/Friesian crossbreds at Kilford, near Denbigh, have been working with Farming Connect to investigate the value of trees and hedges as shade and shelter for their stock.
From February onwards, they rotationally graze the herd on the 330-hectare holding.
The effects of rising temperatures on dairy cows are being monitored as part of an Our Farms study and the first set of data points to a significant loss in milk production if the herd had been exposed to full sun on the hottest days of the summer.
Using a combination of SmaXtec rumen boluses to give real-time insights into cow drinking, rumination, activity, and internal heat stress levels, and sensors measuring temperature and temperature humidity index (THI), the research focused on the 87 days from 5 May – 27 July 2025 when there were frequent periods of intense heat.
On one day, 12 July, the temperature sensor showed that in areas where there was no shelter on the grazing platform the radiant heat was 41.6°C - heat that a black-coated animal will feel on her back when stood in full sun with no shade - and a THI of 32.1°C.
In areas shaded by trees, the radiant heat temperature was significantly lower at 29.8°C and the THI reading was 30.2°C.
Measurements were also taken in the farm’s cubicle buildings where the radiant heat temperature on 12 July was 31.3°C.
Vet and consultant Dr Tom Chamberlain, who analysed this data, calculated a possible milk yield loss of 117.3kg/cow if the herd was at grass under full sun for those 87 days compared to 9.1kg/cow if they were able to access full outdoor shade.
This data is for a cow producing a daily milk yield average of 30kg.
Based on that production level, the potential milk losses at the temperatures recorded at Kilford showed a range of 0-2 litres/cow/day from moderate heat stress, rising to 2-4 litres in a cow experiencing high heat stress, and 4-6 litres in those with severe heat stress.
Gwenan Evans, Dairy Sector Officer at Farming Connect, who is overseeing the project, says this data shows the challenges heat stress presents to the dairy industry.
“Despite the potential of heat stress to significantly impact cow welfare and profitability, it's a topic that has been surprisingly under-researched in Wales,’’ she says.
“With summer temperatures now frequently climbing above 21°C – a threshold for heat stress – and many farms in Wales having open grazing systems, we suspect this could be a major hidden issue.’’
Not only does heat stress decrease milk yield but it results in poorer fertility and reduced lying times having a detrimental impact on cow foot health, resulting in increased lameness.
Integrating trees into livestock grazing systems offers a solution.
Farming Connect Woodland Specialist Geraint Jones describes it as a “powerful, nature-based solution that significantly reduces heat stress in cattle’’.
“As our climate changes, strategic tree establishment becomes a critical tool for farmers,’’ he advises.
Trees create essential microclimates by providing shade and increasing humidity, which directly benefits livestock by lowering air temperature and reducing heat stress.
This combination is highly effective in mitigating the impact of extreme heat on cattle, Geraint adds.
“Providing an environment where cows are as comfortable as possible is paramount, and trees can be a key factor in creating this environment.
“Over the next few years, establishing trees will be crucial for building farm resilience, safeguarding animal welfare, and providing long-term economic benefits in the face of a warming climate.’’
Trees release water vapor through transpiration, increasing local humidity. While people often associate high humidity with feeling hotter, Geraint says it's more complex for animals.
“When this humid air is blown over a hot animal, the animal's sweat or panting evaporates more effectively due to the cooler, more humid air, which helps cool them down.
“Shade provided by trees is the most significant factor, but the increased humidity from transpiration is a contributing factor to the cooler microclimate that helps livestock cope with heat stress.’’
At Kilford, the Our Farms project work will now identify opportunities for integrating trees into the farm’s grazing paddocks, while monitoring temperatures and their impact on cows will continue until the end of September.
The goal is to develop a mutually beneficial agroforestry system that maintains or improves grass productivity while enhancing ecosystem services, says Geraint, ultimately delivering both environmental and economic advantages to the farm.
Integrating trees and hedges into livestock systems
Shade is paramount
Integrating trees into grazing systems provides livestock with significant shade and a natural shield against solar radiation.
For grazing systems, the focus should be on planting trees on the grazing platform, Geraint advises.
Utilising winter housing to provide shade when it is needed is a good practical option too.
Select trees that offer the best potential for shade
Trees with wide canopies and dense foliage offer the most effective solution for providing substantial shade, therefore maximising their role in reducing heat stress risk to livestock.
Planting a diversity of tree species within a hedgerow or a shelterbelt, those with different flowering and fruiting times for example, supports a greater level of biodiversity too.
Plant trees where they have the greatest impact
The level of heat stress mitigation trees offer is influenced by the local microclimate specific to every farm.
Strategic tree planting in open, sun-exposed areas will inevitably yield the greatest benefits.
Climate change resilience
Geraint says the research at Kilford emphasises the critical role trees play in enhancing the resilience of farms to climate change.
“As temperatures continue to rise, the importance of tree cover for livestock welfare will only grow,’’ he points out.
Value existing trees
Existing tree cover provides cattle and sheep with some protection against the adverse impacts of a changing climate.
However, the amount of shade provided by trees already established on a farm can often be limited, especially in systems where livestock numbers have substantially increased in recent years, Geraint suggests.
As well as conserving those trees, measures to enhance cover should therefore be a priority, he adds.
“Approaching tree planting with a long-term vision is essential to ensure it provides shelter and shade for animals for future decades.’’
Tree establishment
Planting new trees offers a practical and natural capital-based approach to climate change mitigation.
“Tree planting is a sound investment in the long-term health and productivity of livestock,’’ says Geraint.
Farming Connect is running a series of farm walks in September, where farmers will hear more about the projects that are underway on farms in the Our Farms network.
Details can be found at https://businesswales.gov.wales/farmingconnect/whats-on