Heat stress in dairy cows is a common problem, even in moderate climates. Due to global warming, it is a growing problem[104]. Milk production[74], and fertility[67,68] in dairy cattle will be negatively affected by heat stress and the incidence of lameness and mastitis will increase[75,80]. Heat stress can be an important potential cause of systemic immune activation[69], which in term can result in involuntary culling of dairy cows[4].

A common way to quantify the risk of heat stress is the Temperature Humidity Index (THI), an index which combined environmental temperature and humidity. Threshold values are often based on studies working with cows that are acclimatized to high environmental temperatures and humidity. In those parts of the world, the risk that cows suffer from heat stress starts to occur at THI level of about 72[107] (see Figure 2). Cows in climate zones in which heat stress occurs regularly will adapt and as a result, threshold values will be much lower, can already show signs of heat stress at a THI of 62[74,78]. The THI is based on environmental temperature and humidity. It does not take into account the level of radiation. Therefore, the THI is suitable to quantify the risk of heat stress in housed dairy cattle but has its limitations in grazing cows that are exposed to direct sunlight.

With the support of Trouw Nutrition, Dr. Tom Chamberlain, veterinarian and owner of Chalcombe Consultancy, is running a project “Raising the awareness of heat stress problems with dairy farmers”. The project has a number of important targets. The first one is to increase awareness that heat stress can also affect cattle in temperate climate zones such as North West Europe and the coastal areas of Latin America and North America. Next to that, a target is to monitor the risk for heat stress with a parameter that is more suitable for grazing cattle, the so-called Dairy Heat Load Index (DHLI).

Dr. Chamberlain comments: “In grazing situations it is preferrable to work with the Dairy Heat Load Index, which is based upon recordings of humidity and temperature in a so called “black globe”. This black globe is placed where the cattle are grazing, giving you an accurate measurement of the conditions a cow really has to face. We now have real-time data for THI and DHLI available from seven farms in the South-West of England (See Figure 2). The results are available in real-time through our webpage.”

The third target of the project is to provide farmers with a better tool to help them with assessing heat stress in their herds. Both THI and DHLI are environment and not animal based. As such, they indicate to what extend animals are at risk, but they do not measure the effect of heat stress in animals. The project will develop a metric based on animal measurements. Dr Chamberlain continues: “Previous research workers have developed animal-based metrics, but these have been too invasive or expensive for commercial use. Our intention is to develop a metric that is simple to apply on commercial dairy farms.”