We've just had a paper published in ERL on heat stress in UK dairy cattle and the effect this has on milk yields. This was initially a short piece of work which at the time was the first application of HadISD to a specific project. However the project grew and took longer than anticipated, and hence has only been published now.
(UK) dairy cattle and heat stress
Many animals are affected by high temperatures and humidities. Think about a hot, humid day, personally I wouldn't have much energy and would prefer just lazing in the shade of a tree. If, for example, I went for a run, I would get pretty warm, which may cause heat-stroke if I went for too long. And if it is humid overnight, I don't sleep as well as normal either, which puts my body under more stress. Cattle aren't often seen going for runs, but if they cannot cool themselves, they also get heat-stressed. This means they divert energy from growing and producing milk to cooling down, and so you can measure the effect of the warm temperatures in the milk yields, which is handy as asking them how they feel is tricky!
We used a measure for heat stress called the "Temperature-Humidity Index" (THI) which combines the air temperature and the relative humidity. Both of these are available from data in 68 of the HadISD stations used in this study. When the THI rises above 70, then cattle experience heat-stress, and if it rises over ~90, then the heat stress is very severe and could be fatal. We looked at the daily average THI, and across the UK, for most of the stations, this threshold of THI > 70 is only crossed in one or two days per year (Fig. 1).
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| Fig 1. Top: The average number of days (over 1973-2012) with THI > 70 for the 68 UK stations. Bottom: The number of days with THI > 70 in 2003. Stations with no days where THI > 70 are shown with grey squares. Note change in colour scale between the two panels. |
The stations inside the M25 (the motorway around London) have more days, but we suspect that there are few cattle being raised that close to London. Looking at the years of 2003 and 2006, which had very warm summers, there were 5-10 days where cattle could be heat-stressed (Fig. 1). Using data provided by the Cattle Information Service we were able to look at data for milk yields on an animal-by-animal basis for a number of herds in the areas most likely to have been strongly affected by the high temperatures.
Factors affecting Milk Yields
There are a number of complicating factors when looking at milk yields from dairy cattle. The amount of milk a cow produces depends on both the number of calves she has had, and also the number of days that have elapsed since her most recent calf was born (Fig 2.)
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| Fig.2 Top: Yield vs days in milk, Bottom: Yield vs lactation number for herd 198 in Devon. Blue points show individual measurements and the red ones show the average (over a 10 day bin or lactation number) with a 1-sigma uncertainty. |
By looking at data on an animal-by-animal basis we were able to select cows in their first lactation and use ranges of 50 days since the birth of their calf to try and reduce the variation from these additional effects. Fig. 3 shows the change in the milk yields for one herd over the entire record. There is a clear decline in milk yields in 2006 for all ranges of days-in-milk. The yield drops from 30 litres to between 15 and 20 litres. There are few cattle contributing prior to 2004, resulting in a noisy curve and no clear indication of an effect in 2003. Combined with the effect of heat-stress is also the reduction in pasture quality for grass fed cattle as the fields tend to dry out during hot weather. However the feed information was not available
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| Fig. 3 Milk yields for one-calf cows in herd 4199 in Somerset. The bands are the 1-sigma ranges for the three ranges of days-in-milk. The dashed lines show the numbers of cows contributing to the mean milk yields. The magenta line shows the Somerset average monthly temperature (Perry & Hollis, 2005). The vertical dashed lines show the heatwaves of 2003 and 2006. |
Climate Projections from UKCP09
To study the effect of any future change in the climate on the number of days with high THI we used the projections from the UK Climate Prediction '09 assessment (UKCP09, Murphy et al 2009). This is an 11-member ensemble of regional climate models runs on a 25 x 25 km grid. The future climate is driven by a medium emissions scenario (A1B). Fig. 4 shows the number of days in each grid box where the THI>70 for the south-west region. Currently, as per the observations, there are only a few days on average where the threshold is exceeded. But by the end of the century, this could have risen to around 30 days per year.
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| Fig. 4. The number of days per grid box where the THI > 70, averaged over the south-west region. Each of the 11 ensemble members are shown in the grey lines, with the median and interquartile range being given by the thick black line and blue envelope respectively. |
This could have a large effect on the milk yields produced by cows, and so impact on the viability of herds and dairy farming in parts of the UK. Although keeping cattle indoors can mitigate the effect of direct solar radiation, the humidity in barns has been found to be always higher than indoors (Erbez et al, 2010). Thought will have to be given in the future how best to keep cattle for their well being and also to ensure that dairy farming remains viable in parts of the UK.
References
Erbez M, Falta D and Chládek G 2010 The relationship between temperature and humidity outside and inside the permanently open-sided cows’ barn Acta Universitatis Agriculturae et Siliviculturae Mendelianae Brunensis (Brno, Česká Republika), LVIII 91–6
Murphy J M et al 2009 UK Climate Projections Science Report: Climate Change Projections Met Office Hadley Centre, Exeter, UK
