“In hot conditions, cattle can become unproductive, overheat or die”.

Those words are from the opening line of a paper in the Journal of Agricultural Engineering Research published in 2000 by two colleagues – Rod McGovern and Jim Bruce. They had produced a model of the thermal balance for cattle in hot conditions and concluded that in hotter conditions, feed intake would be reduced, and production would fall.

Since then, we have also come to understand that there is an earlier negative impact of hotter conditions/heat stress. Namely, that is the reduction in fertility (RIDBA Journal 2021: Vol 22, Edition 1). Their thermal model included air temperature, humidity, wind speed and radiation, together with metabolic heat production from the animal.

Metabolic heat is lost from the body in a variety of ways. They include conduction from the body core to the skin, from the skin by convection, by long-wave heat transfer by radiation, and by latent heat loss via respiration and sweating.

Table 1 shows the results of instantaneous heat balance simulations at four different air temperatures.

Example	1	2	3	4
Air temperature oC	-10	15	30	40
Heat production W/m2	122	122	122	80
Respiratory heat W/m2	47	12	74	73
Stored heat  W/m2	0	0	0	83.5
Evaporation from skin W/m2	17	142	116 (max)	181 (max)
Convective heat loss W/m2	15	59	45	28
Heat flux through coat W/m2	102	11	-24	-75
Respiration rate, breaths per minute	12	12	59	86 (max)
Rise in body temperature oC	0	0	0	1.2

Solar radiation can be added to the model to give results for the expected impact of solar gain (or shading) delivered at stated angles, wind speed, cloud cover, latitude, albedo, the emissivity of the animal and the reflectance. Many of these terms will be familiar to building engineers, but what is the result?

In cold weather – example 1 in Table 1 – convective heat losses are reduced by vasoconstriction but losses through the hair coat are high. In example 3, respiration rate increases to dump more energy as moisture and in example 4 the body starts to acquire heat and body temperature rises. In reality these cattle will already have adapted but with resultant losses to fertility, then feed intake, and then milk production.  The impact of the summer which we’ve just experience in the UK on cattle was, and is, predictable.

The daily – or diurnal – pattern of air temperature can lead to a predictable pattern of heat stress in livestock, and a more recent study with young calves at Iowa State University (Appuhamy, et al. 2021 “The Effects of Diurnal Heat Stress in Dairy Heifer Calves”) describe further impacts.

The study shows that although feed intake increased at night to balance the reduced feed intake during the heat of the day, average daily liveweight gain and feed efficiency decreased significantly during periods of diurnal heat stress. They considered these effects are a likely consequence of nutrients being moved towards an activated immune system and away from productive processes. They also report that water intake per unit of feed consumed also increased significantly during diurnal heat stress.

We do need the science to inform us, and we don’t need the science to inform us. When it gets hot animals consume less food and drink more water and may have reduced immune competence. What a surprise! But heat and water are issues that have been tackled with different levels of precision around the world and this summer will have shaken UK livestock producers – hopefully into an acceptance that what is predictable can also be managed better. We have plenty of information on how to better manage heat and water in the UK’s livestock buildings, but we need to acknowledge the issues first, and work out what it costs to get it wrong.

A recent query involving a new build pig unit threw up the interesting detail that for the month of July 2022 the water consumption in the farrowing rooms increased by 20 litres per pig, per day, compared with the average of the previous three months. That is 37 m³ for the month on this particular site – from drinking water intake increasing to the smart pigs throwing water everywhere to keep cool. Of course, this also adds to slurry volumes. Dairy cows dramatically increase the volume of water intake in warm weather and pigs and poultry show clear preference for cooler drinking water in hot weather conditions. The main production and design questions for any farm – new or existing – are:

• Do we have the correct volumes and flow rates of decent water quality and temperature at the right height and with adequate space for expected competition?

That is no less than six design factors to get right, or wrong.

In food production processes, heat and moisture go hand in hand and the UK agricultural sector has plenty of potential to improve on the management of both factors. Potential gains have been mentioned before as something that need to go into any investment appraisal of our livestock systems. The example of poor fertility – the major reason for culling cows in the UK (NADIS 2022) – will be uppermost in UK dairy statistics this year of high air temperatures and low rainfall. The financial losses of poor fertility accrue from a range of variables (Table 2) and average £250 per cow in the UK herd.

Table 2. The cost of poor fertility.

• genetic gain
• milk production
• veterinary costs
• number of heifers that need to be reared
• cost of AI (or the number of bulls needed)
• Disrupts the pattern of milk production

The chronic health issues linked in part to climate extremes are costing tens of thousands of pounds every year on individual UK livestock farms, which leads to significant potential benefits to offset investment in improved building design and higher specification materials.

And sell rainwater harvesting to the clients and the planners! It should be good business sense and will improve the sustainability image of our industry.

Jamie Robertson
RIDBA Livestock Consultant

The interaction of space and animal welfare has been mentioned before in this section, along with the suggestion that we need to recognise the value of the quality and quantity of space made available to animals.

In the early days of research into animal welfare and the development of animal systems, the defining physical attributes were space (m²) per animal, ambient temperature and not a lot more.

The optimum number of animals in a group (cows can “recognise” up to 60 cohorts, not more) or the height of water troughs and feeders was evaluated (depending on age), and as the science improved the design of animal systems also included factors such as air changes per hour (based on CO₂ production) and the design of sleeping areas (lunge space; cubicle design).

Overall, there emerges an understanding that while we can easily define an equitable unit of space per animal, there is considerable value in the quality of that space, but the quality will often come at a cost. So, the question for the system designer is how much-increased cost does the client value?

The UK cattle sector is the same as any other business in that it needs to invest to remain sustainable, and once the ear tags, fancy feed additives, power, water and all the other consumables have been paid for, there is a need to invest in buildings.

They represent a major, once-in-a-lifetime cost, but the aim is to bring value to the business. The build design process needs to start with the number and size of animals to be housed, group size and feed system. Immediately there will be conflict because the industry experience is “we can/cannot keep 10/15/20 cattle per bay” and “we have always done it like this” and “We tried one of those and it did/didn’t work“. It is not logical to make a once-in-a-lifetime series of technical decisions based on a string of opinions. Instead, refer to the information provided by BS5502 and the RIDBA Farm Buildings Handbook or AHDB guidance material.

The Dairy Housing: A Best Practice Guide (AHDB, 2012) outlines the space requirements in cubicle housing. The book has been freely available for more than ten years and yet producers and builders will still build cattle housing that does not fit industry guidance or – the usual favourite – only use the guidance information they want. The aim is to house a certain number of cattle at a cost and not at a value. We know that wider passageways, cubicles, feed and water spaces, cross-over passages and holes in the roof all increase the cost per cow, and we need to sell the value of these increased costs.

The benefits of good design outlined in the AHDB Dairy Housing booklet include:

• Longer lying times
• Better feet condition
• Reduced aggressive behaviours
• Reduced fouling
• Reduced risk of udder damage or disease
• Optimised feed and water intake
• Improved bedding conditions
• Reduced slurry pooling
• Reduced heat and cold stress

In addition to the above, improved fertility with adequate floor space and surface, lighting quality and impact of well-designed ventilation on the incidence and severity of respiratory diseases, also apply. The positive value of the cost of these design features on the lifetime of a building will run into the tens of thousands of pounds.

Provision of a large area building for single animal systems can be attractive as a means of restraining build costs per housed animal. Build costs per unit of space can be reduced as the total build space increases, up to a limit.

Builders need to be aware the engineering limits on a single build space are far higher than animal system limits on space. This has been mentioned previously whereby an increased number of animals in a single airspace increases the risk of the spread of disease. The poultry sector and, to a lesser extent the pig sector, manage this by designing buildings that match the throughput of animals to facilitate All IN, ALL OUT (AIAO) management of each room. 

The image below shows a beef new build on a top-class farm where there will need to be a significant change in health management before next year to avoid repeat health issues from this past season. The build design and quality is fine, but because the build area is so large that it contains stock of mixed ages, the space quality has been reduced compared with building two separate air spaces. Space cost versus space value.

Jamie Robertson
RIDBA Livestock Consultant

My intention for this article was to present an update from the various British Standards committees on which I represent the interests of the agricultural sector.

Then the Met Office issued a rare red weather warning and Storm Eunice arrived. Once again, the news was filled with fallen trees, displaced trampolines and cladding (roof and wall) ripped from modern buildings.

While the first two can be regarded as the expected norm when storms and gales hit our country, the sight of cladding sheets and panels flying around left me wondering whether the UK had just experienced the one-in-50-year event that buildings are supposed to be designed for, or whether some buildings had not been designed properly.

With this in mind, this article revisits the issue of wind loading on buildings and explains how buildings can be designed to withstand the worst of our winter storms. In the next issue, I shall look specifically at some of the detailing issues that affect the storm resistance of buildings.

Wind forces on buildings

Let’s start with some basic aerodynamics. When the wind blows over or around a building, it is forced to change direction and either speed up or slow down depending on the shape and orientation of the obstruction.

Where the wind blows directly onto a surface, the local external pressure will increase. Where the wind blows parallel to a wall or over a roof, it speeds up causing a decrease in the external air pressure. Unless the building is completely airtight, the wind will also change the internal pressure, either increasing or decreasing it.

The combination of changes to the internal and external air pressure results in either a net positive pressure (on windward facing walls, and the windward slopes of steep roofs) or a net suction (on leeward facing walls, on walls parallel to the direction of the wind and on roofs generally).

From a building design point of view, it is important to understand that wind speed varies enormously with location and building geometry, meaning wind loading is site and building specific, so should be calculated for each and every building project.

Since the magnitude of the wind loading has a direct bearing on the design of the frame, column and rafter sizes for example, it follows that the design of every building is. It should come as no surprise that a building designed for a sheltered location in Oxfordshire may not be adequate if placed on a hilltop on the coast of Cornwall.

Importantly, the wind force on the building is proportional to the square of the wind speed, so doubling the wind speed will produce four times the wind loading on the building.

What are the factors that affect wind speed

1. Location

Some parts of the country tend to experience higher wind speeds than others and this needs to be taken into account when calculating the wind loading on a building.

To enable engineers without specialist meteorological expertise to judge the likely wind speed at a particular location, the available meteorological data has been analysed to produce a contoured wind map of the UK, which is published as part of the UK National Annex to BS EN 1991-1-4. The values shown on the map are magnitudes of the basic wind speed to which correction factors may be applied to take account of wind direction, altitude and exposure conditions.

2. Altitude

Wind speed increases with altitude and this is accounted for by a correction factor that is applied to the basic wind speed. This is especially important for agricultural buildings, since many are constructed at altitudes higher than 200m above sea level, where wind speeds are significantly higher than those in low-lying locations.

3. Distance to sea

The shorter the distance to the sea, the greater the wind speed because the wind loses energy and speed as it blows across land. The greatest reduction in wind speed occurs over the first few miles, meaning that locations on the coast experience much higher wind loading than sites only one or two miles inland. Cliff top sites that combine a coastal location with altitude, experience particularly high wind speeds.

4. Town or country

Agricultural buildings are generally built-in exposed locations that do not benefit from the shelter provided by a surrounding town or city. This results in wind speeds which would be higher than would be experienced by comparable buildings located on an urban site.   

5. Topography

Topographical features such as hills can increase wind speed as the air is forced over them. For this reason, it is important for the person calculating the wind loading to have some familiarity with the site and not simply rely on postcodes. Local obstructions can have a significant impact on the wind speed, by providing shelter, for example, but this effect may vary across the site or even across the building footprint.

6. Wind direction

In the UK, the strongest winds generally blow from the southwest, so a southwest facing coastal location is likely to experience stronger winds that one on the North Sea coast. As wind can – and does – blow from any direction, the factors listed above, in particular distance to the sea and distance into a town, need to be assessed for several points around the compass and the wind speed calculated for each direction.

7. Building height

Taller buildings are exposed to stronger winds and this needs to be reflected in the wind loading calculations. For single storey buildings it is common practice to calculate the wind speed for the ridge height. For a multi-storey building, especially high rises, it is possible to divide the building into zones over its height, so that only the very top of the building is designed for the maximum wind loading.

Design practice

The wind forces acting on a building should be calculated using a recognised code of practice, which in the UK is BS EN 1991-1-4. This is one of the structural Eurocodes and is applicable across Europe, although each country has its own National Annex containing nationally determined parameters and specific national recommendations.

The calculation method in BS EN 1991-1-4 is complex and requires specialist technical knowledge, so it is essential wind loading calculations are undertaken by a qualified structural or civil engineer.

By far the simplest approach is to use one of the many software tools currently available. These range from commercially available packages that take account of all of the factors noted above to free online tools that produce reasonable, but conservative, results with minimal input from the user.

Several steel purlin manufacturers include wind loading tools as part of their specification software which are free to customers. In many cases, the precise site location may be specified in the software by its postcode or grid reference. Alternatively, various online resources may be used to obtain the grid reference, altitude and other location data. Thanks to Google, even the local topography and surrounding terrain may be surveyed without leaving the office. 

The simpler the design approach, the more conservative (i.e., higher) the wind loading, so building designers face a trade-off between design effort and the cost of materials.

Taking a one size fits all approach and designing all buildings for the worst possible wind load will result in over-designed structures and is not recommended. On the other hand, calculating the wind pressures to the n^th degree for a standard industrial unit or agricultural building is an unnecessary expense that will probably give little or no saving in material costs compared to the standard design approaches.

The standard approach presented in BS EN 1991-1-4 and employed by many software tools is a pragmatic way of ensuring that buildings are safe and efficient without needing too much effort at the design stage.

While employing an engineer to calculate the wind loading may seem to be an unnecessary additional cost, the cost of not doing so is almost certain to be greater, either in additional steel or the cost of remedial measures when the building encounters its first storm.    

Dr Martin Heywood
RIDBA Technical Consultant