9 Feb 2015
Sara Pedersen
Job Title
The recent drop in temperatures and snow that came with it may make many of us long for the warmth of the summer months. However, our dairy cows will not be thinking the same and are much more likely to be comfortable in the temperatures we are experiencing now.
I have previously reflected on the effects of heat stress on performance, but what are the options when it comes to combating it? In November I attended the Dairyland Initiative workshops in Wisconsin, US. One of the days was focused entirely on the effects of heat stress in dairy cattle and different heat abatement systems. So what did I learn? In summary, a lot.
The thermoneutral zone for a cow is between 4°C and 21°C, but heat stress is the result of a combination of both temperature and humidity – the temperature humidity index (THI). Historically, we have considered cows to be under stress when THI reaches 72; however, recently this has been lowered to 68. This is long before a human would start to feel uncomfortable, so by the time we start to struggle working in the heat and humidity, the cows have been suffering the effects for some time.
As discussed previously, there are lots of knock-on effects of heat stress. Visual signs include an increased respiratory rate, decreased feed intakes, decreased rumination, reduced yield, increased rectal temperature and increased somatic cell count. Less obvious signs include impaired rumen function and balance, reduced udder health, lower immunity, reduced pregnancy rate (average drop of five per cent during heat stress), increased risk of uterine disease (Figure 1) and lameness.
Some of these effects, such as lameness and uterine disease, are not immediately apparent and may take some time to manifest. In the case of lameness the rise in cases does not occur until eight to 10 weeks after initial heat stress. Longer standing times lead to an increase in sole haemorrhage and ulcers, and a lack of circulation through the hoof reduces healing.
When ambient temperature approaches body temperature the only viable method of heat loss is evaporation, either through sweating or thermal panting.
Cows are not good at sweating, so their main cooling mechanism is panting. When it is heat-stressed a cow’s temperature will increase by 0.5°C per minute while it is lying down. At a critical point (38.9°C body temperature) it will have to stand to be able to thermal pant and reduce its body temperature. Inversely, a cow’s temperature decreases by 0.26°C per minute when it is standing (half the rate it heats up when lying down). It will lie down again when its temperature cools to 37.7°C.
As core body temperature increases, lying time bouts decrease (the cow lies for shorter periods of time). The overall effect is cows lie down for shorter periods of time during heat stress as they have to keep standing up to cool down. Therefore, even with the most optimal cubicle in terms of comfort (sand), heat stress will have a negative impact on lying times (Figure 2). Bearing this in mind, it is important to minimise heat accumulation when the cow is lying down and enhance cooling when the cow is standing.
The collecting yard is a critical area when it comes to alleviating heat stress, since cows are forced to stand during this period and it is important to allow them to cool before being milked and returning to their housing (Figure 3).
In many situations heat stress is actually exacerbated in the collecting yard due to cows being bunched up too tightly or poor use of fans that recirculate hot humid air. As a result, cows actually accumulate rather than dissipate heat. The result is that rather than lie down once they return to their housing they have to continue to stand until they dissipate enough heat.
Ideally, sheds should be ventilated naturally as this is not only very effective, but also sustainable. However, this relies on a number of factors. Firstly, prevailing winds are required so air moves directly into the building’s openings at the eaves or side walls. Air travelling over the open ridge will also create negative pressure or “lift” to aid in ventilation. Secondly, thermal buoyancy (or the “stack effect”) is required with the animals in the building heating up the surrounding air causing it to rise up and out of the ridge opening.
Natural ventilation is always the first choice, even in perfectly designed sheds. However, it will not be achieved unless there are prevailing winds and on those days of the year when wind speed is low or in poorly ventilated sheds, other measures must be considered.
There are three options to alleviate heat stress in dairy cows: cool the cow, cool the air around the cow or a combination of both.
Options to cool the cow include water and ventilation. If using water then large droplets are required to soak the animals since a mist of fine water droplets will not penetrate down to the skin. However, in situations when the air is still, wetting the skin alone does not reduce a cow’s temperature because ventilation is required to evaporate the water and thus cool the skin.
Similarly, as cows don’t sweat, simply moving warm, humid air at higher speeds doesn’t help with cooling since air movement alone creates no cooling effect. Since fans create velocity, but not ventilation, poorly placed fans can actually make a situation worse by recirculating warm air that progressively becomes more humid in the absence of natural ventilation. A common error is to place fans over the feed passage rather than the cubicles. Since lying time drives feed intake it is more effective to keep cows cool while they are in the cubicles, increasing lying times and thus intakes.
Where natural ventilation is not guaranteed, a combination of both soaking and artificial ventilation is optimal. The air velocity at cow level is critical – too fast and the water evaporates too quickly so the cow heats up again; too slow and no effect is seen. The optimum is to provide air velocities of 1m/sec to 2m/sec at cow level, which will evaporate the water within eight to 11 minutes (Berman, 2008; Figure 4).
To achieve optimal air velocities at cow level, the method used to create it is important. Fans are most commonly used; however, the size and position of the fan will influence both throw distance and also air velocity at cow level. For example, the optimal air speed of 1m/sec to 2m/sec will be seven to 11 feet from the fan; however, for a 4ft fan this increases to 10ft to 15ft.
As well as poor positioning, another common problem is insufficient numbers of fans being used. What may feel like a nice breeze to us at a distance from the fan will not be sufficient for cow cooling. In the collecting yard 1,000 cubic feet per minute of air is required per cow – this equates to a lot of fans since one 10,000ft3/m is recommended per 150ft2. Fans must also be angled vertically or at a steep downward angle to ensure the cows reap the benefits.
However, one downside of using fans only is they still rely on natural ventilation, and thus a prevailing wind, to bring in fresh air. So in enclosed spaces where there is little or no natural ventilation, fans alone are not the right solution.
To combat the problem of insufficient natural ventilation, the Dairyland Initiative has been trialling cooling systems involving positive pressure tubes as a way of introducing fresh air and ventilation. Coupled with a sprinkler system this has shown to be very effective at aiding in cooling cows in the collecting yard. However, as with fan placement, design is critical to ensure air velocity at cow level is at target levels. Tube diameter, height and hole size and placement all have a bearing on throw distance and thus air velocity at cow level.
The air around the cow can either be cooled through the use of water or evaporative pads. In contrast to the use of water to cool the cow, when cooling the air, fine droplets of mist must be produced, also known as “fogging”. As the very fine water droplets evaporate they remove energy from the air cooling it and helping cows lose body heat. This system can be fitted to fans to help direct the mist, but it is important beds or feed do not become wet as a result.
Evaporative cooling pads are commonly used in mechanically ventilated sheds in the poultry and pig industries and also in the dairy industry, where heat stress is an issue for most of the year. They are completely closed systems with no reliance on natural ventilation. They work by drawing outdoor air in through wet porous pads and cooling it with fans located along the opposite wall. Commonly, baffles are placed over the cow’s beds to redirect air flow to cow level. These are expensive systems to maintain and it is unlikely they will be installed on UK dairy farms.
Due to their origins as grazing animals on the African plains, cows associate light with heat and darkness with coolness. This is why on a warm day they will bunch together in a shaded area, even if this is actually the warmest part of the shed. Another reason for bunching is fly worry. Regardless of the cause, bunching increases standing time and reduces cooling since humid air accumulates around the animals (in a similar manner to an overcrowded collecting yard). Therefore, the use of sunscreens to darken the shed can help reduce bunching as well as ensuring adequate fly control.
Heat stress is an underestimated problem on UK dairy farms and considering future modifications/installations now before we enter the summer months will be time well spent. By the time we consider it to be warm, the cows will have been suffering for longer, by which time the damage is done and the knock-on effects will be impacting on productivity.
Figure 4. Changes in hair surface radiant temperature (°C) after wetting when exposed to still air or air velocities of 0.5m/sec, 1m/sec, and 2m/sec. (Taken from Berman, 2008).
