Beef Cattle's Adjustment to Cold Weather
Winter = snow, wind, cold, hockey and booster cables.
Dealing with winter is an unpleasant fact of life in the temperate region of North America. For most beef cattle, winter brings a very significant change to their environment. And unlike farmers, they can't jump in the truck to warm up on nasty days, nor enjoy a Thermos of hot coffee in the tractor cab, with the heater running full blast and Shania on the cd player!
However, similar to farmers who don insulated coveralls and thick gloves, cattle also prepare for colder weather. As temperatures decline during fall, cattle grow a longer hair coat, which provides a much higher level of insulation by trapping more pockets of air within the hair. Still (non-moving) air is a poor conductor of heat, so a thick layer of micro air bubbles within the coat provide the animal with a heat retaining blanket. In contrast,, cattle which are housed in a warm barn are not stimulated to grow a thick winter coat. If these cattle are suddenly moved out into cold temperatures, they will quickly get chilled and show signs of discomfort by bawling and roaming restlessly instead of bedding down. If left in the cold environment, these cattle will respond by growing a winter coat in a few weeks. But letting cattle adapt naturally to cooling temperatures is a good strategy which reduces overall stress on the animals.
Figure 1: Bush lots can provide excellent wind protection
Animals have a certain environmental temperature range in which they can function without expending extra energy to maintain their core body temperature. This range is call the thermo neutral zone. The temperature at the lower end of this zone is called the lower critical temperature. Below this temperature, the animal has to expend extra energy to keep warm, by shivering (initial stage) or producing heat in muscle tissue without shivering (later stages). The lower critical temperature is affected by hair coat depth and whether the hair is wet or dry, along with other factors. Table 1 gives values of lower critical temperatures for various scenarios.
*ref. Oklahoma State University Mesonet Weather Model, 2003
As environmental temperature drops below their thermoneutral zone, cattle respond by increasing their resting metabolic rate. This produces more internal body heat, just like cranking up the furnace in your house. Of course, producing heat requires fuel. For cattle, the fuel required is supplied by feed energy or from burning stored body fat. For each 1 degree Celsius decline in temperature below their lower critical temperature, cattle require an additional 2% energy in the diet. This can be accomplished by an increase in feed intake, but if they are already at maximum intake, then the energy density of the diet must be increased.
So far, we have just considered air temperature as measured by a regular thermometer. But as you will agree after a winter stroll across the yard on a windy January day, there is more to feeling cold than just the temperature of the air. The amount of heat lost to the environment on a cold day is also affected by wind speed. This effect is known as wind chill. Wind chill occurs when there is relative movement between a warm surface (like a cow) and the surrounding air. Although individual air molecules are relatively poor conductors of heat, when they move in a steady stream over a surface, millions of molecules each pulling away a small amount of heat add to significant heat loss. In addition to this, the air movement disrupts the cow's boundary layer, a thin layer of still air which surrounds objects and acts as a insulative layer. When the boundary layer is pulled away, its insulative value is lost and heat loss increases rapidly. In fact, the first 10 kilometers per hours (kph) of wind speed (from 0 to 10 kph) has a greater effect than going from, say, 30 to 40 kph, due to the initial loss of the boundary layer. The effect of wind chill is shown in Table 2 This gives the heat loss equivalent of still air compared with moving air at the same temperature but at various wind speeds. For example, if the thermometer read -20 C', and wind speed was 15 kph, it would feel like it was actually almost 30 C'.
an L, T or triangle shape will allow cattle to choose the best side for protection as wind direction varies. Since cattle spend a great deal of time at feeders, placing them in areas sheltered from the wind is important. If the feeders are in a comfortable zone, the cattle will spend the maximum amount of time there and consume more feed, which is beneficial for both growing cattle on high energy diets and wintering cows on base quality roughages. Interestingly, not all open front barns make good windbreaks. Unless the influence of wind swirling and tunneling are taken into account, an open front barn can feel colder on a moderately windy day than being outside in the yard ! If you see cattle crowding into the corners of the barn on a windy day, or staying totally outside, they are telling you that the main barn areas are not comfortable. You need to consider some additional wind control structures.
Figure 2: Plastic wind barrier screening makes an excellent wind break in open areas
Moisture also plays a role in heat loss. In contrast to air, water is a very good conductor of heat, and if the air spaces in the hair coat are filled with moisture, heat loss increases dramatically. So if an animal's hair coat is wetted by rain outdoors, or condensation drip in the barn, its insulative value plummets, and heat loss is rapid. Similarly, mud or manure glued onto the hair coat compresses it, squeezing out most air pockets and filling the rest with conductive material. With wet cattle, the need for extra feed energy to compensate for cold stress increases to 4% for each degree Celsius temperature drop below their lower critical temperature.
The key is to keep cattle clean prior to freeze up. Fresh paddocks, concrete yards and bedded packs go a long way to maintaining clean hides. After freeze up, mud becomes much less of a problem, and rain turns to snow, which has much less wetting power. As air temperatures drop, snow becomes drier and a better insulator. If cattle have a relatively large area with good snow cover, they can lay down comfortably without bedding. If the weather goes through freeze and thaw cycles, some bedding may still be required.
Understanding the principles of heat loss helps us help Bossy to manage winter conditions. The effects of air temperature, wind speed and moisture all work together in negative synergy to increase heat loss. Making sure cattle are acclimatized to winter, providing protection from wind, and minimizing the impact of wet conditions will dramatically improve cattle comfort and feed utilization.
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