The Challenge of Feeding Beef Cows in Ontario Winters Part 1: Cows and Their Thermal Environment
Ontario winters are great for ice fishing, snowmobiling and skiing, but they can be tough on beef cows (and beef farmers). Even with advanced grazing management to stretch the pasture season, cows will be on costly stored forage for most of the winter. We want to ensure nutrient requirements are met and that the cattle are not being overfed or underfed, while minimizing wastage1. We know that the cows will need more feed energy to compensate for cold temperatures - but how much? Determining this can be very challenging, since several animal, environmental and feed-related factors impact on both energy requirements and feed consumption. In Part 1 of this series we'll look at the effects of various environmental factors on cow heat loss. In Part 2 , we'll combine these and other effects to calculate the dietary energy needs of beef cows during winter, and how that translates into feeding management.
Thermal Energy Exchange Between Cows and Their Environment
While air temperature is the most common measure of the thermal environment, it can be a very inaccurate means of assessing how the environment actually impacts on livestock outdoors. In addition to gaining or losing heat from the air surrounding them, animals exchange thermal energy with their environment through a number of different mechanisms, including:
In order to better account for all of these factors, the concept of Effective Ambient Temperature (EAT) was developed. This estimates the combined effect of 2 or more thermal factors and calculates what the equivalent temperature of calm air, dry air would be. The most common uses of the EAT concept are "Wind Chill Factor", used in winter to express the cooling effect of wind, and summer's "Humidex", which quantifies the impact that high humidity levels have on limiting the evaporation of sweat, thus decreasing an animal's ability to cool itself. In this article, we will use a simple EAT based on air temperature and wind speed, and address the other factors separately. In Part 2, we will combine several factors to estimate the net effect on cows, and predict what changes are required to their feeding program.
The Thermoneutral Zone
Cattle, like all mammals (and birds) need to maintain a constant core temperature for body systems to operate normally. For healthy cattle, this corresponds to a rectal temperature of 38 C. Cows living outdoors are exposed to a large variation in EAT. However, they have the ability to maintain a stable core temperature over a range of EAT, using behaviour and other means, without having to expend additional metabolic energy, This is called their thermoneutral zone (TNZ). The high end of this zone is called the upper critical temperature (UCT), while the bottom end is the lower critical temperature (LCT). When temperatures move outside of the TNZ, the animal has to start expending extra energy to either cool down (heat stress) or warm up (cold stress) (see Fig.1).
Figure 1. Schematic of Relationships of Temperature and Thermal Zones1
An animal's thermoneutral zone can shift over time. For example, shedding a winter hair coat and losing body fat would cause a cow's TNZ to shift to the right on the diagram, making the animal more comfortable in hotter temperatures but less able to cope with cold.
Adaptation to Cold Weather
As temperatures decline during fall, beef cows adapt by growing a thick hair coat. This increases the insulation value of the layers between the body core and the environment. This extra insulation helps them better conserve body heat, and shifts their thermoneutral zone down the temperature scale. Their LCT drops from a summer value of around 15 C to a winter value of about -5 to -10 C.
Other important cow factors are body condition score and hide thickness. Cows in better body condition have a thicker insulative layer of subcutaneous fat which helps to slow core heat loss to the environment. Hide thickness is also important, with thicker hides improving insulation (see Fig. 2). Cows with dairy breeding have thin hides, most European origin breeds have moderate hide thickness, and most British origin breeds have thick hides.
Figure 2. Schematic cross section of a beef cow showing body layers and heat flow to a cold environment
As the effective ambient temperature drops below their LCT, cows respond by increasing their basal metabolic rate to generate more heat - heart rate, blood flow, respiration and cellular activity are all elevated. When temperatures get really cold, they go into short term shivering phase to generate heat from muscular contractions. If the cold stress continues, they switch over to a phase where muscles generate heat without movement, efficiently converting the energy in nutrients into heat. This is called non-shivering thermogenesis (thermo = heat, genesis = origin). All of these responses require additional metabolic energy, which must come either from the diet or from the cow's fat reserves.
Animals are surrounded by a thin layer of air molecules, called the "boundary layer" that "sticks" to their outer surface. This layer warms up to about the same temperature as the outer surface of the animal and helps to slow heat loss to the surrounding air. Even a low wind speed of 5 km/hr disrupts this layer and accelerates the loss of heat. Heat loss increases with increasing wind speed, and this effect is proportionally larger at lower air temperatures (Table 1). The amount of protection cows have from the wind is an important factor in determining their dietary energy requirements.
Table 1. Effective ambient temperature (EAT) for various air temperatures and wind speeds (°C eqivalent)1
1 Data from Environment Canada Wind Chill Calculator
Example: with an air temperature of -20C and a wind speed of 20km/hr, heat loss is equivalent to a still air temperature of -30C.
Effect of Precipitation
The insulative value of the hair coat is based on the many tiny air pockets which it traps. Still air is a good insulator. If the coat gets wetted by rain and slicked down, the pockets get reduced in number and filled with water, which is a good conductor of heat. This dramatically reduces the insulative value of the coat, increasing heat loss from the animal. If the precipitation is in the form of snow, and conditions are fairly cool, it usually has little effect as it doesn't melt on the cows
Applying Environmental Factors to Beef Cow Feed Requirements
In the next edition of OMAFRA Virtual Beef, Part 2 of this series will quantify the effects outlined in this article and use them to calculate the dietary energy needs of beef cows and ration adjustments for various winter scenarios.
1 for more on wastage and utilization see previous Virtual Beef article:
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