Source: Manitoba Agriculture, Food and Rural Initiatives
Manitoba cattle are subject to considerable environmental stress during the winter months. They must adapt to prolonged cold temperatures and also survive acute conditions such as blizzards and sudden, extremely cold weather. Animals in good body condition with access to shelter or a windbreak are better able to withstand both the acute and prolonged stresses.
During a sudden snap of very cold weather, the temperature may drop below the animal’s “lower critical temperature” (LCT). At this point, heat production of the animal must increase immediately to prevent a drop in body temperature. The LCT refers to the effective temperature. Rain, melting snow or wind will significantly increase the coldness of the environment. For example, a -5oC temperature with a 40 km/hour wind speed is equivalent to -20oC with no wind.
The LCT varies depending upon the weight, condition and energy intake of the cattle. Dairy cows and feedlot cattle on high grain rations have a very low LCT (-30oC to -45oC) because of their high feed intake and metabolic rate. The LCT for pregnant beef cows, dry dairy cows and growing calves, in good body condition, is between -10oC and -25oC. Young calves, from birth to 3 weeks of age have a LCT of +13°C. Dairy calves from 3 weeks old to weaning have a LCT of +1°C.
Animals in good body condition are better able to tolerate extreme cold. For example, thin cows with a BCS of 1-2 have a LCT of -17oC while cows in good condition with a BCS of 3 have a LCT of close to -25oC. This is another good reason for feeding thin cows separately from the rest of the herd. Providing a wind break and/or shelter, ensuring cattle have good body fat stores and feeding additional energy (i.e. grain) will help all cattle survive acute cold stress with a minimum of suffering and long term effects.
The effect of acute cold stress on the feed bill is relatively small compared to the effects of acclimatization. These effects last for the entire winter period even when conditions are relatively mild. When the temperature drops and remains below the thermoneutral, or comfort, zone (15oC – 20oC), certain adaptive changes occur. Hair coat becomes thicker and longer. The metabolic rate increases. Feed intake can increase by up to 30%. A faster rate of passage, however, means that the digestibility of forages is reduced in cold weather (cattle are getting less energy from a given amount of feed).
The overall result of acclimatization is to increase the energy requirements by approximately 10% for every 10oC drop in effective temperature below the comfort zone. Cows in good condition can increase their consumption of good quality hay by about 10 lbs maximum and this helps to meet part of the increased energy requirement. After this, the energy must be supplied as grain or as better quality forage.
Milking dairy cows are typically housed indoors and fed high energy diets. They are therefore relatively immune to the effects of cold stress. The acclimatization process which occurs in all cattle may increase the dry matter intake by 2-3 kg/cow/day. If milking cows are already consuming their limit for feed, milk production may drop if the energy density of the diet is not increased. There is very little research into the effect of cold stress on lactating dairy cows. Replacement heifers and dry cows experience cold stress in a similar way to beef cattle.