Newborn dairy calves, until about a month old raised in an unheated or cold facilities, often struggle with poor growth rates and are more susceptible to disease. By assuring a high plane of dietary milk energy, dairy producers can largely avoid bringing a winter energy crisis inside the calf barn.
In order to survive the cold, a pre-weaned dairy calf must maintain its core body temperature to sustain heat-requiring body functions such as digestion, respiration, circulation, immune protection and other metabolic tasks. When temperatures in the calf barn are moderate, the calf has little trouble meeting these maintenance requirements (NEm), which by definition is the amount of metabolizable dietary energy (ME) that will result in no loss or gain in body energy. When more ME is consumed than needed for NEm, this energy surplus allows the calf to grow (protein/fat deposition), which is dependent upon a net energy requirement for growth (NEg).
For example, a 50-kg pre-weaned dairy calf requires about 7.9 megajoules (MJ) of dietary ME per day in order meet its NEm. It requires an addition 6.2 MJ to meet the NEg for a daily growth rate of 500g/head/day. Therefore, this calf needs a total of about 14 MJ to stay alive and support modest growth performance.
More milk needed as temperature drops
The ambient temperatures of a calf barn, where a young pre-weaned dairy calf do not need to expend extra ME to stay warm (or cool), is referred as the “thermal neutral zone” (TNZ). The comfort zone for most indoor-housed dairy calves range: between 10 – 25C.
In an unheated calf barn or hutch (or due to a low-temperature thermostat setting), a low critical point of the TNZ is often reached and the pre-weaned calf may not be able to maintain its body temperature and other body functions unless it obtains and then expends additional ME (as heat) just to keep warm. For calves 0-3 weeks of age, this lower critical temperature is 20 C and for calves older than three weeks of age, it’s 10 C.
In order to meet this extra demand in NEm under cold temperatures outside the TNZ, it becomes a straightforward matter of providing more dietary energy to the baby calf!
Since whole milk (3.5 per cent b.f.) contains about 2.80 MJ/l (22.4 MJ/kg, dm basis), about five litres per head per day should be fed to a less-than-three week old dairy calf housed in a calf barn at 20 C (in the TNZ). If the calf barn temperature falls to 0 C (outside the TNZ) then a total of 6.5 litres of milk should be put in front of this calf. At both temperatures, the dietary energy of the respective quantity of milk should satisfy the total ME and provide enough extra energy to support a daily growth rate of 500 g per head per day.
On the other hand, if no more than the initial five litres of whole milk were supplied in the second situation (still outside the TNZ), the ME of the same calf would be satisfied but not enough dietary energy would meet its NEg requirements. As a result, the growth rate of this calf would suffer. In this case, a rule of thumb in order to provide enough nutrition for both NEm and NEg is to increase the amount of milk (or milk replacer) fed by about two per cent for every 1 C when the temperature drops below 20 C.
Good-quality milk replacer
Further advice is warranted for those dairy producers that purchase calf milk replacers rather than feed whole milk. It is recommended that producers buy only a high-quality commercial product with a 20 per cent fat content.
Despite widespread belief, a 20 per cent high fat milk replacer contains approximately 95 per cent of the energy content of whole milk (21.0 – 21.4 MJ/kg). Milk replacers’ dietary energy is provided in the protein, fat and lactose (milk carbohydrate) found in its ingredients such as skim milk, whey protein concentrates, whey and edible bleached fats. If we use the same whole milk feeding protocol, when feeding nutritious milk replacer to zero to three-week old calves housed in a calf barn at 0 C (outside the TNZ), we should provide about 855 grams (18.2/21.4) of milk replacer powder on a daily basis.
Reconstituted at the rate of 125 g/litre of water (same dm of whole milk), then we would should provide about seven litres of mixture per calf per day. Since this daily volume of liquid might constitute a significant belly-full for some of the younger dairy calves in two routine feedings, a third feeding should be warranted in order to assure that calves drink enough milk replacer and to prevent digestive upsets and nutritional scours.
Although milk-fed baby calves do not pick up or digest energy-containing calf starter, it is a good idea to set out a small meal at one to two weeks of age.
Regardless of calf starter acceptance, baby calves raised under cold winter housing conditions must still rely upon a good whole milk or milk replacer-feeding program. Freezing temperatures, particularly in unheated or cold calf barns means young pre-weaned dairy calves need extra energy in their diets. Supply enough milk-based energy goes a long way in reducing cold stress upon promising dairy calves that remain healthy and continue to grow.