WITH MUCH of Australia blanketed by a cold and wet winter, our attention once again turns to calves and the challenges associated with rearing during these chilly months. Some farms will observe an increase in morbidity (sick calves) and mortality (dead calves) during inclement weather. A lot of this is due to pathogen survival under these favourable conditions but other factors such as housing and nutrition also play an important role in outcome for the calf. This article discusses the nutritional requirements of the young calf and how these are affected by environmental temperature.
Maintenance vs growth?
The protein and energy requirements of a calf are divided into two categories based on their use for maintenance and growth.
Maintenance describes the amount of energy and protein needed to support normal bodily functions, including maintaining body temperature. Maintenance requirements are related to body size with bigger animals having higher maintenance needs.
Growth requirements account for the nutrients required to build body tissues. The nutrients the calf consumes are used to support maintenance first with any nutrients fed in excess of maintenance needs being used for growth. Nutrients needed to support growth logically increase as growth rate increases.
Calf growth is affected by many factors but daily intake of protein and energy are the most important.
Frequently, energy intake is the first limiting factor to growth. If a calf consumes more energy than she needs for maintenance, the “extra” energy can be used to convert dietary protein into body tissue. However, if a calf consumes less energy than required for maintenance, there is no energy available for growth. Diets must provide enough energy to support growth and enough protein to be used for that growth.
Feeding too little of either nutrient, or feeding the wrong ratio of energy to protein, will limit growth. Nutrients are provided by liquid feeds (milk and milk replacer) and calf starter, with intake and composition of both these feeds affecting growth potential.
Keeping warm
The ‘normal’ core body temperature of a young calf is 38.6°C (range 38.5–39.5°C). There is an environmental temperature range at which a calf is comfortable and does not require any additional energy to actively warm or cool its body. This zone is called the thermoneutral zone and, for a dairy calf up to three weeks of age, is between 10 and 25°C.
This zone changes as a calf grows due to the production of heat from fermentation in the rumen and increase in body mass. For example, the thermoneutral zone for a one month old dairy calf is 0°C to 25°C.
Within this zone the amount of body heat produced by the calf equals or exceeds the heat lost from the body through various means. Breed, hair coat, wind, precipitation, mud and solar radiation will also affect the thermoneutral zone.
Environmental conditions also affect maintenance energy requirements. Calves housed in drafty, wet, cold conditions have higher maintenance energy requirements than those housed in draft-free, dry environments. Newborn calves are particularly vulnerable to these changes in temperature.
A calf is born with a supply of brown fatty tissue that releases energy as heat, but in extreme cold conditions this energy reserve may be used up within hours of birth. When the brown fat energy reserve runs out, a calf uses the energy designed to maintain its body temperature and may fail to gain weight.
If the environmental temperature falls below the thermoneutral zone, the calf uses more energy from the diet to maintain its core body temperature. The table (right) shows that when the environmental temperature drops from 20°C to 5°C, calves less than 3 weeks of age require 40 per cent more energy from the diet just to maintain core body temperature.
This does not allow any partitioning of nutrients for growth. Required increase in energy needs as environmental temperature decreases
Environmental temperature
It is recommended that young calves should be fed extra energy during cold weather to satisfy the increase in maintenance energy requirements and improve the chances of increased growth during this critical period. This can be accomplished by increasing the volume of liquid diet being fed or by adding additional milk solids to the diet (fortifying the milk).
Australian research has shown important increases in growth rates during the pre-weaning period in calves fed fortified milk compared to calves fed on conventional diets. Research has also shown that calves fed on a higher plane of nutrition are less likely to succumb to sickness than those fed inadequately in relation to their environment. This supports the need to provide additional nutrition to young calves during winter when the risk of disease and exposure to the elements is greatest.
Always speak with your veterinarian prior to embarking on a change in calf nutrition as inconsistent feeding can result in gastrointestinal upset and diarrhoea.
Required increase in energy needs as environmental temperature decreasesEnvironmental temperature °C% increase in metabolizable energy required for maintenanceBirth to 3 weeks>3 weeks2000151301027054013-56840
Source: Nutrient Requirements for dairy Cattle, National Research Council, 2001