Álvaro García
Two calves can be born into the same herd, receive the same colostrum, and follow the same feeding program, yet their early growth can look very different simply because they were born in different seasons. Temperature, humidity, bedding conditions, pathogen pressure, and ventilation all change throughout the year, creating very different biological challenges for the newborn calf. While producers often recognize these seasonal differences as management issues, recent research suggests they may also shape calf growth and development in ways that extend far beyond the preweaning period.
The first weeks of life are among the most critical in the development of a dairy calf. During this period, nutrients must support not only maintenance, but also immune function, organ maturation, skeletal growth, and mammary development. Any factor that increases maintenance requirements reduces the nutrients available for growth. Season of birth may be one of the most important and most overlooked of those factors.
Growth is about nutrient partitioning
Early growth in calves is fundamentally a matter of nutrient partitioning. Once maintenance requirements are met, remaining nutrients can be directed towards structural growth, muscle development, fat accretion, and immune function. But when calves are born into environmental conditions that increase thermoregulatory demands, that equation changes immediately.
For newborn calves, the thermoneutral zone is relatively narrow. Cold stress can begin when temperatures fall below approximately 10 to 15°C (50 to 59°F), particularly under wet or windy conditions. Heat stress may begin above 25°C (77°F), especially when humidity is high. In either case, calves must spend more energy simply to maintain body temperature. That means less energy is left for growth. This helps explain why season of birth can create different growth trajectories, even when nutrition appears identical.
What the research says
A recent study by von Riedheim and colleagues offers some of the clearest evidence yet that season of birth affects calf development in ways that begin before birth and continue after birth. Researchers followed neonatal calves born in winter and summer during their first 21 days of life while standardizing nutrition after the initial colostrum and transition milk period. This design allowed them to isolate the effects of season itself, separate from later feeding differences.
The results revealed an important biological paradox. Winter-born calves entered life with greater tissue reserves, showing greater longissimus dorsi muscle thickness, backfat thickness, and skinfold thickness at birth compared with summer-born calves. This suggests that environmental conditions during gestation may shape fetal development, possibly as an adaptive response to colder conditions. But after birth, the pattern reversed. Despite starting with greater muscle and fat reserves, winter-born calves showed slower gains in body weight, skeletal growth (withers height, body length, and heart girth), muscle thickness, and backfat during the first three weeks of life, whereas summer-born calves consistently grew faster. The explanation appears to lie in maintenance energy demand.
Why winter calves grow more slowly
Cold weather increases maintenance requirements because calves must generate more heat to maintain body temperature. This process, largely driven by non-shivering thermogenesis and brown adipose tissue activation, is essential for survival but metabolically expensive. As environmental temperatures fall below the thermoneutral zone, a greater share of available nutrients must be directed toward heat production rather than tissue accretion. In simple terms, every calorie spent on thermoregulation is a calorie no longer available for growth.
This helps explain the paradox observed in the study. Winter-born calves may enter life with greater tissue reserves, but they quickly begin using more nutrients simply to maintain body temperature. Summer-born calves, in contrast, face lower thermoregulatory costs and can allocate a greater proportion of nutrients toward structural growth. In practical terms, winter calves may be born better prepared, but summer calves often grow more efficiently. This distinction is important because birth condition and postnatal growth rate are not necessarily the same thing.
Season shapes more than weather
The study also examined whether the fat concentration of colostrum could modify the seasonal effect on growth. Calves received either full-fat or low-fat colostrum during the first feedings and then transitioned to identical diets. The effect of colostral fat was real, but secondary. Full-fat colostrum improved gluteal fat deposition during early life, particularly under winter conditions, but it did not override the broader seasonal effect on overall growth. This reinforces an important concept: while early colostrum energy matters, it operates within the larger energetic framework created by environmental conditions. In other words, better colostrum can help, but it cannot fully erase the biological cost of thermal stress.
One of the most interesting implications of this work is that season of birth may not simply reflect postnatal weather. It may also reflect the gestational environment. Winter-born calves likely experienced a different maternal metabolic environment during late gestation than summer-born calves. Differences in temperature, photoperiod, and maternal energy balance may influence fetal nutrient partitioning and tissue development before birth, which could help explain why winter calves were born with greater tissue reserves. This suggests that part of the “season effect” may already be programmed before the calf is even born, reinforcing the growing evidence that prenatal environmental conditions can influence postnatal performance.
For producers, these findings carry important practical implications. A calf born in winter may require more nutritional support simply to achieve the same growth targets as a calf born under milder conditions. If feeding programs remain unchanged across seasons, winter calves may fall behind even under excellent management. This means calf programs should not be identical year-round. Winter-born calves may benefit from greater energy density in milk or milk replacer, better bedding insulation, improved wind protection, dry resting surfaces, and closer monitoring of growth. Summer-born calves bring their own challenges, particularly heat stress and dehydration, but under moderate conditions they may convert nutrients into growth more efficiently because less energy is diverted toward thermoregulation. The message is clear: season does more than change the weather, it changes biology.
More than a calendar date
Season of birth is often treated as little more than a calendar event, but biologically it may be one of the earliest environmental forces shaping a calf’s developmental trajectory. The work by von Riedheim and colleagues reminds us that growth does not begin at birth. It begins in utero, where seasonal conditions can already influence tissue development, and continues after birth under the immediate energetic demands of thermoregulation. Together, these prenatal and postnatal forces help determine how nutrients are partitioned between survival and growth during one of the most vulnerable periods of life.
For producers, this carries an important message: calves born in different seasons may not have the same biological starting point or the same nutritional needs. Recognizing those differences can help refine calf management, improve early growth efficiency, and reduce developmental setbacks that may compromise future performance. Early growth is not simply about reaching weaning targets. It is about building the structural, metabolic, and physiological foundation of the future dairy cow. If season helps shape that foundation from the very beginning, then adapting calf management to seasonal biology may be one of the earliest opportunities to influence lifetime performance.
The full list of references used in this article is available upon request.
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