In Korea, dairy consumption trends are shifting from fluid milk toward processed dairy products such as cheese and butter. Holstein cows dominate the Korean dairy industry, but climate stress and rising feed costs are increasingly challenging farm sustainability (
KOSIS, 2024). Jersey cows are gaining attention as an alternative due to their high milk solids content, feed efficiency andheat tolerance (
US JERSEY, 2021).
Despite their potential, studies on Jerseys under Korean conditions are limited. Birth weight is a fundamental indicator of calf viability and future productivity
(Ugurlu et al., 2014). Heavier calves are typically healthier, more robust andyield better growth and milk production outcomes (
Lorenz, 2021).
Identifying breed-specific factors influencing calf birth weight can inform tailored breeding and management strategies
(Soberon et al., 2012). Therefore, this study compares the impact of dam age, parity, gestation length, season andcalf sex on birth weight in Holstein and Jersey calves raised under uniform conditions.
All procedures were approved by the Institutional Animal Care and Use Committee of the National Institute of Animal Science, Republic of Korea (approval number: NIAS-20210502).
Animals, diet and data collection
This study analyzed 635 dairy cows (Holstein: 572; Jersey: 63) raised under identical management on a farm in Cheonan, Korea, from March 2016 to March 2024. All cows conceived via artificial insemination and were relocated to a single pen 21 days before calving, receiving a standardized close-up TMR diet (13 kg DM·cow
-1·day
-1; CP 11.1%, NDF 48.2%, ADF 26.9%, Ca 0.4%, P 0.15%) with ad libitum water. Calves were of the same breed as their dams; abortions and twin births were excluded.
Data collected included breed, dam’s age and parity, gestation deviation (DBE), birth season (spring, summer, autumn, winter), sex and birth weight. Birth seasons were categorized as spring (March-May), summer (June-August), autumn (September-November) and winter (December-February).
Statistical analyses
Statistical analyses were conducted using SPSS v27.0 (IBM, Chicago, IL, USA). Stepwise multiple regression identified key predictors of calf birth weight among dam age, parity, DBE, birth season andsex. Variables were added or removed based on statistical significance (entry: p<0.05; removal: p>0.10) to construct a parsimonious model while minimizing multicollinearity. Model assumptions-including linearity, independence, homoscedasticity andnormality-were verified. Multicollinearity was assessed via tolerance (> 0.1) and variance inflation factor (VIF < 10). Final models were selected after stepwise refinement.
Breed characteristics and calf birth weight
Holstein dams had a mean age of 5.1 years (SD = 1.8), parity of 2.5 (SD = 1.2) and a DBE of -1.5 days (SD = 5.3). Their calves had an average birth weight of 42.7 kg (SD = 5.8). Jersey dams were slightly younger, with a mean age of 4.4 years (SD = 1.8), parity of 2.1 (SD = 1.2) and a DBE of -0.2 days (SD = 6.6). Jersey calves were considerably lighter, with an average birth weight of 25.3 kg (SD = 4.1). The calf sex ratio showed a slight male predominance in both breeds (Holstein: 51.6%; Jersey: 54.0%) (Table 1). This considerable difference in birth weight between breeds reflects their physiological and genetic characteristics. The values obtained align with previously reported ranges for each breed
(Ugurlu et al., 2014), confirming breed-specific size differences at birth.
Regression analysis and key factors affecting birth weight
Stepwise multiple regression revealed that Holstein calf birth weights were significantly affected by DBE (β = 0.350), sex (β = 0.316), birth season (β = -0.112) and dam age (β = 0.083) (p<0.05). The model explained 27.3% of the variance (adj R
2 = 0.268) (Table 2). For Jerseys, DBE (β = 0.448), sex (β = 0.277) and dam age (β = 0.228) were significant predictors (p<0.05), explaining 31.4% of the variance in birth weight (Table 3). No seasonal effect was observed for Jerseys.
Impact of dam age
Dam age positively influenced birth weight in both breeds, with a stronger effect in Jerseys. This supports findings that older cows, especially multiparous, provide better uterine environments and nutrient supply for fetal development
(Duncan et al., 2023). Previous studies have shown that calves born to multiparous cows can weigh 3-4 kg more than those from primiparous cows (
Johanson and Berger, 2003;
Olson et al., 2009).
Gestation length (DBE)
DBE was the most influential factor for both breeds, indicating that extended gestation allows for greater fetal growth. This observation is consistent with nutritional and developmental principles reported in previous studies
(Micke et al., 2010).
Seasonal effects
Only Holstein calves showed significantly lower birth weights when born in autumn. This may reflect heat stress during late gestation in summer, which can impair maternal metabolism and reduce feed intake, thereby limiting nutrient availability to the fetus (
West, 2003;
Rhoads et al., 2009). Related studies have linked high THI prior to calving with reduced birth weight and placental efficiency
(Reynolds et al., 2006).
Jersey calves were not significantly affected by season, suggesting greater resilience to environmental stress. Jerseys are known for their heat tolerance, with lower metabolic heat production and more efficient respiratory cooling than Holsteins
(Lim et al., 2021; Cook et al., 2007). This physiological adaptation may explain their stable birth weights across seasons.
Effect of calf sex
Male calves were heavier than females in both breeds, with holstein males averaging 3.7 kg more and Jersey males 2.2 kg more. These results are consistent with sex-based fetal growth patterns observed in cattle
(Wilson et al., 2017), reinforcing previous findings
(Olson et al., 2009).
