Factors Affecting Birth Weight in Thoroughbred Foals Born in Kentucky, United Kingdom, and Australia

Kentucky (KY), United Kingdom (UK), and Australia (AUS) are major breeding regions for Thoroughbreds. Birth weights (BW) are commonly measured in each region and are perceived as important parameters for assessing foal health and future athletic ability. This study was conducted to evaluate how foal BW is affected by gender, mare age, parity, month of birth (MOB), and geographic location. Birth weight was measured with electronic scales in 3,062 foals over multiple years in KY (12 farms, 2013–2018, n = 801), UK (6 farms, 2007–2019, n = 934) and AUS (1 farm, 2013–2019, n = 1,327). Differences between gender were measured using unpaired t-tests and differences related to month of birth, mare age, parity, and location were measured using a one-way ANOVA. All results are reported as mean ± SD. BW was greater in KY foals (56.5 ± 6.6 kg) compared with UK (55.3 ± 6.5 kg; P < 0.01), or AUS (53.9 ± 6.6 kg; P < 0.01). AUS foals were also smaller than UK foals (P < 0.01). Fillies were on average 1.2 kg smaller than colts in each region (P < 0.01). Parity significantly affected BW in each region. Primiparous (para 1) mares produced smaller foals in all regions compared with multiparous mares that produced 2 foals (para 2) or multiparous mares that produced morethan 2 foals (para > 2). Foals out of para 2 mares were smaller than foals out of para > 2 mares (P < 0.01). In all regions combined, foals out of para 1 mares (48.7 ± 5.8 kg) were 5.4 kg lighter than para 2 (54.1 ± 5.8 kg) (P < 0.01) and 8.6 kg lighter than foals out of para > 2 mares (57.2 ± 5.8 kg) (P < 0.01). Mare age was highly correlated with parity (r = 0.89). Age of primiparous mares (5.7 ± 1.3 y) was unaffected by region. In all regions, month of birth affected BW with foals born earlier in the season being lighter than those born in subsequent months. However, most of this difference was due to a larger proportion of para 1 and para 2 mares foaling earlier in the season. BW in multiparous (para > 2) mares was not affected by MOB in any region except the UK where para > 2 foals born in January were smaller than in all subsequent MOB (P < 0.01). Although region and gender affected BW, by far the greatest effect was due to parity. Foals out of para 1 and para 2 mares were on average 15% and 5% lighter at birth than para > 2 mares in each region. These large differences warrant further investigation.