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Simple Summary: Recently: more attention has been given to low-birth-weight calves, often without considering gestation length. Similar to human medicine, calves classified as small for gestational age (SGA) weigh below the 10th percentile at birth. The present study aimed to investigate the long-term effects on fertility, productive performance, and overall survival in Holstein Friesian (HF) heifers born SGA. We found that while SGA calves showed initial catch-up growth at six months of age, they were still significantly lighter at twelve months. SGA heifers required significantly more inseminations to conceive compared to their average (AGA) and large (LGA) counterparts. Additionally, more SGA heifers were culled during their first lactation and had lower survival rates to the second calving. SGA heifers also had a higher risk of leaving the herd prematurely. Although overall lifespan and total milk yield were similar among all groups, SGA heifers produced less milk per productive day. This study reveals that HF calves born SGA face significant long-term health and productivity challenges, emphasizing the need for further research on the economic impact of rearing SGA heifers. Recently: more attention has been given to low-birth-weight calves, often without considering gestation length. Calves can be classified as small for gestational age (SGA) when their birth weight is below the 10th percentile, similar to the definition in human medicine. While SGA babies face various health risks, it remains unclear if SGA calves face similar long-term health consequences. This study aimed to investigate the long-term effects on fertility, productive performance, and overall survival in Holstein Friesian (HF) heifers born SGA. Chi-squared analysis assessed culling and survival rates, and linear mixed-effect models evaluated the impact of gestational age group (small, average, or large for gestational age, respectively, SGA, AGA, and LGA) on growth, fertility, milk yield, and lifespan. SGA calves showed catch-up growth at six months but weighed significantly less at twelve months (p = 0.003). Age at first insemination and calving did not differ significantly, although SGA heifers required more inseminations (2.3 ± 1.50) compared to AGA and LGA heifers (1.7 ± 0.98 and 1.5 ± 0.89, respectively, p = 0.006). SGA calves tended to be culled more during the first lactation than AGA calves (25.0% vs. 11.9%, p = 0.078) and showed lower survival to second calving (p = 0.019) compared to AGA and LGA heifers. The Kaplan–Meier analysis indicated a tendency for gestational age to affect overall survival (p = 0.1), with SGA heifers having a higher risk of leaving the herd prematurely (p = 0.035, hazard ratio = 1.53). Milk yield per productive day was significantly lower in SGA heifers (21.2 ± 8.73 kg) compared to AGA and LGA heifers (26.9 ± 5.01 kg and 26.3 ± 3.38 kg, respectively, p = 0.006). This study reveals that HF calves born SGA suffer long-term consequences, although further research is needed to understand the economic impact of rearing SGA heifers. [ABSTRACT FROM AUTHOR]

Simple Summary: Poor fertility is the main reason for premature culling of cattle. Dam factors and external variables can both influence how the calf fetus develops, known as fetal programming. Dam factors include age, parity, body condition, health and milk yield. External variables include nutrition and the environment during pregnancy. These all affect placental growth and the nutrient supply to the fetus, which in turn influence the size, shape and body composition of the calf. Postnatal growth rates, organ structure and immunity can all be affected. The impact on organs such as ovaries, liver, pancreas, lungs, spleen and thymus is dependent on the stage of pregnancy during which the fetal environment is sub-optimal. Regulatory systems which influence growth, metabolism and fertility can be permanently reprogrammed. Most changes affecting fertility are probably indirect. For example, calf health, the timing of puberty, age and size at first calving, and the ability to adapt to lactation after calving can all alter future reproductive potential. The size of these effects is hard to quantify due to practical difficulties in obtaining data over a sufficient time period. Nevertheless, there is compelling evidence that the fertility of some cows is compromised by events happening before their birth. Cattle fertility remains sub-optimal despite recent improvements in genetic selection. The extent to which an individual heifer fulfils her genetic potential can be influenced by fetal programming during pregnancy. This paper reviews the evidence that a dam's age, milk yield, health, nutrition and environment during pregnancy may programme permanent structural and physiological modifications in the fetus. These can alter the morphology and body composition of the calf, postnatal growth rates, organ structure, metabolic function, endocrine function and immunity. Potentially important organs which can be affected include the ovaries, liver, pancreas, lungs, spleen and thymus. Insulin/glucose homeostasis, the somatotropic axis and the hypothalamo-pituitary-adrenal axis can all be permanently reprogrammed by the pre-natal environment. These changes may act directly at the level of the ovary to influence fertility, but most actions are indirect. For example, calf health, the timing of puberty, the age and body structure at first calving, and the ability to balance milk production with metabolic health and fertility after calving can all have an impact on reproductive potential. Definitive experiments to quantify the extent to which any of these effects do alter fertility are particularly challenging in cattle, as individual animals and their management are both very variable and lifetime fertility takes many years to assess. Nevertheless, the evidence is compelling that the fertility of some animals is compromised by events happening before they are born. Calf phenotype at birth and their conception data as a nulliparous heifer should therefore both be assessed to avoid such animals being used as herd replacements. [ABSTRACT FROM AUTHOR]