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A runt is an animal that is unusually small for its species. In veterinary medicine, a runt may also be described using terms such as low-birth weight, intrauterine growth restriction, and small for gestational age. An animal may be defined as small for gestational age (SGA) depending on different criteria, such as size in comparison to littermates', as percent of maternal body weight, as a specific neonate weight threshold for the breed or species, and as different body proportions displayed by runts.
Runts face many challenges in comparison to their normal birth weight peers - they are more likely to contract diseases, and die in the neonatal period, have lower glycogen stores, suffer from developmental delays, insulin resistance, hypothermia, and low Hypotension. Runts are also associated with economic losses in farm animals - SGA adult Cattle give smaller milk yields and suffer from infertility, intrauterine growth restricted (IUGR) piglets have modifications in their muscle tissue that may affect the taste of their Pork, adult IUGR Pig have smaller litter sizes and lower birth weight piglets in their litters and adult low birth weight Sheep may have poorer quality Wool.
In cattle and sheep, an additional reason may be Heat wave during pregnancy. In dairy cows, a contributing factor may be lactating while pregnant, which can overtax the cow's ability to provide sufficient nutrients to the fetus. Nulliparous cows are more at risk of giving birth to SGA calves, and on average give birth to calves of a lower birth weight.
In , a larger litter size may cause more low weight puppies to be born. Typically low weight puppies, like piglets, have smaller in comparison to their normal body weight littermates.
In , younger mothers are more likely to give birth to kittens with lower body weight. Likelihood of giving birth to low body weight kittens increases if there is at least one stillbirth in the litter.
However, it is the identification of at-risk puppies that presents a unique challenge in dogs, as dog breeds can vary in weight from less than Toy dog to 120 kg. This discrepancy in size can make it hard to create a uniform guideline for care which Dog breeding and can implement in practice. Several identifying tools have been proposed, such as puppy weight - mother weight ratio, which can help identify low birth weight mongrel puppies or breed-specific thresholds, which can be more useful in identifying underweight purebred puppies, as the birth weight of puppies can vary quite a bit among same adult size large and giant breeds.
For livestock like swine, labor-intensive birth assistance has been identified as a major mitigating factor in runt mortality and future outcomes, however such a strategy is cost ineffective in intensive animal farming. Instead, the recommended strategy is managing the sow's nutritional intake and not breeding IUGR piglets.
In cows, it has been found that runt calves are less likely to be effective milk producers and also tend to produce smaller calves in turn. It has been suggested that it would be more effective to redirect SGA calves to veal production, and preferentially breed calves of an average size and good productivity.
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