When to cull cattle selectively in a wild state?

The ultimate goal of breeding-back is to release the herds into nature, opposing them to natural selection so that they become dedomesticated wild animals after a sufficient amount of time. However, there is the problem that breeding-back herds are not genetically uniform and when artificial selection is stopped, their phenotype becomes increasingly heterogeneous in the starting phase. This is what happened to Heck cattle in Oostvaardersplassen. All possible phenotypes may appear. Therefore there is the question whether selective culling should be applied at the start of the dedomestication phase in order to assist natural selection to stabilize a wild animal-like phenotype and prevent very domestic phenotypes. 

Concerning adaptions necessary for survival, cattle and aurochs probably differ in a similar manner as wild and domestic yak. That means we can assume that in domestic cattle, aspects such as fat storage, digestion, metabolism, endurance and respiration are probably different from aurochs and less adapted for a survival in nature, leading to a higher death rate (this is what we see when comparing domestic to wild yaks), and perhaps this is true for other physiological aspects too. Natural selection prefers those individuals with the better genes for a survival in nature. Not to forget, we have intraspecific selection factors such as sexual selection and combat that will influence the cattle (predators, if present, can be a selection factor as well). These selection factors will prefer those individuals that have a higher chance for winning intraspecific fights and being chosen by mating partners, and therefore have a higher reproductive success. This can be caused by genes influencing behaviour traits and certain morphological traits. Not to forget, both factors are connected. Individuals with a more aware and active behaviour as much as the higher readiness to take risks will have a higher chance of winning fights and thus have a higher reproductive success. These behaviour traits are influenced by the corticosteroid hormones, which also influence morphology and are involved in producing the typical domestic morphology. Some individuals might have behaviour traits that are connected to hormonal levels that would enforce a wildtype like body conformation although they do not look “good” in overall appearance. We would have to acknowledge the fact that nature knows better concerning natural selection. 

What selective culling would do is mainly to remove individuals whose horns have the wrong shape or size or that have the wrong colour. This would, in turn, slow down the process of dedomestication as this could often also eliminate individuals that might have alleles for physiological traits advantageous for survival. We cannot know if an individual that has the wrong colour or small horns has alleles for better fat storage, metabolism or other “invisible” factors crucial for survival, or if it has a more wildtype like behaviour. Thus, selective culling based on such optical factors at the beginning of dedomestication would be counteractive. 

And there is the problem of balancing traits. Most people tend to overemphasize colour because it is the first and strongest impression. Yet it is only regulated by very few genes, while other aspects such as skeletal proportions, body shape and body size have a much more complicated background. For example, the Oostvaardersplassen herd has a bull that has a very aurochs-like morphology in build and shape like no other Heck bull has, it almost looks like a Spanish fighting bull. However, it has a grey colour and upright horns. I, personally, would never cull this individual as a lot of genes are involved in skeletal proportions and morphology but only one allele causes this kind of dilution. But someone else might consider this individual to be one of those with the “undesired traits” because of its grey colour and would cull it. 

Thus, my opinion is that breeding back cattle that have just been released into wilderness should not be culled selectively at all in the first decades. At first, the cattle would have to seize their genetic potential without disturbance by artificial selection in order to get dedomesticated and have a chance of survival that is as high as possible. They would have to have get uniform for the traits enforced by natural and sexual selection as quickly as possible and the result will undoubtedly be aurochs-like due to the regression to the wildtype, as wildtype traits (be it morphological, physiological or anything else) are more beneficial under natural circumstances as the wildtype is a product of nature. Relying on pleiotropic effects might also lead to some surprises just like in the Farm fox experiment, only in the opposite direction. For traits that would take longer to be influenced by natural selection, such as colour variants or horn shape, selective culling could be used after a few decades. But I, personally, would not cull selectively within the first thirty years of dedomestication.