All animals change during individual development (ontogeny), and this process of changing continues until death. For “breeding-back”, it is important to look at how a bull or cow might change during its life as it is relevant for selection.
Usually, it is impossible to judge an the bull or cow properly before the age of 3 years. The bodily proportions and shape are still going to change considerably, and the horns are not nearly developed enough.
Usually, all calves are rather long-legged animals, proportioned like deer. During its life, the legs always get shorter and the trunk longer. Also, the trunk gets heavier. A bull usually does not have its full bulk until the age of 6 years (Frisch, 2010), and bulk continues increasing. So if a young bull at the age of 3 years already is proportioned and shaped like an aurochs, you can assume that it will end up heavier and more longish at reproductive age. Also, the hump is a trait that increases with age. A bull that shows no hump at the age of 3 might still develop one later on.
Also, horn seem to continue to grow all life long, and they also slightly alter their shape by continuing the curve. Horns that might not look much aurochs-like at the age of three might end up satisfying by the age of five years.
I give you some examples for changing body shape and horns in Taurus cattle now that I was able to find thanks to an extensive photo archive I was provided by Matthias Scharf from the ABU. Most of the photos are courtesy of Matthias Scharf so please do not replicate without permission.
On the upper photo, you see Lamarck at the age of three years. Although one would assume this bull is more or less adult based on its looks, and simply lacks the inwards-curve in its horns, he developed very clearly inwards-facing horn tips later on. Not quite as spiral-shaped as in an aurochs, but satisfying. The second photo shows him at the age of eight years.
This nameless and extremely beautiful cow had horns that did not curve that much either when I saw her in 2013, when she was two years old. One year later, her horns developed a nice shape – in most aurochs cows, they would probably still be more curled, but those are satisfying horns.
Lerida is and always has been one of my favourite Taurus cows. The upper photo shows her with a nice slender body at the age of five. Three years later, she “put on some weight”, but still is a beautiful individual.
42 623 is the current breeding bull at Hellinghauser Mersch. The upper photo shows him at the age of one and a half year, he has squarely built proportions and a body of acceptable bulk. In 2015, the year the lower photo was taken, he developed a rather massive body, but the trunk-height ratio still seems to be ok, the shoulder hump is more pronounced and he is also one of the largest bulls.
We should not forget the ontogenic changes of wild-type colour as well. Phenotypical E+ is the only colour variant that shows ontogenic changes. All wildtype coloured calfs are born in a chestnut colour – during their first life months the colour changes (or: should change) to black in bulls, and cows darken only on certain areas (depending on the degree of sexual dichromatism). If the wildtype coloured calf is going to show dilution factors, it will show them later on, when the change in colour appears. Therefore, also Chianina or Podolian calfs are born in a chestnut brown colour, but their fur looses the pigment in the aftermath.