It is not all positive that has come out of animal breeding practices. There are examples where selective breeding has been taken too far. There are also examples where selective breeding has not only improved certain performances, but simultaneously and unintendedly also deteriorated other performances that were not under selection: the so-called negative correlated responses. Both types of negative responses to selective breeding are difficult to predict and are usually only noticed afterwards. This is the case because it takes a while to realise that the negative effects are structural and not coincidence, and that they occur at increasing frequency throughout the population. Even then it sometimes takes stepping back to realise the negative consequences. Changes are going slowly, so you get used to them.
1.13.1 Negative effects in dog breeding
Some clear examples of selection that has gone too far can be found in dog breeding. This is partly because selective breeding in dogs has a long history, but mainly because some dog breeds are selected mainly on looks. And most extreme looks tend to be considered the best, so selection in those breeds has been, and still is, on extreme looks (for some examples see Figure 7). The fact that shape of the skull in some breeds make it difficult for them to eat normal food because of the upper jaw being much shorter than the lower jaw, such as in case of the Boxer or the Bulldog, or breathing, such as in case of all breeds with a short upper jaw resulting in a flat face, or give birth or even mate without medical intervention (e.g. Bulldog), or where there is a risk that the eyes pop out of the socket because the skull is too small for their eyes (e.g. Pekinese, Chihuahua), are clear examples of selection being taken too far. And most of these examples are only related to the skull. Other breed characteristics that are not increasing the dog's wellbeing are, for example, too long ears so that infections are common (e.g. Basset Hound), or long back and neck so that intervertebral disc disease has become common (e.g. Dachshund), or too much skin so that inflammation in between the folds becomes common (e.g. Bulldog), or sloping back so that hip problems are common (e.g. German Shepherd Dog). All examples relate to selective breeding and taking breeds more and more too an extreme, because that is what you win the show with. Looking back only we realise that we have gone too far. And that realisation comes only very slowly because people get used to animals with certain features. They don't consider them abnormal for a very long time. Important is to realise that these effects can be reversed by selecting in the opposite direction.
1.13.2 Negative effects of selection for performance
It is not just dogs where we have taken selective breeding a step too far. Selection for large offspring has resulted in a high fraction of difficult births, sometimes requiring caesarean sections in the Texel sheep, and it was even almost as a standard way of delivering in the beef cattle breeds Belgian White-and-Blue cattle and the Dutch Improved Red-and-White. In the Texel sheep selection against difficult births has resulted in a decreasing fraction of birth requiring assistance. In this case the process could be reversed. But in the Belgian White-and- Blue and the Dutch Improved Red-and-White cattle the situation is more complex and the repair process will take many generations.
Birth problems are not the only unintended negative consequence of selective breeding in farm animals. Remember that the intention was to produce a lot of food, but cheaply so that it would be available for everyone. This has resulted in intensive farming systems, such as in pigs and poultry farming, where the animal products should be produced with as little costs as possible. So fast growth or more eggs with less feed. This has gone very well for many years and breeders really thought that there would be no limits to the genetic improvements as production increased linearly. Unfortunately, in the 1980’s it became more clear that there were also some negative consequences of strong selection for performance traits. For example, broilers started to show metabolic health problems due to the fast growth, laying hens started to have increased bone fractures because they couldn’t manage sufficient calcium intake to deposit into the increasing number of eggs. However, in animal breeding the awareness raised that these negative effects should be corrected and avoided in the future. In introduction DNA techniques in animal breeding and the increased capacity of computers facilitates a more balanced improvement of desired traits and the avoidance of negative effects.
1.13.2.1 Fertility in dairy cows as an example of corrective breeding
Dairy cows started to show reduced fertility during the high production period. This is illustrated in the figures below, where the trends in the breeding value for milk production and for calving interval in dairy cows is given. Since those problems became apparent around 2000 selection pressure has shifted around from mainly milk production traits to much more attention to animal health and reproductive performance. This shift has been the trend in all farm animal species. The introduction of genomic selection around 2010 facilitates the balanced improvement of milk production and fertility.
