The aim of this literature review is to give an overview of the differences between pure- and crossbreeding systems. Crossbreeding is a mating system with individuals of different lines or breeds. It is one of several breeding strategies in dairy production used to increase the economic profit. The use of crossbreeding increases due to changes in the dairy market and an increase of inbreeding among purebred Holstein. The main benefit of crossbreeding is Heterosis, which is the improvement in genetic level in a hybrid offspring above the average of the parent breeds.

Farmers? main reason for crossbreeding dairy cows is that they have seen a decline in functional traits, especially in the Holstein breed. Throughout crossbreeding, the farmers can combine functional and production traits of different dairy breeds and utilize the Heterosis effects which emerge when two or more breeds are crossed. Results from different studies with Holstein crossed with Jersey, Brown Swiss, Normande and Scandinavian Red have shown that the crosses had higher fertility, longer productive life, had fewer stillborn calves and had easier calvings than purebred Holstein. The Holstein crosses have shown very good results and sometimes even better than the purebred Holstein for fat production, which gave higher amount of energy corrected milk (ECM) and higher income for the farmers.

The Heterosis effect and breed group effect of crossbreds between Swedish Holstein (SH) and Swedish Red (SRB) was estimated in this study. Observations on cows born between 1990 and 2012 were used to estimate the Heterosis effect and breed group effect for several traits included in the Nordic breeding goals: production, fertility, udder health, calving performance, survival and other diseases. Breeding within the Holstein dairy breed has earlier been focused on increased milk production and conformation. This, combined with an intensive use of individual animals, has resulted in a radical increase of milk yield in the Holstein dairy cow population all over the world. However, this breeding strategy has resulted in increased inbreeding and several functional traits have impaired.

Artificial insemination (AI) has had a great influence on the development of pig production and is today widely practiced (Gerrit et al., 2005). With AI, genetic progress can at a short period of time be spread worldwide compared with using natural service and the best boars can be intensively used. This is also one of the biggest risks with AI; the impact of semen containing a genetic disease or contamination of pathogens can be enormous. Fortunately, it is a small risk of disease transmission due to many tests of the boars and the semen before use (Maes et al, 2008). It is important that the boars used for insemination have good breeding values along with good reproduction features and have good fertility results (Robinson et al., 2005).

Artificial insemination (AI) has had a great influence on the development of pig production and is today widely practiced (Gerrit et al., 2005). With AI, genetic progress can at a short period of time be spread worldwide compared with using natural service and the best boars can be intensively used. This is also one of the biggest risks with AI; the impact of semen containing a genetic disease or contamination of pathogens can be enormous. Fortunately, it is a small risk of disease transmission due to many tests of the boars and the semen before use (Maes et al, 2008). It is important that the boars used for insemination have good breeding values along with good reproduction features and have good fertility results (Robinson et al., 2005).