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Problems in functional traits of Holsteins are likely partly because of increased inbreeding. Increased inbreeding has been associated with a number of fitness problems, including increased risk of disease occurrence and even reduced survival. Studies have also shown the risk of disease occurrence is significantly increased 3 weeks before calving to 3 weeks after calving. Therefore, comparatively highly inbred Holsteins may be at an extremely high risk for disease because of associations with increased inbreeding and the known increased risk of disease occurrence shortly after calving.
The problems associated with inbreeding in Holsteins could possibly be resolved by crossbreeding with breeds like the Norwegian Red, which has been specifically bred over a number of generations for disease resistance. Crossbreeding may be an effective way to improve health by introducing favourable alleles (i.e., heterosis) and alleviating inbreeding depression. Studies have shown crossbreeding in dairy cattle is associated with decreased disease occurrence and infection and enhanced antibody response, which is particularly relevant to controlling pathogens such as those causing mastitis. Other studies have shown crossbreeding in dairy cattle significantly increases survival, which is likely also due to heterosis. It is not clear whether the response of crossbreeding with Norwegian Red is simply due to heterosis or the additional effect of the breed being selected particularly for disease resistance.
Studies show that immune response traits are moderately heritable (h2 between 0.20 and 0.40) which is satisfactory. However, relatively few studies have evaluated immune response traits in younger animals and then re-evaluated these traits in older animals. It is important to evaluate this to determine if the ability of an animal to acquire an immune response remains constant over its lifetime or if other factors like age have a large effect on the ability. These issues were studied by Dr Cartwright and co-workers and published in the Journal of Animal Science, Volume 90 of 2012, pages 2970 to 2978 with the title: Adaptive immune response, survival, somatic cell score between postpartum Holstein and Norwegian Red x Holstein first-calf heifers.
The objective of their study was to evaluate immune response, survival, and somatic cell score (SCS) between previously immunized Holstein and Norwegian Red x Holstein crossbreds first-calf heifers after calving. Also, they correlated the immune response traits observed in calves in a previous study with their immune response traits observed as first-calf heifers.
Crossbreds had a tendency to have a greater immunoglobulin antibody response to the test antigen, as well as a significantly greater hypersensitivity response after calving compared with Holsteins. This suggests that they may have enhanced defense against pathogens, which may result in increased resistance to a number of diseases compared with the Holsteins. In general, crossbreds also had increased survival, which may be related to their ability to elicit an enhanced immune response compared with the Holsteins. No significant difference was observed between breeds for SCS, which could mean either their resistance to mastitis was not different or that the association of SCS with mastitis may be different in the breeds (which is likely). It was also observed that the correlations between calf and heifer traits were positive and significant, suggesting that these traits are stable and an animal would only have to be evaluated once in its life to predict its future immune response. The results of whether the Norwegian Red had an influence were inconclusive as without access to the purebred animals in this study, it is difficult to determine whether the results observed are due to heterosis in the crossbred animals or due to differences in the breed of sire. Overall, the enhanced immune response observed in the crossbreds compared with the Holsteins indicate that during times of increased incidence of disease the crossbred animals could have enhanced defense against pathogens. This could translate into increased disease resistance and survival in crossbred animals and could therefore further result in an economic gain to producers with crossbred dairy cattle compared with purebred dairy cattle.