Differences between dairy cows descending from artificial insemination bulls vs. dairy cows descending from natural service bulls on organic farms in Switzerland

• Daughters of natural service (NS)-bulls were compared to daughters of artificial insemination (AI)-bulls on organic dairy farms.
• Daughters of NS-bulls had lower somatic cell scores (SCS) and by trend shorter calving intervals (CI).
• Daughters of NS-bulls had by trend lower daily milk yields (DMY).
• DMY was by trend lower in cows with higher SCS.
• Differences might be due to better local adaptation of NS-bulls.

This study investigated differences between phenotypes of daughters of artificial insemination (AI)-bulls and daughters of natural service (NS)-bulls, respectively, on organic dairy farms in Switzerland. Organic rules recommend the use of natural mating. Therefore it is of interest whether those two groups of phenotypes show different characteristics on organic farms. Only farms using both AI and NS with sires of the same dairy breed as the inseminated cows were included in the study. First lactations of 594 cows from 29 farms were analysed for daily milk yield (DMY), somatic cell score (SCS), and calving interval (CI). Furthermore, veterinary treatments and fat/protein ratios >1.5 and <1.1 in milk samples during the first 100 days of lactation indicating risks for metabolic disorders were investigated. General linear models and-for binary variables-logistic regression models were calculated to explore differences in health parameters and DMY. Service method (AI or NS) and DMY (except when DMY was the target variable) were included in the models as fixed effects and farms as random effects. Distances between farms where the bulls had been bred and farms where their daughters lived were calculated with a common route planning tool. Additionally it was examined whether the bull's farm of origin was organic or not. Dairy cows descending from NS-bulls showed a lower SCS and tended to have shorter CI and a trend to lower DMY compared to cows descending from AI-bulls. No effects of service method on other health parameters were found. Around 70% of NS-bulls and 26% of AI-bulls had been bred in the same regions (<100 km distance) as their daughters. No NS-bull, but 35% of AI-bulls came from abroad (≥300 km within Switzerland or from another country). 1.8% of AI-bulls and 30.8% of NS-bulls had been bred on an organic farm. One explanation for the effects found in cows descending from NS-bulls might lie in their better adaptation to local conditions. However, effects of the service method and the bull’s environment of origin cannot be distinguished and it cannot be excluded that the chosen NS bulls had incidentally higher genetic merits for CI and SCS than AI-bulls. Further research on differences between NS- and AI-bulls and also on genotype x environment-interactions between organic and conventional environments is necessary.