Associations among BCS, milk production and days open in Walloon primiparous dairy cows

Many studies showed that the reproductive performance of dairy cows may be affected by negative energy balance (NEB). The evolution of the body condition score (BCS) in early lactation may reflect the NEB. For this study, BCS of 1,296 primiparous dairy cows were recorded in 76 Walloon dairy herds between April 2006 and December 2008. Associations among milk production, BCS and days open (DO) were studied using logistic regression models in order to find NEB indicators. DO were transformed in binary data (0.1) based on days in milk (DIM) intervals. Two dependant variables were created: DO3579 for DO between 35 and 79 DIM and DO80119 for DO between 80 and 119. Two sets of independent variables were built. The milk production set contained variables such as milk production/composition and breeding values associated, peak yield, estimated 305-d yields, nadir milk protein and fat content, DIM at nadir/peak, etc. The second set concerned the BCS information. It contained BCS and BCS changes during the 120 firsts d of lactation, nadir BCS and DIM at which it occurred, BCS at calving, etc. Two logistic regression models were developed, one for each dependant variables by using SAS stepwise logistic regression. Significant associations were identified. Except for the herd effect, the only measure significantly associated with the likelihood of DO3579 was the BCS loss between calving and 75 d of lactation. Regarding the second model, two genetic milk yield measures and the mean BCS between 60 and 105 d of lactation were significantly associated with the likelihood of DO80119. In both models, the association between DO and the severity of NEB was negative. These preliminary results show that novel functional trait such as BCS could help Walloon breeders to identify cows that present a risk of poor fertility.