High somatic cell counts and changes in milk fat and protein contents around insemination are negatively associated with conception in dairy cows

The fertility of dairy cows has decreased dramatically worldwide over the last few decades, and several causes of this trend have been reported. Several studies have associated compromised udder health with deteriorating reproduction performance. Subclinical ketosis (SCK) has also been reported to be a risk factor for decreased conception. The objective of the present study was to describe how SCK might interact with the reported association between udder health and conception in dairy cows. Data from the French Milk Control Program and data on 8,549,667 instances of artificial insemination (AI) and their corresponding preceding and subsequent test-days from 5,979,701 Holstein cows were examined over a 5-year period (2008-2012). The effect of udder health was evaluated through a low (L) or high (H) somatic cell count (SCC) before and after AI using a threshold of 200,000 cells/mL, and transformed into four groups (LL, LH, HI, and HH). Three proxies for defining SCK were proposed based on the milk fat and protein content (or their ratio) before AI. Statistical analysis first included a generalized additive model to help define the optimal threshold values. Next, a logistic regression with a Poisson correction was performed. On average, the risk of conception at first AI was reduced by 14% for LH or HH cows (relative risk [and 95% CI] = 0.86 10.85-0.871) when the SCC increased or remained high within 40 days before and after AI, relative to LL group. The reduction of conception success associated with SCK (fat and protein contents changes) varied from 3% to 17% depending on the used SCK proxy. Including the interaction term SCC*SCK clearly showed that the association of increased SCC around AI with conception success was modified by the presence of SCK. A cow that already has SCK and experiences an increase in SCC around or after AI exhibits up to 2 times further decrease in conception success compared with a cow with a high SCC and no SCK. In conclusion, this study reinforces the previously described association between intramammary inflammation around or after AI and a decreased rate of conception. These findings highlight how SCK interacts with the above mentioned relationship by strengthening the negative association between mastitis and conception success. In addition, the present work supports the theory that local inflammation may affect the whole-body response and alter the functions of other organs, such as the reproductive tract.