The Effect of Abnormal Reproductive Tract Discharge on the Calving to Conception Interval of Dairy Cows

Abstract

Prolonged calving-to-conception interval (CCI) can increase economic loss in cattle. We investigated the effect of post-calving abnormal reproductive tract discharge (ARTD) on CCI in dairy cows and quantified the relationship of ARTD and associated risk factors with CCI. The source population was dairy cows that calved in the study period on three pasture-based, year-round calving farms in the Riverina, NSW, Australia. Farm records and records from veterinarians' visits were analyzed. ARTD was defined as the presence of reproductive tract discharge according to the following classification: per vaginum purulent discharge ≥21 days post-calving or mucopurulent discharge >26 days post-calving. The incidence of ARTD was calculated. A Kaplan-Meier survivor function was used to estimate median time to conception post-calving dependent on the presence or absence of ARTD. Mixed effects Cox-proportional hazard models were used to estimate the direct and indirect effects of ARTD, and other potential risk factors on CCI such as body condition score (BCS), ambient temperature, and milk yield. Model structures were guided by a directed acyclic graph of potential risk factors for ARTD. The incidence of ARTD in lactations was 16% (95% CI 13.8-18.5%) and did not differ significantly between the three farms (P > 0.05). The median CCI was 176 and 118 days for lactations with and without ARTD, respectively (P < 0.01). The rate of pregnancy following calving in cows with ARTD was significantly decreased relative to the rate of pregnancy in cows without (total effect hazard ratio = 0.62, se = 0.18, P = 0.01). High peak milk yield (>32 L) and parity >2 also significantly extended CCI. We did not observe an effect of BCS or ambient temperature on CCI. The incidence of ARTD in the current study was consistent with clinical endometritis (considered a major source of ARTD) reported in other studies. In contrast-and despite regular veterinary assessment and treatment of on the farms in this study-ARTD extended CCI. Evaluation of the economic impact of ARTD on dairy farms in this region is warranted, and methods to identify high risk cows and develop effective interventions are required.

Keywords: Friesian; Holstein; calving to conception; cow; dairy; endometritis; survival analysis.

Figure 1

Directed acyclic graph used to inform survival analyses to identify the influence of endometritis and other potential risk factors in the peri- and post-partum period on calving to conception interval in a study of dairy cows in the Riverina, New South Wales, Australia.

Figure 2

Distributions of body condition score (BCS), parity and peak mild yield of cows from farms 1 and 2 in a study to identify the influence of abnormal reproductive tract discharge and other potential risk factors on the calving to conception interval in dairy cows in the Riverina, NSW, Australia.

Figure 3

Kaplan-Meier survival curve of the probability of remaining not in-calf, dependent on the presence of abnormal reproductive tract discharge (ARTD) during lactation in a study to identify the influence of ARTD and other potential risk factors on the calving to conception interval in dairy cows on three farms in the Riverina, NSW, Australia. “Number of cows at risk” shows the number of cows that remained not in-calf at time points during lactation. “Number of censored observations” shows the number of cows that did not conceive or were not detected as having conceived during the lactation. Orange = cows with ARTD.