Effect of subclinical mastitis detected in the first month of lactation on somatic cell count linear scores, milk yield, fertility, and culling of dairy cows in certified organic herds

摘要

It is well established that subclinical mastitis (SCM),characterized by somatic cell count (SCC) >200,000cells/mL, has a negative effect on the productivity, reproductiveperformance, and survivability of cows fromconventional dairy herds. However, in organic herds,where the use of antimicrobial drugs is restricted forthe treatment and control of intramammary infections(IMI) in dairy cows, little is known about the effect ofSCM on performance and survivability. The objectiveof this study was to evaluate whether SCM diagnosedduring the first month of lactation was associated withSCC linear score dynamics, milk production, fertility,and culling of dairy cows in USDA-certified organicherds. We collected data from 2 organic herds in NewMexico and Texas. A total of 1,511 cows that calvedbetween June 2018 and May 2019 were included in thestudy and were followed until month 10 of the currentlactation. Cows with SCC >200,000 cells/mL in thefirst month of lactation were considered to have SCM.We used mixed linear regression models accountingfor repeated measures to assess the effect of SCM onmonthly milk production and SCC linear scores. Weused Cox proportional hazards models to evaluate theeffect of SCM on the risk of pregnancy and culling. Weconsidered parity, farm, previous gestation length, stillbirth,twinning, dystocia, and 2- and 3-way interactionsas potential confounders. Cows diagnosed with SCMduring the first month of lactation produced less milkthan cows without SCM. Cows with SCM had elevatedSCC linear scores during their previous lactation andthroughout the subsequent months of lactation comparedto cows without SCM. The effect of SCM onSCC linear scores was more pronounced in multiparousthan primiparous cows. Subclinical mastitis during thefirst month of lactation did not affect the likelihood ofpregnancy during the first 300 d in milk. Cows withSCM in the first month were more likely to die or beculled during the 300 d of lactation than cows withoutSCM. We observed that elevated SCC in the firstmonth of lactation had detrimental effects on the milkyield and survivability of dairy cows in USDA organicherds, but it did not affect reproductive performance.We demonstrated that cows with SCM diagnosed in thefirst month of lactation continued to have elevated SCClinear scores throughout their entire lactation, and thatelevated SCC was carried over from the previous lactation.